Article pubs.acs.org/JPCA
Unraveling the Conformational Landscape of Triallyl Phosphate: Matrix Isolation Infrared Spectroscopy and Density Functional Theory Computations N. Ramanathan, C. V. S. Brahmmananda Rao, K. Sankaran, and K. Sundararajan* Chemistry Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamil Nadu, India S Supporting Information *
ABSTRACT: The conformations of triallyl phosphate (TAP) were studied using matrix isolation infrared spectroscopy and density functional theory (DFT) calculations. TAP was trapped in N2, Ar, and Xe matrixes at 12 K using an effusive source and the resultant infrared spectra recorded. The computational analysis on conformers of TAP is a challenging problem due to the presence of the large number of conformations. To simplify this problem, conformational analysis was performed on prototypical molecules such as dimethyl allyl phosphate (DMAP) and diallyl methyl phosphate (DAMP), to systematically arrive at the conformations of TAP. The above methodology discerned 131 conformations for TAP, which were found to contribute to the room temperature population. The computations were performed using B3LYP/6-311++G(d,p) level of theory. Vibrational wavenumber calculations were performed for the various conformers to assign the experimental infrared features of TAP, trapped in solid N2, Ar, and Xe matrixes.
1. INTRODUCTION Nuclear reprocessing technology has played a key role in the development of liquid−liquid extraction for the industrial-scale separation of metals.1 The high degree of purity of materials needed for nuclear applications favor solvent extraction to be the method of choice for the separation when compared to other methods like precipitation and ion exchange.2 Tri-n-butyl phosphate (TBP) as an extractant is considered as the work horse of the nuclear reprocessing industry because it has all the desirable properties of an extractant in the solvent extraction process and hence has wide applications in various stages of the nuclear fuel cycle.3−5 Although reprocessing of spent nuclear fuel is performed predominantly by the solvent extraction process, currently, the solid-phase extraction (SPE) technique has been emerging as the active field of research as an alternate to the conventional solvent extraction procedure, which involves partitioning between a liquid and solid (sorbent) phase.6 This sample treatment technique enables the purification of analytes from solution by sorption on a solid sorbent. In comparison to liquid−liquid extraction, SPE has notable advantages in the reduction of large volumes of organic waste and in tuning the selectivity of the solid-phase extractant for a specific metal ion.7 Because there is no organic phase; the requirement of phase modifier is eliminated and the third phase formation is avoided. Thus, SPE is likely to reduce the complexity of reprocessing relative to solvent extraction, which in turn makes the procedure cost-effective. It is either important to incorporate extractants used in solvent extraction process into solid-phase materials to make the sorbent materials for separations (extraction chromatography) or to modify the extractant itself to make it © 2015 American Chemical Society
suitable as a sorbent material for extraction. A number of reports exist for organic molecules (including organophosphorous compounds) incorporated onto the polymer based ion-exchange sorbent materials.7−12 Because organophosphate (TBP) is used as an extractant for solvent extraction in nuclear reprocessing; it will be prudent to make a polymer out of phosphate to make use of the phosphoryl group in exploring the extraction of metals of interest in solid phase. In this context, poly triallyl phosphate (PTAP) derived from monomeric triallyl phosphate (TAP) by virtue of the double bond was found to be a promising solid-phase extractant of the future. Polymerization of TAP could be achieved through emulsion polymerization with halogenated methanes.13 The studies on suitability of PTAP as a solid-phase extractant are in progress in our laboratory. Because physicochemical behavior of the extractant is integrated from the molecular level, understanding the conformations of the monomeric extractant assumes significance. The analysis of different conformations of TAP can be expected to throw light on the bulk-phase properties of TAP, such as dipole moment, solubility, mass density, self-diffusion coefficient, etc., which eventually can be extrapolated to the polymeric species. Importantly, the conformations have a strong control over physical, chemical, and biological properties of materials.14−16 The conformational analysis of large molecules poses a serious challenge due to the large multitude of conformations present in it. Reva, Fausto and co-workers reported the extensive study of conformations of 1,4-, 1,3-, and 1,2-butanediols, which possessed Received: January 28, 2015 Revised: April 7, 2015 Published: April 7, 2015 4017
DOI: 10.1021/acs.jpca.5b00889 J. Phys. Chem. A 2015, 119, 4017−4031
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were performed to ensure that the computed structures corresponded to minima on the potential surface and also to assign the vibrational features observed in the experiments. Computed frequencies were assigned to the experimental infrared features after incorporation of a scaling factor. To arrive at the scaling factor, the experimentally observed strongest feature at 1280.6 cm−1 in the PO stretching region was correlated with the strongest computed feature of the ground state conformer in this region. A scaling factor of 1.022 would be required to bring the two values in agreement. This scaling factor was used to scale other features of the ground and higher energy cluster of conformers, in this region. The scaling factor is believed to be necessary, to account for the influence of the host matrix on the vibrational wavenumbers of the infrared spectrum.45−48 The SYNSPEC program was used to simulate a vibrational spectrum using the computed wavenumbers and intensities, by assuming a Lorentzian line profile with a line width of 4 cm−1.49 Calculations were performed using the Onsager self consistent reaction field (SCRF) model,50,51 as implemented in the Gaussian program, to determine quantitatively the effect of the matrix on the energies of a few selected conformers of TAP. In this model, the solute molecule is placed in a spherical cavity surrounded by a continuum, the continuum being originated from the influence of solvent. The radius of the spherical cavity occupied by the solute was estimated on the basis of molecular volumes obtained from the Gaussian calculations. Subsequently in the calculations, the radius of the spherical cavity and the dielectric constant of the matrix material were specified as additional inputs in addition to the molecular specification. Using this model, different dielectric constants52−55 were investigated to examine the effect of surroundings on the conformational distribution of TAP. To understand the nature of the electronic effects in determining conformational preferences, Natural Bond Orbital analysis (NBO, Version 3.1), invoked through Gaussian was performed.56 The NBO has been reported to untangle the conformational preferences in several systems.57−68
65, 73, and 81 unique conformational minima, respectively, using matrix isolation infrared and ab initio calculations.17−19 The vast number of conformations in these systems was reported to be due to a presence of conformationally relevant 3-fold rotational axes. The conformational analysis of TAP is a complex problem, because TAP possesses nine carbon atoms, which results in a large number of conformations. It is, therefore, imperative to find out the entire conformational picture of TAP through a simplified approach. On the basis of our earlier work on trimethyl phosphate (TMP),20−24 triethyl phosphate (TEP),25,26 acetals/ketals,27−32 silanes,33−36 carbonates,37,38 and phosphite,39 we concluded that a combination of hyperconjugative (arises due to the delocalization interactions) and steric interactions decides the conformational preferences in these molecules. It is interesting to probe the conformations of TAP as the knowledge on conformations would serve as a platform to understand its chemistry in the condensed phase. In this work, therefore, the conformations of TAP were studied to understand its conformational landscape using matrix isolation infrared spectroscopy. The DFT calculations were used to arrive at the various possible conformations of TAP and also to correlate our experimental results.
2. EXPERIMENTAL DETAILS Matrix isolation experiments were carried out using a Leybold AG helium-compressor-cooled closed cycle cryostat. The details of the vacuum system are described elsewhere.20−23 TAP was prepared through a simple one-step synthesis from phosphoryl chloride (POCl3; 1 mol) and allyl alcohol (3 mol) precursors by mixing them in a round-bottom flask, which was maintained at −10 °C. The synthetic procedure is analogous to the one reported for the preparation of various trialkyl phosphates.40 The TAP was vacuum distilled to remove traces of water and unreacted allyl alcohol. Furthermore, the sample was subjected to several freeze−pump−thaw cycles before performing experiments. The infrared spectrum of liquid TAP was recorded using ZnSe window, which showed an excellent agreement with the earlier report.41−43 TAP and matrix gas were deposited by streaming them separately through a twin jet nozzle system. TAP was maintained at room temperature during the deposition. N2 (INOX, 99.9995%), Ar (INOX, 99.9995%), and Xe (Chemtron, 99.997%) were used as a matrix gases. Deposition was carried out at the rate of ∼3 mmol/h, and a typical deposition lasted for ∼1 h. The spectra were recorded using a BOMEM MB 100 FTIR spectrometer with a spectral resolution of 1 cm−1. The spectrum was recorded at 12 K and subsequently, the matrix was warmed to 30 K (N2), 35 K (Ar), and 60 K (Xe), maintained at this temperature for ∼15 min and recooled to 12 K and the spectra were again recorded.
4. RESULTS AND DISCUSSION 4.1. Experimental Section. Figure 1 (1350−1200 cm−1, grid A, and 1100−800 cm−1, grid B) compares the infrared spectra of neat TAP liquid (trace a) with TAP isolated in N2, Ar, and Xe matrixes (traces b−d). The region spanned in the figure corresponds to PO (grid A), CC, CO, and PO stretching (grid B) vibrational modes of TAP. Almost all of these vibrations are either coupled with CH or CH2 deformation vibrations. The comparison of the bulk liquid and matrix isolated spectra reveals the relatively broad nature of the infrared spectrum in the bulk phase, which clearly precludes the conformational analysis in the liquid TAP. Our computations indicated that, of the different PO, CC, CO, and PO stretching regions of TAP, the features due to the PO stretching region (1350−1200 cm−1) show the maximum separation (by more than 20 cm−1) for the different conformations; hence, the analyses of TAP for different conformations were focused on the PO stretching region (Figure 1, grid A). An analysis of the normal modes using Gauss View, revealed that the 1350−1200 cm−1 features were not pure PO stretching vibrations of TAP but were mixed with CH2 twisting modes. Figure 2 compares the simulated spectra of a few representative conformers in the CC, CO, and PO stretching
3. COMPUTATIONAL DETAILS Computations were performed using a GAUSSIAN 03W package,44 on a Pentium 4 machine using DFT methods. The structures of the conformers of allyl phosphates ((dimethyl allyl phosphate (DMAP), diallyl methyl phosphate (DAMP), and TAP) were computed using a 6-311++G(d,p) basis set. DFT methodology was adopted through the B3LYP hybrid exchange−correlational functional. All the structures were optimized without imposing any symmetry restrictions during the optimization process. Vibrational wavenumber calculations 4018
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Figure 1. Infrared spectra of TAP spanning the region 1350−1200 cm (grid A) and 1100−800 cm−1 (grid B): (a) neat liquid; (b) N2 matrix (12 K); (c) Ar matrix (12 K); (d) Xe matrix (12 K).
Figure 3. Comparison of computed and experimental spectra of TAP, spanning the region 1350−1200 cm−1: (a) scaled computed spectra of all 131 conformers of TAP; (b) matrix isolation infrared spectra of TAP in a N2 matrix recorded using an effusive source at 12 K; (c) annealed at 30 K.
Figure 2. Comparison of computed and experimental spectra of TAP, spanning the region 1100−800 cm−1. (a) Scaled computed spectra of a few representative G±(tc)G±(tc)G±(tc), G±(te±)G±(te±)G±(te±), G±(te∓)G±(te∓)G±(te∓), G±(g±e∓)G±(g±e∓)G±(g±e∓), G±(g±e±)G ± (g ± e ± )G ± (g ± e ± ), G ± (g ± c)G ± (g ± c)G ± (g± c), G ± (tc)G ± (te ± )G±(g±e±), and G±(te∓)G±(g±e∓)G±(g±c) conformers of TAP. (b) Scaled computed spectra of a few representative T(tc)G±(tc)G±(tc), T(te±)G±(te±)G±(te±), T(tc)G±(tc)G±(te∓), T(te∓)G±(te∓)G±(te∓), and T(g±e±)G±(g±e±)G±(g±e±) conformers of TAP. (c) Matrix isolation infrared spectrum of TAP in a N2 matrix recorded using an effusive source at 12 K.
regions of TAP (discussed in detail in section 4.4) and the infrared spectrum of TAP in the N2 matrix. Figure 3 shows the comparison of the simulated spectrum of TAP (trace a; more details are given in section 4.2) in the PO stretching + CH2 twisting mode and the experimental infrared spectrum in the N2 matrix (trace b). The main spectral features of TAP in a N2 matrix (in the PO stretching + CH2 twisting mode) occur at 1303.7, 1280.6, and 1267.4 cm−1. The features observed at 1291.7 and 1250.4 cm−1 could probably be due to TAP−H2O adducts, because H2O is an inevitable impurity in a matrix isolation experiment. When the matrix was annealed at 30 K, the splitting of the features near 1280.6 was observed (Figure 3, trace c). Figure 4 (traces a−d) shows the infrared spectra of TAP in the Xe matrix at different annealing temperatures. The spectral features of TAP in the Xe matrix (in the PO stretching + CH2 twisting mode) occur at 1302.6, 1299.9, 1277.2, 1272.9, and 1267.1 cm−1. The features observed at 1289.3 and 1248.5 cm−1 are likely due to TAP−H2O adducts in the Xe matrix. 4.2. Computational Exploration of the Conformations of TAP. Because TAP is the phosphoric acid ester of allyl alcohol, the knowledge on the conformations of allyl alcohol becomes important in systematically arriving at the conformations of TAP. Recent work by Durig et al. reported five possible conformations for allyl alcohol, which arises due to the rotation around CC and CO bonds by changing the relative position of the hydroxy group with respect to the CC single and double bonds.69 This eventually results in 0°, +120°, and −120° orientations of the terminal carbon atom (connected to the next nearest carbon atom through double bond) with respect to the oxygen atom of allyl alcohol. When the allyl
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Figure 4. Infrared spectra of TAP in the Xe matrix spanning the region 1350−1200 cm−1. Spectra recorded at (a) 12 K and annealed at (b) 40 K, (c) 50 K, and (d) 60 K.
alcoholic moiety replaces the hydrogen atom of phosphoric acid to form TAP, the determination of the relative positions of all carbon atoms with respect to the next three neighboring atoms become important, which eventually leads to many possible conformations. The conformational analysis on a single allyl chain would simplify the problem of TAP to arrive at all possible orientations in TAP. To systematically approach the conformational problem of TAP, calculations were first performed on DMAP (Figure 5), i.e., a molecule where one of the methyl groups of TMP was replaced by an allyl group. It is clear that the basic orientations of the conformations of allyl phosphates can be derived from the conformations of TMP molecule, as the allyl chain can be considered to arise from the replacement of one of the hydrogens of the methyl group by a vinyl moiety. For TMP molecule, computations identified three minima, corresponding to conformers with C3 (G±G±G±), C1 (TG±G±), and Cs (TG+G−) structures, given in the order of increasing energy.20−23 The preference for the “Gauche” orientation in the different conformers of TMP was attributed to the operation of both geminal and vicinal delocalization hyperconjugative interactions.39 In TAP too, the carbon atoms attached to the oxygen should prefer to have the “Gauche” orientation with varying orientations of rest of the carbon atoms (of the vinyl moiety) of the allyl chain. Similarly, the higher energy cluster should arise with the basic orientation of TG±G± for varying orientations of the rest of the carbon atoms of the allyl chain. A closer examination of DMAP revealed that the first carbon attached to oxygen atom is restricted to have a “Gauche (60°)” orientation in its ground state (GGG orientation due to hyperconjugation). Given the orientation of the first carbon as G±, the second and third carbons adopt “gauche (±60°)”/“trans (180°)” and “cis (0°)”/“gauche (±120°)”/“gauche (∓120°)” orientations, respectively, which results in 1 × 2 × 3 = 6 orientations for the G±(xy)G±G± cluster of DMAP. To differentiate the “gauche (±60)” and “gauche (±120°)”/“gauche
Figure 5. Computed structures at B3LYP/6-311++G(d,p) level of theory of a few conformers of DMAP. The ZPE corrected energy with respect to the ground state G±(tc)G±G± conformer of DMAP is also given alongside of the structures.
(∓120°)” conformations, the latter is designated as “±e”/“∓e” orientations. It can be noted that the terminology “Gauche, 60° (G)/Trans, 180° (T)” and “gauche, 60° (g)/gauche, 120° (e)/trans, 180° (t)/cis, 0° (c)” are used to denote the hyperconjugative (first carbon attached to oxygen) and nonhyperconjugative carbon atoms (second and third carbons), respectively. Table 1 shows the absolute and relative energies of conformers of DMAP, computed at the B3LYP/6-311++G(d,p) level of theory. In the calculations of DMAP, we have observed certain important exclusions. In the G±(xy)G±G± cluster, the second carbon does not adopt “g∓” orientation if the first hyperconjugative carbon attached to the oxygen orients “G±”, probably due to the steric interaction operating between the terminal carbon and hydrogen atoms with phosphoryl group. For example, the G±(g∓c)G±G± conformer was interconverted to the G±(tc)G±G± conformer during optimization. This eliminates three conformers in the G±(xy)G±G± cluster for DMAP, which otherwise would have resulted in a total of nine conformations. Likewise, the T(xy)G±G± cluster also showed a similar behavior for the orientation, when, x = g∓. Additionally, in the higher energy T(xy)G±G± cluster of DMAP, the “g±” orientation for the second carbon gets interconverted to the “trans” orientation if the third carbon orients “e∓” as the T(g±e∓)G±G± conformer of DMAP did optimize to T(te∓)G±G± in the potential energy surface. Likewise, the T(g±c)G±G± conformer was found to 4020
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have been probed for each G±(xy)G±(xy)G±(xy) and T(xy)G±(xy)G±(xy) clusters of conformations of TAP. It can be reiterated that the clusters of conformations of TAP correlate with the ground state, G±G±G±, and the first higher energy TG±G± conformers of TMP. Because the second higher energy TG+G− conformer has been computed to contribute to only a few percent of the total population in TMP,20−23 this orientation of the hyperconjugative carbon has not been considered for TAP. In other words, the T(xy)G+(xy)G−(xy) cluster was not calculated for different conformations. Based on the computations performed on DMAP and DAMP, an important rule was formulated in arriving at the conformations of TAP, which is discussed in the subsequent section. 4.3. Additivity Rule. It is clear that as the number of allyl chains replacing the methyl group of TMP gets increased, the conformational analysis becomes complicated. For example, the number of conformations for the GGG cluster, which in DMAP is 6, increases to 36 in DAMP and 216 in TAP. As the number of conformations becomes larger and larger, the brute-force search of the conformations would be extremely demanding on time and the computational resources. The examination of the conformers of DAMP and TAP by making a comparison with DMAP unfolds an interesting “additivity rule” in deriving the relative energies the conformations of multi chain phosphate without needing extensive computations. Consider the conformation of the diallyl phosphate (DAMP), where one chain has the “te∓” orientation and the second chain has the “g±c” structure. For G±(te∓)G±G± and G±(g±c)G±G± conformers of DMAP, in the monoallyl chain, the energy was 0.086 and 0.647 kcal/mol, respectively relative to the G±(tc)G±G± conformer. In the diallyl chain of DAMP, where one of the two chains has “te∓” and the other “g±c” orientations, the predicted energy of the G±(te∓)G±(g±c)G± conformer was found to be 0.733 kcal/mol (0.086 + 0.647; results from the addition of relative energy of G±(te∓)G±G± and G±(g±c)G±G± conformers of DMAP molecule), whereas the calculated energy of the conformer G±(te∓)G±(g±c)G± relative to the ground state G±(tc)G±(tc)G± form was 0.695 kcal/mol. Similarly, for TAP with three allyl chains, the predicted energy of the G±(te±)G±(g±e∓)G±(g±e±) conformer was found to be 0.862 kcal/mol, the calculated energy being 0.781 kcal/mol, which in turn is reasonably close to the predicted energy. Likewise, for the T(tc)G±(te∓)G±(g±e∓) conformer of TAP, the predicted energy (1.110 kcal/mol) and its computed value (1.142 kcal/mol) are in fair agreement with each other. The “additivity rule” for predicting the relative energy of multichain allyl phosphates hence can be defined as “the relative energy of the multi chains is the sum of relative energies of the individual chains”. The comparison of the computed and predicted energies of the conformers of DAMP employing the “additivity rule” using a model DMAP molecule are given in the Supporting Information. To rigorously test for the agreement between the predicted and computed energies of the different conformers, all possible conformers of TAP were computed at the B3LYP level of theory using the 6-311++G(d,p) basis set. Not unexpectedly, for a few of the conformers, there is a large deviation between the computed and predicted energies derived from DMAP and the deviation is likely due to the interaction between the inter chains. Nevertheless, the employment of the “additivity rule” results in the prediction of the relative energies of multichain phosphate with reasonable accuracy, without demanding timeconsuming extensive computations. Of course, unraveling all
Table 1. Absolute (Hartrees) and Relative (kcal/mol) Energies of Conformers of DMAP Calculated at the B3LYP/ 6-311++G(d,p) level of theory s. no.
structure
1 2 3 4 5 6
G±(tc)G±G± G±(te±)G±G± G±(te∓)G±G± G±(g±e∓)G±G± G±(g±e±)G±G± G±(g±c)G±G±
7 8 9 10
T(tc)G±G± T(te∓)G±G± T(te±)G±G± T(g±e±)G±G±
a
absolute energya (Hartrees) G±(xy)G±G± Cluster −839.458227 −839.458162 −839.458090 −839.457652 −839.457494 −839.457195 T(xy)G±G± Cluster −839.457171 −839.456855 −839.456706 −839.455434
relative energya (kcal/mol) 0.000 0.041 0.086 0.361 0.460 0.647 0.663 0.861 0.954 1.753
Energies corrected for ZPE.
interconvert to the T(tc)G±G± structure during optimization. The elimination of these conformers is also likely due to the steric interaction between the allyl chain and the methyl group connected to the adjacent oxygen (inter chain interaction). The effective conformational minima for the T(xy)G±G± cluster then turns out to be four against six of the G±(xy)G±G± cluster of DMAP. The above exercise clearly helped us to eliminate certain conformers in DMAP (three out of nine in the G±(xy)G±G± cluster and f ive out of nine in the T(xy)G±G± cluster), which greatly simplified the conformational search of TAP. All optimized conformers of DMAP along with the energies calculated at B3LYP level of theory using 6-311++G(d,p) basis set are given in Table 1 and the structures of a few selected conformers are shown in Figure 5. The calculations were then performed on the conformers of DAMP, which had two allyl chains and a methyl group. These computations were accomplished to examine the variation of the conformational orientations, if any, due to the inter chain interaction. The number of conformers for the G±(xy)G±(xy)G± and the T(xy)G±(xy)G± clusters turn out to be 36 (6 × 6) and 24 (4 × 6), respectively. Out of 60 (36 for G±(xy)G±(xy)G± and 24 for T(xy)± G (xy)G±) conformers in total, 58 minima were obtained for DAMP, confirming that there is minimal interchain interaction between the two allyl chains. Of the 36 G±(xy)G±(xy)G± clusters, 15 conformers were found to be degenerate with the already existing 21 conformers, leading to an effective total of 45 unique conformations in the potential energy surface of DAMP. The conformers with the geometries T(te±)G±(g±c)G± and T(g±e±)G±(g±c)G± interconverted to T(te±)G±(tc)G± and T(g±e±)G±(tc)G± conformers, respectively, indicating the interconversion is likely due to the steric interaction between the two allyl chains. Calculated absolute and relative energies of the conformers of DAMP computed at the B3LYP level of theory using the 6-311++G(d,p) basis set are given in the Supporting Information. The conformational analysis was then performed on TAP whose conformations were calculated to be 216 (6 × 6 × 6) and 144 (4 × 6 × 6) for G±(xy)G±(xy)G±(xy) and T(xy)G±(xy)G±(xy) clusters, respectively. Clearly, the conformational analysis performed on DMAP and DAMP molecules eliminated the complexity in probing the conformations of TAP; otherwise, 729 (9 × 9 × 9) conformers should 4021
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Table 2. Structures, Computed and Predicted (Based on the “Additivity Rule”) Relative Energies (kcal/mol), Vibrational Wavenumbers, and Room Temperature Population of All 131 Conformers of TAPa calculated (PO stretch + CH2 twist) vibrational wavenumber (cm−1)
relative energy (kcal/mol) s. no.
structure
computedb ±
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53
G±(tc)G±(tc)G±(tc) G±(tc)G±(te±)G±(te∓) G±(te±)G±(te±)G±(te±) G±(tc)G±(te±)G±(te±) G±(te±)G±(te±)G±(te∓) G±(tc)G±(tc)G±(te±) G±(tc)G±(tc)G±(te∓) G±(te±)G±(te∓)G±(te∓) G±(te∓)G±(te∓)G±(te∓) G±(tc)G±(te∓)G±(te∓) G±(tc)G±(tc)G±(g±e∓) G±(tc)G±(te±)G±(g±e∓) G±(te±)G±(te±)G±(g±e∓) G±(tc)G±(te∓)G±(g±e∓) G±(te±)G±(te±)G±(g±e±) G±(te±)G±(te∓)G±(g±e∓) G±(te∓)G±(te∓)G±(g±e∓) G±(tc)G±(tc)G±(g±e±) G±(te±)G±(te∓)G±(g±e±) G±(tc)G±(te∓)G±(g±e±) G±(tc)G±(te±)G±(g±e±) G±(te∓)G±(te∓)G±(g±e±) G±(tc)G±(te±)G±(g±c) G±(te∓)G±(te∓)G±(g±c) G±(te±)G±(te±)G±(g±c) G±(te±)G±(g±e∓)G±(g±e±) G±(tc)G±(g±e∓)G±(g±e∓) G±(tc)G±(g±e∓)G±(g±e±) G±(te±)G±(g±e∓)G±(g±e∓) G±(te∓)G±(g±e∓)G±(g±e∓) G±(te±)G±(te∓)G±(g±c) G±(te∓)G±(g±e∓)G±(g±e±) G±(tc)G±(g±e∓)G±(g±c) G±(te±)G±(g±e±)G±(g±e±) G±(te∓)G±(g±e∓)G±(g±c) G±(te∓)G±(g±e±)G±(g±e±) G±(te±)G±(g±e∓)G±(g±c) G±(tc)G±(g±e±)G±(g±e±) G±(te∓)G±(g±e±)G±(g±c) G±(g±e∓)G±(g±e∓)G±(g±e±) G±(te±)G±(g±e±)G±(g±c) G±(g±e∓)G±(g±e∓)G±(g±e∓) G±(g±e∓)G±(g±e±)G±(g±e±) G±(te∓)G±(g±c)G±(g±c) G±(g±e±)G±(g±e±)G±(g±e±) G±(tc)G±(g±c)G±(g±c) G±(te±)G±(g±c)G±(g±c) G±(g±e∓)G±(g±e∓)G±(g±c) G±(g±e∓)G±(g±e±)G±(g±c) G±(g±e±)G±(g±e±)G±(g±c) G±(g±e∓)G±(g±c)G±(g±c) G±(g±e±)G±(g±c)G±(g±c) G±(g±c)G±(g±c)G±(g±c)
0.000 0.041 0.042 0.046 0.061 0.070 0.081 0.121 0.158 0.170 0.366 0.414 0.426 0.440 0.457 0.484 0.485 0.515 0.525 0.540 0.541 0.619 0.692 0.708 0.747 0.781 0.782 0.786 0.789 0.789 0.798 0.880 0.910 0.914 0.922 1.001 1.034 1.051 1.182 1.226 1.258 1.261 1.288 1.312 1.406 1.411 1.414 1.502 1.536 1.624 1.709 1.780 2.162
54 55 56
T(tc)G±(tc)G±(te∓) T(tc)G±(te∓)G±(te∓) T(tc)G±(te±)G±(te∓)
0.507 0.541 0.621
unscaledc
predicted ±
scaledd
population (%)
(165) (170) (142) (147) (168) (151) (181) (197) (234) (211) (160) (149) (146) (205) (168) (175) (207) (185) (197) (211) (176) (234) (156) (226) (159) (174) (155) (185) (149) (176) (176) (201) (183) (195) (196) (233) (163) (207) (225) (171) (185) (139) (202) (216) (230) (196) (179) (163) (192) (222) (185) (210) (203)
1285.5 1282.3 1280.6 1282.0 1280.6 1283.4 1284.2 1281.4 1281.6 1283.0 1283.0 1280.9 1279.7 1282.5 1279.3 1280.0 1280.1 1282.8 1279.7 1281.3 1280.5 1280.6 1283.4 1282.3 1281.2 1279.1 1280.4 1280.1 1278.6 1278.8 1281.0 1279.2 1283.2 1279.1 1280.9 1280.2 1280.5 1280.9 1282.1 1278.8 1280.9 1276.8 1278.6 1283.3 1279.5 1285.9 1282.9 1279.0 1280.9 1280.8 1281.3 1283.1 1282.8
0.89 4.98 0.83 2.49 2.43 2.37 2.34 2.19 0.69 2.01 1.44 2.64 1.32 2.58 1.23 1.60 1.17 1.11 2.22 2.16 2.16 0.93 1.68 0.81 0.75 1.44 0.72 1.44 0.72 0.72 1.38 1.20 1.14 0.57 1.14 0.48 0.96 0.90 0.72 0.33 0.55 0.11 0.30 0.30 0.08 0.24 0.24 0.21 0.42 0.18 0.15 0.12 0.02
(211) (186) (210)
1317.8 1315.3 1316.6
2.28 1.08 1.88
±
G (xy)G (xy)G (xy) Cluster 0.000 1257.8 0.127 1254.7 0.123 1253.0 0.082 1254.4 0.168 1253.0 0.041 1255.8 0.086 1256.6 0.213 1253.8 0.258 1254.0 0.172 1255.4 0.361 1255.4 0.402 1253.3 0.443 1252.2 0.447 1254.9 0.542 1251.8 0.488 1252.4 0.533 1252.5 0.460 1255.2 0.587 1252.2 0.546 1253.7 0.501 1252.9 0.632 1253.0 0.688 1255.8 0.819 1254.7 0.729 1253.6 0.862 1251.6 0.762 1252.8 0.821 1252.5 0.763 1251.1 0.808 1251.3 0.774 1253.4 0.907 1251.7 1.008 1255.6 0.961 1251.6 1.094 1253.3 1.006 1252.6 1.049 1252.9 0.920 1253.3 1.193 1254.5 1.182 1251.3 1.148 1253.3 1.083 1249.3 1.221 1251.1 1.380 1255.7 1.380 1252.0 1.294 1258.2 1.335 1255.3 1.369 1251.5 1.408 1253.3 1.567 1253.2 1.655 1253.7 1.754 1255.5 1.941 1255.2 T(xy)G±(xy)G±(xy) Cluster 0.749 1289.4 0.835 1287.0 0.790 1288.3 4022
DOI: 10.1021/acs.jpca.5b00889 J. Phys. Chem. A 2015, 119, 4017−4031
Article
The Journal of Physical Chemistry A Table 2. continued calculated (PO stretch + CH2 twist) vibrational wavenumber (cm−1)
relative energy (kcal/mol) s. no.
structure
computedb
±
57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74
T(te∓)G±(te∓)G±(te∓) T(te∓)G±(tc)G±(te∓) T(tc)G±(tc)G±(te±) T(te±)G±(tc)G±(te∓) T(tc)G±(tc)G±(tc) T(te±)G±(te∓)G±(te∓) T(tc)G±(te±)G±(te±) T(te∓)G±(tc)G±(te±) T(te∓)G±(te±)G±(te∓) T(te±)G±(te±)G±(te∓) T(te∓)G±(tc)G±(tc) T(te∓)G±(te±)G±(te±) T(te±)G±(tc)G±(te±) T(tc)G±(te∓)G±(g±e±) T(te±)G±(tc)G±(g±c) T(te±)G±(tc)G±(tc) T(tc)G±(te∓)G±(g±e∓) T(tc)G±(tc)G±(g±e±)
0.705 0.717 0.721 0.786 0.812 0.817 0.838 0.892 0.892 0.991 1.011 1.015 1.022 1.072 1.089 1.089 1.142 1.153
75 76 77 78
T(te±)G±(te±)G±(te±) T(tc)G±(te±)G±(g±e±) T(tc)G±(tc)G±(g±e∓) T(te∓)G±(te∓)G±(g±e±)
1.170 1.187 1.259 1.262
79
T(te∓)G±(tc)G±(g±e±)
1.329
80
T(te∓)G±(te±)G±(g±e±)
1.331
81 82 83 84
T(tc)G±(g±e±)G±(g±e±) T(te∓)G±(te∓)G±(g±e∓) T(tc)G±(g±e±)G±(g±e∓) T(te±)G±(te∓)G±(g±e±)
1.340 1.349 1.351 1.400
85 86 87 88 89 90 91 92 93
T(tc)G±(te∓)G±(g±c) T(te±)G±(tc)G±(g±e±) T(tc)G±(te±)G±(g±e∓) T(te∓)G±(g±e±)G±(g±e±) T(te±)G±(te∓)G±(g±e∓) T(te∓)G±(tc)G±(g±e∓) T(te±)G±(te±)G±(g±e±) T(te±)G±(tc)G±(g±e∓) T(g±e±)G±(tc)G±(te∓)
1.424 1.431 1.435 1.443 1.448 1.471 1.472 1.481 1.491
94 95 96 97 98 99 100 101 102 103 104 105 106 107
T(g±e±)G±(te∓)G±(te∓) T(tc)G±(g±e∓)G±(g±e∓) T(te∓)G±(g±e±)G±(g±e∓) T(te±)G±(g±e±)G±(g±e±) T(g±e±)G±(g±e±)G±(g±e∓) T(te±)G±(g±e±)G±(g±e∓) T(te∓)G±(te±)G±(g±e∓) T(te∓)G±(te∓)G±(g±c) T(g±e±)G±(tc)G±(te±) T(te±)G±(te±)G±(g±e∓) T(g±e±)G±(te±)G±(te∓) T(g±e±)G±(tc)G±(tc) T(te±)G±(g±e∓)G±(g±e∓) T(tc)G±(te±)G±(g±c)
1.535 1.541 1.594 1.603 1.660 1.662 1.676 1.700 1.708 1.734 1.736 1.752 1.765 1.766
unscaledc
predicted
scaledd
population (%)
1314.1 1317.1 1321.1 1317.1 1321.2 1314.2 1320.8 1320.0 1317.2 1317.1 1320.9 1320.0 1320.2 1318.3 1320.7 1320.7 1311.8 1313.7 1320.2 1320.4 1318.9 1314.2 1303.3 1310.8 1318.9 1313.2 1319.0 1312.1
0.82 1.60 1.58 1.42 0.68 0.68 0.65 1.20 1.20 1.00 0.49 0.48 0.96 0.88 0.86 0.43 0.78 0.76
±
T(xy)G (xy)G (xy) Cluster 1.033 1285.8 (219) 0.947 1288.7 (224) 0.704 1292.7 (211) 1.040 1288.7 (231) 0.663 1292.8 (217) 1.126 1285.9 (226) 0.745 1292.4 (198) 0.902 1291.6 (212) 0.988 1288.8 (212) 1.081 1288.7 (206) 0.861 1292.5 (228) 0.943 1291.6 (202) 0.995 1291.8 (219) 1.209 1289.9 (170) 1.601 1292.3 (237) 0.954 1292.3 (237) 1.110 1283.6 (171) 1.123 1285.4 (138) 1291.8 (102) 1.039 1292.0 (211) 1.164 1290.5 (177) 1.024 1285.9 (179) 1.493 1275.2 (84) 1282.6 (69) 1290.5 (132) 1.321 1284.9 (139) 1290.6 (112) 1.362 1283.9 (103) 1290.3 (136) 1.583 1279.0 (204) 1.404 1282.2 (180) 1.484 1278.1 (210) 1.500 1282.4 (120) 1290.6 (133) 1.396 1283.9 (186) 1.414 1285.4 (165) 1.065 1286.6 (164) 1.801 1278.1 (206) 1.401 1282.2 (182) 1.242 1285.9 (183) 1.455 1290.2 (160) 1.315 1285.2 (194) 1.839 1285.4 (118) 1292.9 (123) 1.925 1282.6 (153) 1.985 1278.7 (195) 1.702 1277.4 (222) 1.874 1278.2 (206) 2.574 1276.4 (229) 1.775 1277.2 (225) 1.263 1285.5 (178) 1.680 1282.9 (204) 1.794 1293.8 (180) 1.356 1285.0 (186) 1.880 1286.1 (136) 1.753 1294.4 (204) 1.676 1277.6 (212) 1.351 1287.0 (185) 4023
1307.1 1310.4 1306.2 1310.6 1319.0 1312.1 1313.7 1314.9 1306.2 1310.4 1314.2 1318.6 1313.5 1313.7 1321.3 1310.8 1306.8 1305.5 1306.3 1304.5 1305.3 1313.8 1311.1 1322.3 1313.3 1314.4 1322.9 1305.7 1315.3
0.37 0.72 0.64 0.64
0.56 0.56 0.28 0.56 0.54 0.50 0.48 0.48 0.48 0.23 0.46 0.44 0.44 0.44 0.44 0.20 0.20 0.36 0.18 0.32 0.32 0.32 0.30 0.30 0.28 0.28 0.14 0.14 0.28
DOI: 10.1021/acs.jpca.5b00889 J. Phys. Chem. A 2015, 119, 4017−4031
Article
The Journal of Physical Chemistry A Table 2. continued calculated (PO stretch + CH2 twist) vibrational wavenumber (cm−1)
relative energy (kcal/mol) s. no.
structure
computedb
±
108 109 110 111 112
T(te∓)G±(g±e∓)G±(g±e∓) T(tc)G±(g±e±)G±(g±c) T(te±)G±(te∓)G±(g±c) T(tc)G±(g±e∓)G±(g±c) T(g±e±)G±(te±)G±(te±)
1.784 1.785 1.785 1.924 1.928
113 114 115 116 117 118
T(te∓)G±(g±e±)G±(g±c) T(te∓)G±(te±)G±(g±c) T(te±)G±(g±e±)G±(g±c) T(g±e±)G±(g±e±)G±(g±c) T(te±)G±(te±)G±(g±c) T(g±e±)G±(tc)G±(g±e±)
1.981 2.016 2.049 2.053 2.059 2.093
119 120 121 122
T(g±e±)G±(te∓)G±(g±e±) T(te±)G±(g±e∓)G±(g±c) T(g±e±)G±(te∓)G±(g±e∓) T(g±e±)G±(te±)G±(g±e±)
2.098 2.150 2.173 2.228
123 124 125 126 127 128 129 130
T(te∓)G±(g±e∓)G±(g±c) T(g±e±)G±(tc)G±(g±e∓) T(g±e±)G±(g±e±)G±(g±e±) T(g±e±)G±(te∓)G±(g±c) T(g±e±)G±(te±)G±(g±e∓) T(g±e±)G±(g±e∓)G±(g±e∓) T(g±e±)G±(te±)G±(g±c) T(g±e±)G±(g±e∓)G±(g±c)
2.234 2.280 2.458 2.576 2.608 2.945 2.957 3.344
131
T(tc)G+(tc)G−(tc)
1.871
unscaledc
predicted
scaledd
population (%)
1305.6 1307.0 1312.2 1307.9 1316.7 1322.6 1306.4 1314.3 1306.1 1306.2 1314.3 1311.0 1318.7 1321.8 1308.0 1306.5 1308.6 1309.7 1317.8 1306.4 1310.9 1303.7 1309.6 1310.7 1303.3 1311.3 1303.0
0.13 0.26 0.26 0.20 0.10
±
T(xy)G (xy)G (xy) Cluster 1.583 1277.5 (211) 1.770 1278.9 (234) 1.687 1284.0 (194) 1.671 1279.7 (217) 1.835 1288.4 (94) 1294.1 (111) 1.988 1278.3 (247) 1.549 1286.0 (192) 2.061 1278.0 (247) 2.860 1278.1 (247) 1.642 1286.0 (196) 2.213 1282.8 (107) 1290.3 (70) 1293.3 (86) 2.299 1279.8 (114) 1.962 1278.4 (234) 2.200 1280.4 (143) 2.254 1281.5 (77) 1289.4 (81) 1.869 1278.3 (233) 2.114 1282.7 (131) 2.673 1275.6 (188) 2.486 1281.4 (156) 2.155 1282.5 (153) 2.475 1275.2 (179) 2.441 1283.1 (124) 2.761 1275.0 (211) T(xy)G+(xy)G−(xy) Cluster 1288.4 (191)
1316.7
0.18 0.18 0.16 0.16 0.16 0.16
0.16 0.14 0.14 0.12 0.12 0.12 0.04 0.06 0.06 0.02 0.04 0.02 0.06
The computations were performed at the B3LYP/6-311++G(d,p) level of theory. bEnergies corrected for ZPE. cIntensity in km mol−1 given in parentheses. dScaling factor = 1.022. a
were established to be 53. Similarly, for the T(xy)G(xy)G±(xy) cluster, of the 144 conformers, 60 conformers were found to be degenerate with the existing 84 conformers. Out of 84 conformers, 7 conformers with the “g±c” orientation interconvert to “tc” orientation, resulting in 77 conformational minima in the potential energy surface of TAP. Therefore, as a combined consequence of degeneracy and probable interconversion of a few conformers, the total number of conformations contributing to the room temperature population of TAP in the gas phase was found to be 130. Only one T(tc)G + (tc)G − (tc) conformer was considered from the T(xy)G +(xy)G−(xy) family whose energy was calculated to be 1.87 kcal mol−1 with respect to the ground state G±(tc)G±(tc)G±(tc) conformer of TAP. Figures 6 and 7 show a few representative G ± (xy)G±(xy)G±(xy) and T(xy)G±(xy)G±(xy) conformers of TAP. The cumulative population of the G±(xy)G±(xy)G±(xy) cluster was calculated to be 62.3%, and for the T(xy)G±(xy)G±(xy) cluster, it was 37.6%. The population calculated is a representative of all 360 + 1 conformers of TAP, which were essentially compressed to 130 + 1 conformers due to degeneracy and the probable interconversion of a few conformers during optimization. For a few conformers of TAP, the selected structural parameters computed at the B3LYP/6-311++(d,p) level of theory
the conformations is still a laborious task, and once they are resolved, the “additivity rule” just follows and simplifies the prediction of the relative energies of the conformers. The structures, relative energies in increasing order, vibrational wavenumbers, and population of the different TAP conformers calculated at B3LYP/6-311++G(d,p) level of theory are tabulated in Table 2. Predicted relative energies using the “additivity rule” are also given in Table 2 for comparison. The comparison of percentage errors between the calculated and the predicted relative energy values of all conformers of TAP are given in Supporting Information. The structures of all possible 216 conformers of G±(xy)G±(xy)G±(xy) and 144 conformers of T(xy)G±(xy)G±(xy) clusters of TAP (in total, 360) are presented in the Supporting Information. A closer examination revealed that in the G±(xy)G±(xy)G±(xy) cluster, of the 216 conformers, 160 conformers were found to be degenerate with the already existing 56 conformers. Consequently, 56 conformers corresponding to G±(xy)G±(xy)G±(xy) structures alone do exist in the potential energy surface of TAP. The G±(tc)G±(tc)G±(g±c), G±(tc)G±(te∓)G±(g±c), and G±(tc)G±(g±e±)G±(g±c) conformers respectively interconvert to G±(tc)G±(tc)G±(tc), G±(tc)G±(te∓)G±(tc), and G±(tc)G±(g±e±)G±(tc) presumably due to the interchain interaction. Therefore, the effective conformational minima for the G±(xy)G±(xy)G±(xy) cluster 4024
DOI: 10.1021/acs.jpca.5b00889 J. Phys. Chem. A 2015, 119, 4017−4031
Article
The Journal of Physical Chemistry A
Figure 6. Computed structures at B3LYP/6-311++G(d,p) level of theory of a few representative conformers of G±(xy)G±(xy)G±(xy) clusters of TAP. The ZPE corrected energy with respect to the ground state G±(tc)G±(tc)G±(tc) clusters of TAP is also given alongside of the structures.
Figure 7. Computed structures at B3LYP/6-311++G(d,p) level of theory of a few representative conformers of T(xy)G±(xy)G±(xy) clusters of TAP. The ZPE corrected energy with respect to the ground state G±(tc)G±(tc)G±(tc) clusters of TAP is also given alongside of the structures.
are presented in Table 3 and the computed vibrational wavenumbers in the different regions are given in Table 4. 4.4. Vibrational Assignments. When TAP was codeposited with the N2 matrix at 12 K (Figure 3), the spectral features were observed in the PO stretching region at 1303.7, 1280.6, and 1267.4 cm−1. The simulated spectrum of TAP in the PO stretching region was obtained by taking all 131 conformers into consideration and correcting the intensity of each of the conformers for their population at room temperature (Figure 3, trace a). As can be seen from the figure, the scaled computed wavenumbers of the G±(xy)G±(xy)G±(xy) and T(xy)G±(xy)G±(xy) clusters (Table 2) agreed well with the set of experimental wavenumbers observed at 1280.6/1267.4 and 1303.7 cm−1, respectively. After all, the wavenumbers in a given cluster are contiguous to each other, resulting in the broadening of spectral width. It should be noted that computed and experimental wavenumbers of G±(xy)G±(xy)G±(xy) and T(xy)G±(xy)G±(xy) clusters are well separated by ∼20 cm−1. One cannot miss noting that the corresponding vibrational wavenumbers are very broad and overlapping in the liquid spectrum, making the conformational analysis in the bulk phase practically impossible (Figure, 1, trace a). The features observed at 1291.7 and 1250.4 cm−1 are probably due to G±(xy)G±(xy)G±(xy) cluster−H2O and T(xy)G±(xy)G±(xy) cluster−H2O adducts, respectively. Interestingly, the relative
shifts of these water associates are in good agreement with the TMP (G±G±G±)−H2O and TG±G±−H2O adducts.70 The annealing experiments of TAP in the N2 matrix lead to the splitting of the 1280.6 cm−1 band characteristic of G±(xy)G±(xy)G±(xy) cluster to 1282.0, 1279.6, and 1276.3 cm−1. The splitting of the spectral band could probably be due to the matrix site effect. Because the experimental spectra for the vibrational modes in the C−C, P−O, and C−O stretching regions for the different conformations of TAP were not resolved, no attempt was made to firmly assign the features observed in these regions to the different cluster of conformations. It is apparent from the simulated spectra (Figure 2) that there is a significant overlapping of the features of both the clusters in this vibrational region. All the observed experimental features in C−C, C−O, and P−O stretching vibrational regions, therefore, can be cumulatively assigned to both the G±(xy)G±(xy)G±(xy) and T(xy)G±(xy)G±(xy) clusters of conformers of TAP. Table 5 lists the experimental vibrational wavenumbers in the different vibrational regions of TAP in N2, Ar, and Xe matrixes. 4.4.1. Conformational Behavior in Xe Matrix. Although the spectra recorded for TAP in the Ar matrix exhibit a behavior similar to that of the N2 matrix, Xe presents an interesting 4025
DOI: 10.1021/acs.jpca.5b00889 J. Phys. Chem. A 2015, 119, 4017−4031
Article
The Journal of Physical Chemistry A
Table 3. Selected Structural Parametersa of Representative Conformers from G±(xy)G±(xy)G±(xy), T(xy)G±(xy)G±(xy), and T(tc)G+(tc)G−(tc) Clusters of TAP Calculated at the B3LYP/6-311++G(d,p) Level of Theory G±(xy)G±(xy)G±(xy) cluster G±(tc)G±(tc)G±(tc) G±(te±)G±(te±)G±(te±) G±(te∓)G±(te∓)G±(te∓) G±(g±e∓)G±(g±e∓)G±(g±e∓) G±(g±e±)G±(g±e±)G±(g±e±) G±(g±c)G±(g±c)G±(g±c) G±(tc)G±(te±)G±(g±e±) G±(te∓)G±(g±e∓)G±(g±c) T(xy)G±(xy)G±(xy) cluster
P1O2 1.48 1.48 1.48 1.48 1.48 1.48 1.48 1.48 P1O2
P1O3 1.60 1.60 1.60 1.60 1.60 1.60 1.60 1.60 P1O4
∠O2P1O3 116.1 115.9 115.8 115.8 115.7 116.2 116.1 116.8 ∠O2P1O4
1.47 1.60 113.5 T(tc)G±(tc)G±(tc) T(te±)G±(te±)G±(te±) 1.47 1.60 113.4 T(tc)G±(tc)G±(te∓) 1.47 1.60 113.4 T(te∓)G±(te∓)G±(te∓) 1.47 1.60 113.5 T(g±e±)G±(g±e±)G±(g±e±) 1.47 1.59 113.6 T(xy)G+(xy)G−(xy) cluster P1O2 P1O4 ∠O2P1O4 T(tc)G+(tc)G−(tc)
1.47
1.61
111.7
∠C6O3P1O2
∠C7O4P1O2
∠C15C6O3P1
∠C17C15C6O3
dipole moment (Debye)
42.5 45.4 45.7 32.9 45.5 33.5 48.8 51.4 ∠C6O3P1O2
42.6 45.2 45.7 33.0 43.1 31.1 47.6 35.9 ∠C7O4P1O2
−171.3 −176.3 −158.1 81.6 104.2 110.1 −171.2 −158.8 ∠C20C7O4P1
−0.9 127.0 −123.9 −128.4 123.9 −3.2 −1.0 −124.6 ∠C22C20C7O4
0.76 1.26 1.15 0.46 1.27 0.27 1.14 0.91 dipole moment (Debye)
34.5 33.4 35.1 36.0 24.1 ∠C6O3P1O2
−179.6 176.3 177.8 176.2 173.0 ∠C7O4P1O2
175.5 170.3 173.9 176.4 101.3 ∠C20C7O4P1
0.9 125.6 0.8 −127.8 119.9 ∠C22C20C7O4
3.53 3.92 3.51 3.76 3.95 dipole moment (Debye)
23.6
−179.9
−178.7
−0.63
2.63
a
Bond distances in Å; bond angles and torsion angles in degrees. Torsion angles of the fragment ABCD denote the angle between ABC and BCD planes.
Table 4. Computed Vibrational Wavenumbers of Representative Conformers from G±(xy)G±(xy)G±(xy), T(xy)G±(xy)G±(xy), and T(tc)G+(tc)G−(tc) Clusters of TAP at the B3LYP/6-311++G(d,p) Level of Theorya vibrational wavenumbers (cm−1) conformer G±(tc)G±(tc)G±(tc)
870.9 (64)
936.6 (77)
G±(te±)G±(te±)G±(te±)
879.4 (95)
936.9 (44)
G±(te∓)G±(te∓)G±(te∓)
871.5 (99)
941.9 (26)
G±(g±e∓)G±(g±e∓)G±(g±e∓)
840.7 (89)
928.6 (41)
G±(g±e±)G±(g±e±)G±(g±e±)
843.2 (94)
937.7 (34)
G±(g±c)G±(g±c)G±(g±c)
833.1 (65)
920.0 (56)
G±(tc)G±(te±)G±(g±e±)
856.6 878.9 838.5 863.9 862.2 874.3 863.7 876.5 859.2 876.1 858.6 876.6 824.5 838.1 846.5 874.0
939.5 (40)
G±(te∓)G±(g±e∓)G±(g±c) T(tc)G±(tc)G±(tc) T(te±)G±(te±)G±(te±) T(tc)G±(tc)G±(te∓) T(te∓)G±(te∓)G±(te∓) T(g±e±)G±(g±e±)G±(g±e±) T(tc)G+(tc)G−(tc) a
PO stretch + CH2 rock + −CC stretch + in plane CH bend CH2 rock
(61) (114) (66) (120) (118) (101) (183) (125) (130) (104) (161) (124) (150) (132) (48) (174)
928.5 (32) 930.4 935.4 930.7 940.7 931.7 940.3 937.6 940.2 934.4 934.6 930.4 935.3
(57) (54) (16) (16) (33) (21) (20) (29) (19) (27) (36) (86)
CO stretch + CH2 rock + out of plane CH bend 1024.9 1025.4 1008.3 1009.2 1011.5 1012.3 1006.5 1007.4 1006.2 1006.8 1008.2 1008.9 1008.7 1018.3 1009.4 1010.6 1019.1 1030.4 1002.6 1014.5 1010.9 1015.3 1006.5 1014.6 1001.5 1015.4 1018.6 1029.0
(438) (430) (734) (737) (546) (531) (597) (598) (575) (563) (261) (253) (654) (443) (338) (572) (317) (356) (567) (492) (371) (122) (579) (579) (452) (584) (253) (396)
−CH2 rock + out of plane CH bend
PO stretch + −CH2 twist
1036.3 (100)
1257.8 (165)
1032.8 (64)
1253.0 (142)
1034.0 (120)
1254.0 (234)
1028.2 (13)
1249.3 (139)
1033.1 (53)
1252.0 (230)
1044.2 (106)
1255.2 (203)
1032.7 (74)
1252.9 (176)
1030.2 (40)
1253.3 (196)
1035.9 (49)
1292.8 (217)
1033.1 (175)
1292.0 (211)
1026.9 (369)
1289.4 (211)
1033.0 (102)
1285.8 (219)
1033.3 (31)
1275.6 (188)
1036.0 (110)
1288.4 (191)
The intensities (in km/mol) are given in parentheses.
wavenumbers observed at 1277.2/1272.9/1267.1 cm−1 and 1302.6/1299.9 cm−1, respectively, are due to G±(xy)G±(xy)G±(xy) and T(xy)G±(xy)G±(xy) clusters of TAP. As mentioned
variation (Figure 4). The main spectral features of TAP in the Xe matrix (in the PO stretching + CH2 twisting mode) occur at 1302.6, 1299.9, 1277.2, 1272.9, and 1267.1 cm−1. The set of 4026
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The Journal of Physical Chemistry A Table 5. Experimental Vibrational Wavenumbers of TAP in Different N2, Ar, and Xe Matrixes experimental vibrational wavenumbers (cm−1) matrix
PO stretch + CH2 rock + in plane CH bend
−CC stretch + CH2 rock
CO stretch + CH2 rock + out of plane CH bend
−CH2 rock + out of plane CH bend
N2 Ar Xe
891.0, 925.8, 939.3 889.1, 923.8, 933.5 894.9, 919.0, 930.1, 935.9, 939.7
994.2 990.9 986.5
1027.5 1029.4 1026.0, 1031.8
1058.8 1059.8 1062.2
PO stretch + −CH2 twist 1267.4, 1280.6, 1303.7 1265.6,1280.6, 1305.7 1267.1, 1272.9, 1277.2, 1299.9, 1302.6
Table 6. NBO Analysis of Some of the Important Geminal and Vicinal Hyperconjugative Interactions, Showing the Donor and Acceptor Orbitals and the Second-Order Perturbation Energies, E2, in the Representative G±(tc)G±(tc)G±(tc), T(tc)G±(tc)G±(te∓), and T(tc)G+(tc)G−(tc) Conformers of TAP, Calculated at the B3LYP/6-311++G(d,p) Level of Theory second-order perturbation energies, E2 (kcal/mol)
second-order perturbation energies, E2 (kcal/mol) donor orbital
acceptor orbital
σP1O2 σP1O2 σP1O2 πP1O2 πP1O2 πP1O2 πP1O2 π1P1O2 π1P1O2 π1P1O2 σP1O3 σP1O3 σP1O3 σP1O3 σP1O4 σP1O4 σP1O4 σP1O5 σP1O5 σP1O5 σP1O5 σP1O5 π*P1O2 π*P1O2 π*P1O2 π1*P1O2 π1*P1O2 π1*P1O2 σ*P1O4 σ*P1O5
π*P1O2 σ*P1O3 σ*P1O5 σ*P1O2 σ*P1O3 σ*P1O4 σ*P1O5 σ*P1O3 σ*P1O4 σ*P1O5 π1*P1O2 π*P1O2 σ*P1O4 σ*P1O5 π*P1O2 σ*P1O3 σ*P1O5 σ*P1O2 π*P1O2 π1*P1O2 σ*P1O3 σ*P1O4 σ*P1O3 σ*P1O4 σ*P1O5 σ*P1O3 σ*P1O4 σ*P1O5 σ*P1O5 σ*P1O3
lp(2)O2 lp(2)O2 lp(2)O2 lp(1)O3
σ*P1O3 σ*P1O4 σ*P1O5 σ*P1O2
G±(tc)G±(tc) G±(tc)
T(tc)G±(tc) G±(te∓)
T(tc)G+(tc) G−(tc)
donor orbital
acceptor orbital
16.32 1.57 11.13 7.21
Unraveling the Conformational Landscape of Triallyl Phosphate: Matrix Isolation Infrared Spectroscopy and Density Functional Theory Computations N. Ramanathan, C. V. S. Brahmmananda Rao, K. Sankaran, and K. Sundararajan* Chemistry Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamil Nadu, India S Supporting Information *
ABSTRACT: The conformations of triallyl phosphate (TAP) were studied using matrix isolation infrared spectroscopy and density functional theory (DFT) calculations. TAP was trapped in N2, Ar, and Xe matrixes at 12 K using an effusive source and the resultant infrared spectra recorded. The computational analysis on conformers of TAP is a challenging problem due to the presence of the large number of conformations. To simplify this problem, conformational analysis was performed on prototypical molecules such as dimethyl allyl phosphate (DMAP) and diallyl methyl phosphate (DAMP), to systematically arrive at the conformations of TAP. The above methodology discerned 131 conformations for TAP, which were found to contribute to the room temperature population. The computations were performed using B3LYP/6-311++G(d,p) level of theory. Vibrational wavenumber calculations were performed for the various conformers to assign the experimental infrared features of TAP, trapped in solid N2, Ar, and Xe matrixes.
1. INTRODUCTION Nuclear reprocessing technology has played a key role in the development of liquid−liquid extraction for the industrial-scale separation of metals.1 The high degree of purity of materials needed for nuclear applications favor solvent extraction to be the method of choice for the separation when compared to other methods like precipitation and ion exchange.2 Tri-n-butyl phosphate (TBP) as an extractant is considered as the work horse of the nuclear reprocessing industry because it has all the desirable properties of an extractant in the solvent extraction process and hence has wide applications in various stages of the nuclear fuel cycle.3−5 Although reprocessing of spent nuclear fuel is performed predominantly by the solvent extraction process, currently, the solid-phase extraction (SPE) technique has been emerging as the active field of research as an alternate to the conventional solvent extraction procedure, which involves partitioning between a liquid and solid (sorbent) phase.6 This sample treatment technique enables the purification of analytes from solution by sorption on a solid sorbent. In comparison to liquid−liquid extraction, SPE has notable advantages in the reduction of large volumes of organic waste and in tuning the selectivity of the solid-phase extractant for a specific metal ion.7 Because there is no organic phase; the requirement of phase modifier is eliminated and the third phase formation is avoided. Thus, SPE is likely to reduce the complexity of reprocessing relative to solvent extraction, which in turn makes the procedure cost-effective. It is either important to incorporate extractants used in solvent extraction process into solid-phase materials to make the sorbent materials for separations (extraction chromatography) or to modify the extractant itself to make it © 2015 American Chemical Society
suitable as a sorbent material for extraction. A number of reports exist for organic molecules (including organophosphorous compounds) incorporated onto the polymer based ion-exchange sorbent materials.7−12 Because organophosphate (TBP) is used as an extractant for solvent extraction in nuclear reprocessing; it will be prudent to make a polymer out of phosphate to make use of the phosphoryl group in exploring the extraction of metals of interest in solid phase. In this context, poly triallyl phosphate (PTAP) derived from monomeric triallyl phosphate (TAP) by virtue of the double bond was found to be a promising solid-phase extractant of the future. Polymerization of TAP could be achieved through emulsion polymerization with halogenated methanes.13 The studies on suitability of PTAP as a solid-phase extractant are in progress in our laboratory. Because physicochemical behavior of the extractant is integrated from the molecular level, understanding the conformations of the monomeric extractant assumes significance. The analysis of different conformations of TAP can be expected to throw light on the bulk-phase properties of TAP, such as dipole moment, solubility, mass density, self-diffusion coefficient, etc., which eventually can be extrapolated to the polymeric species. Importantly, the conformations have a strong control over physical, chemical, and biological properties of materials.14−16 The conformational analysis of large molecules poses a serious challenge due to the large multitude of conformations present in it. Reva, Fausto and co-workers reported the extensive study of conformations of 1,4-, 1,3-, and 1,2-butanediols, which possessed Received: January 28, 2015 Revised: April 7, 2015 Published: April 7, 2015 4017
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were performed to ensure that the computed structures corresponded to minima on the potential surface and also to assign the vibrational features observed in the experiments. Computed frequencies were assigned to the experimental infrared features after incorporation of a scaling factor. To arrive at the scaling factor, the experimentally observed strongest feature at 1280.6 cm−1 in the PO stretching region was correlated with the strongest computed feature of the ground state conformer in this region. A scaling factor of 1.022 would be required to bring the two values in agreement. This scaling factor was used to scale other features of the ground and higher energy cluster of conformers, in this region. The scaling factor is believed to be necessary, to account for the influence of the host matrix on the vibrational wavenumbers of the infrared spectrum.45−48 The SYNSPEC program was used to simulate a vibrational spectrum using the computed wavenumbers and intensities, by assuming a Lorentzian line profile with a line width of 4 cm−1.49 Calculations were performed using the Onsager self consistent reaction field (SCRF) model,50,51 as implemented in the Gaussian program, to determine quantitatively the effect of the matrix on the energies of a few selected conformers of TAP. In this model, the solute molecule is placed in a spherical cavity surrounded by a continuum, the continuum being originated from the influence of solvent. The radius of the spherical cavity occupied by the solute was estimated on the basis of molecular volumes obtained from the Gaussian calculations. Subsequently in the calculations, the radius of the spherical cavity and the dielectric constant of the matrix material were specified as additional inputs in addition to the molecular specification. Using this model, different dielectric constants52−55 were investigated to examine the effect of surroundings on the conformational distribution of TAP. To understand the nature of the electronic effects in determining conformational preferences, Natural Bond Orbital analysis (NBO, Version 3.1), invoked through Gaussian was performed.56 The NBO has been reported to untangle the conformational preferences in several systems.57−68
65, 73, and 81 unique conformational minima, respectively, using matrix isolation infrared and ab initio calculations.17−19 The vast number of conformations in these systems was reported to be due to a presence of conformationally relevant 3-fold rotational axes. The conformational analysis of TAP is a complex problem, because TAP possesses nine carbon atoms, which results in a large number of conformations. It is, therefore, imperative to find out the entire conformational picture of TAP through a simplified approach. On the basis of our earlier work on trimethyl phosphate (TMP),20−24 triethyl phosphate (TEP),25,26 acetals/ketals,27−32 silanes,33−36 carbonates,37,38 and phosphite,39 we concluded that a combination of hyperconjugative (arises due to the delocalization interactions) and steric interactions decides the conformational preferences in these molecules. It is interesting to probe the conformations of TAP as the knowledge on conformations would serve as a platform to understand its chemistry in the condensed phase. In this work, therefore, the conformations of TAP were studied to understand its conformational landscape using matrix isolation infrared spectroscopy. The DFT calculations were used to arrive at the various possible conformations of TAP and also to correlate our experimental results.
2. EXPERIMENTAL DETAILS Matrix isolation experiments were carried out using a Leybold AG helium-compressor-cooled closed cycle cryostat. The details of the vacuum system are described elsewhere.20−23 TAP was prepared through a simple one-step synthesis from phosphoryl chloride (POCl3; 1 mol) and allyl alcohol (3 mol) precursors by mixing them in a round-bottom flask, which was maintained at −10 °C. The synthetic procedure is analogous to the one reported for the preparation of various trialkyl phosphates.40 The TAP was vacuum distilled to remove traces of water and unreacted allyl alcohol. Furthermore, the sample was subjected to several freeze−pump−thaw cycles before performing experiments. The infrared spectrum of liquid TAP was recorded using ZnSe window, which showed an excellent agreement with the earlier report.41−43 TAP and matrix gas were deposited by streaming them separately through a twin jet nozzle system. TAP was maintained at room temperature during the deposition. N2 (INOX, 99.9995%), Ar (INOX, 99.9995%), and Xe (Chemtron, 99.997%) were used as a matrix gases. Deposition was carried out at the rate of ∼3 mmol/h, and a typical deposition lasted for ∼1 h. The spectra were recorded using a BOMEM MB 100 FTIR spectrometer with a spectral resolution of 1 cm−1. The spectrum was recorded at 12 K and subsequently, the matrix was warmed to 30 K (N2), 35 K (Ar), and 60 K (Xe), maintained at this temperature for ∼15 min and recooled to 12 K and the spectra were again recorded.
4. RESULTS AND DISCUSSION 4.1. Experimental Section. Figure 1 (1350−1200 cm−1, grid A, and 1100−800 cm−1, grid B) compares the infrared spectra of neat TAP liquid (trace a) with TAP isolated in N2, Ar, and Xe matrixes (traces b−d). The region spanned in the figure corresponds to PO (grid A), CC, CO, and PO stretching (grid B) vibrational modes of TAP. Almost all of these vibrations are either coupled with CH or CH2 deformation vibrations. The comparison of the bulk liquid and matrix isolated spectra reveals the relatively broad nature of the infrared spectrum in the bulk phase, which clearly precludes the conformational analysis in the liquid TAP. Our computations indicated that, of the different PO, CC, CO, and PO stretching regions of TAP, the features due to the PO stretching region (1350−1200 cm−1) show the maximum separation (by more than 20 cm−1) for the different conformations; hence, the analyses of TAP for different conformations were focused on the PO stretching region (Figure 1, grid A). An analysis of the normal modes using Gauss View, revealed that the 1350−1200 cm−1 features were not pure PO stretching vibrations of TAP but were mixed with CH2 twisting modes. Figure 2 compares the simulated spectra of a few representative conformers in the CC, CO, and PO stretching
3. COMPUTATIONAL DETAILS Computations were performed using a GAUSSIAN 03W package,44 on a Pentium 4 machine using DFT methods. The structures of the conformers of allyl phosphates ((dimethyl allyl phosphate (DMAP), diallyl methyl phosphate (DAMP), and TAP) were computed using a 6-311++G(d,p) basis set. DFT methodology was adopted through the B3LYP hybrid exchange−correlational functional. All the structures were optimized without imposing any symmetry restrictions during the optimization process. Vibrational wavenumber calculations 4018
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Figure 1. Infrared spectra of TAP spanning the region 1350−1200 cm (grid A) and 1100−800 cm−1 (grid B): (a) neat liquid; (b) N2 matrix (12 K); (c) Ar matrix (12 K); (d) Xe matrix (12 K).
Figure 3. Comparison of computed and experimental spectra of TAP, spanning the region 1350−1200 cm−1: (a) scaled computed spectra of all 131 conformers of TAP; (b) matrix isolation infrared spectra of TAP in a N2 matrix recorded using an effusive source at 12 K; (c) annealed at 30 K.
Figure 2. Comparison of computed and experimental spectra of TAP, spanning the region 1100−800 cm−1. (a) Scaled computed spectra of a few representative G±(tc)G±(tc)G±(tc), G±(te±)G±(te±)G±(te±), G±(te∓)G±(te∓)G±(te∓), G±(g±e∓)G±(g±e∓)G±(g±e∓), G±(g±e±)G ± (g ± e ± )G ± (g ± e ± ), G ± (g ± c)G ± (g ± c)G ± (g± c), G ± (tc)G ± (te ± )G±(g±e±), and G±(te∓)G±(g±e∓)G±(g±c) conformers of TAP. (b) Scaled computed spectra of a few representative T(tc)G±(tc)G±(tc), T(te±)G±(te±)G±(te±), T(tc)G±(tc)G±(te∓), T(te∓)G±(te∓)G±(te∓), and T(g±e±)G±(g±e±)G±(g±e±) conformers of TAP. (c) Matrix isolation infrared spectrum of TAP in a N2 matrix recorded using an effusive source at 12 K.
regions of TAP (discussed in detail in section 4.4) and the infrared spectrum of TAP in the N2 matrix. Figure 3 shows the comparison of the simulated spectrum of TAP (trace a; more details are given in section 4.2) in the PO stretching + CH2 twisting mode and the experimental infrared spectrum in the N2 matrix (trace b). The main spectral features of TAP in a N2 matrix (in the PO stretching + CH2 twisting mode) occur at 1303.7, 1280.6, and 1267.4 cm−1. The features observed at 1291.7 and 1250.4 cm−1 could probably be due to TAP−H2O adducts, because H2O is an inevitable impurity in a matrix isolation experiment. When the matrix was annealed at 30 K, the splitting of the features near 1280.6 was observed (Figure 3, trace c). Figure 4 (traces a−d) shows the infrared spectra of TAP in the Xe matrix at different annealing temperatures. The spectral features of TAP in the Xe matrix (in the PO stretching + CH2 twisting mode) occur at 1302.6, 1299.9, 1277.2, 1272.9, and 1267.1 cm−1. The features observed at 1289.3 and 1248.5 cm−1 are likely due to TAP−H2O adducts in the Xe matrix. 4.2. Computational Exploration of the Conformations of TAP. Because TAP is the phosphoric acid ester of allyl alcohol, the knowledge on the conformations of allyl alcohol becomes important in systematically arriving at the conformations of TAP. Recent work by Durig et al. reported five possible conformations for allyl alcohol, which arises due to the rotation around CC and CO bonds by changing the relative position of the hydroxy group with respect to the CC single and double bonds.69 This eventually results in 0°, +120°, and −120° orientations of the terminal carbon atom (connected to the next nearest carbon atom through double bond) with respect to the oxygen atom of allyl alcohol. When the allyl
−1
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Figure 4. Infrared spectra of TAP in the Xe matrix spanning the region 1350−1200 cm−1. Spectra recorded at (a) 12 K and annealed at (b) 40 K, (c) 50 K, and (d) 60 K.
alcoholic moiety replaces the hydrogen atom of phosphoric acid to form TAP, the determination of the relative positions of all carbon atoms with respect to the next three neighboring atoms become important, which eventually leads to many possible conformations. The conformational analysis on a single allyl chain would simplify the problem of TAP to arrive at all possible orientations in TAP. To systematically approach the conformational problem of TAP, calculations were first performed on DMAP (Figure 5), i.e., a molecule where one of the methyl groups of TMP was replaced by an allyl group. It is clear that the basic orientations of the conformations of allyl phosphates can be derived from the conformations of TMP molecule, as the allyl chain can be considered to arise from the replacement of one of the hydrogens of the methyl group by a vinyl moiety. For TMP molecule, computations identified three minima, corresponding to conformers with C3 (G±G±G±), C1 (TG±G±), and Cs (TG+G−) structures, given in the order of increasing energy.20−23 The preference for the “Gauche” orientation in the different conformers of TMP was attributed to the operation of both geminal and vicinal delocalization hyperconjugative interactions.39 In TAP too, the carbon atoms attached to the oxygen should prefer to have the “Gauche” orientation with varying orientations of rest of the carbon atoms (of the vinyl moiety) of the allyl chain. Similarly, the higher energy cluster should arise with the basic orientation of TG±G± for varying orientations of the rest of the carbon atoms of the allyl chain. A closer examination of DMAP revealed that the first carbon attached to oxygen atom is restricted to have a “Gauche (60°)” orientation in its ground state (GGG orientation due to hyperconjugation). Given the orientation of the first carbon as G±, the second and third carbons adopt “gauche (±60°)”/“trans (180°)” and “cis (0°)”/“gauche (±120°)”/“gauche (∓120°)” orientations, respectively, which results in 1 × 2 × 3 = 6 orientations for the G±(xy)G±G± cluster of DMAP. To differentiate the “gauche (±60)” and “gauche (±120°)”/“gauche
Figure 5. Computed structures at B3LYP/6-311++G(d,p) level of theory of a few conformers of DMAP. The ZPE corrected energy with respect to the ground state G±(tc)G±G± conformer of DMAP is also given alongside of the structures.
(∓120°)” conformations, the latter is designated as “±e”/“∓e” orientations. It can be noted that the terminology “Gauche, 60° (G)/Trans, 180° (T)” and “gauche, 60° (g)/gauche, 120° (e)/trans, 180° (t)/cis, 0° (c)” are used to denote the hyperconjugative (first carbon attached to oxygen) and nonhyperconjugative carbon atoms (second and third carbons), respectively. Table 1 shows the absolute and relative energies of conformers of DMAP, computed at the B3LYP/6-311++G(d,p) level of theory. In the calculations of DMAP, we have observed certain important exclusions. In the G±(xy)G±G± cluster, the second carbon does not adopt “g∓” orientation if the first hyperconjugative carbon attached to the oxygen orients “G±”, probably due to the steric interaction operating between the terminal carbon and hydrogen atoms with phosphoryl group. For example, the G±(g∓c)G±G± conformer was interconverted to the G±(tc)G±G± conformer during optimization. This eliminates three conformers in the G±(xy)G±G± cluster for DMAP, which otherwise would have resulted in a total of nine conformations. Likewise, the T(xy)G±G± cluster also showed a similar behavior for the orientation, when, x = g∓. Additionally, in the higher energy T(xy)G±G± cluster of DMAP, the “g±” orientation for the second carbon gets interconverted to the “trans” orientation if the third carbon orients “e∓” as the T(g±e∓)G±G± conformer of DMAP did optimize to T(te∓)G±G± in the potential energy surface. Likewise, the T(g±c)G±G± conformer was found to 4020
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have been probed for each G±(xy)G±(xy)G±(xy) and T(xy)G±(xy)G±(xy) clusters of conformations of TAP. It can be reiterated that the clusters of conformations of TAP correlate with the ground state, G±G±G±, and the first higher energy TG±G± conformers of TMP. Because the second higher energy TG+G− conformer has been computed to contribute to only a few percent of the total population in TMP,20−23 this orientation of the hyperconjugative carbon has not been considered for TAP. In other words, the T(xy)G+(xy)G−(xy) cluster was not calculated for different conformations. Based on the computations performed on DMAP and DAMP, an important rule was formulated in arriving at the conformations of TAP, which is discussed in the subsequent section. 4.3. Additivity Rule. It is clear that as the number of allyl chains replacing the methyl group of TMP gets increased, the conformational analysis becomes complicated. For example, the number of conformations for the GGG cluster, which in DMAP is 6, increases to 36 in DAMP and 216 in TAP. As the number of conformations becomes larger and larger, the brute-force search of the conformations would be extremely demanding on time and the computational resources. The examination of the conformers of DAMP and TAP by making a comparison with DMAP unfolds an interesting “additivity rule” in deriving the relative energies the conformations of multi chain phosphate without needing extensive computations. Consider the conformation of the diallyl phosphate (DAMP), where one chain has the “te∓” orientation and the second chain has the “g±c” structure. For G±(te∓)G±G± and G±(g±c)G±G± conformers of DMAP, in the monoallyl chain, the energy was 0.086 and 0.647 kcal/mol, respectively relative to the G±(tc)G±G± conformer. In the diallyl chain of DAMP, where one of the two chains has “te∓” and the other “g±c” orientations, the predicted energy of the G±(te∓)G±(g±c)G± conformer was found to be 0.733 kcal/mol (0.086 + 0.647; results from the addition of relative energy of G±(te∓)G±G± and G±(g±c)G±G± conformers of DMAP molecule), whereas the calculated energy of the conformer G±(te∓)G±(g±c)G± relative to the ground state G±(tc)G±(tc)G± form was 0.695 kcal/mol. Similarly, for TAP with three allyl chains, the predicted energy of the G±(te±)G±(g±e∓)G±(g±e±) conformer was found to be 0.862 kcal/mol, the calculated energy being 0.781 kcal/mol, which in turn is reasonably close to the predicted energy. Likewise, for the T(tc)G±(te∓)G±(g±e∓) conformer of TAP, the predicted energy (1.110 kcal/mol) and its computed value (1.142 kcal/mol) are in fair agreement with each other. The “additivity rule” for predicting the relative energy of multichain allyl phosphates hence can be defined as “the relative energy of the multi chains is the sum of relative energies of the individual chains”. The comparison of the computed and predicted energies of the conformers of DAMP employing the “additivity rule” using a model DMAP molecule are given in the Supporting Information. To rigorously test for the agreement between the predicted and computed energies of the different conformers, all possible conformers of TAP were computed at the B3LYP level of theory using the 6-311++G(d,p) basis set. Not unexpectedly, for a few of the conformers, there is a large deviation between the computed and predicted energies derived from DMAP and the deviation is likely due to the interaction between the inter chains. Nevertheless, the employment of the “additivity rule” results in the prediction of the relative energies of multichain phosphate with reasonable accuracy, without demanding timeconsuming extensive computations. Of course, unraveling all
Table 1. Absolute (Hartrees) and Relative (kcal/mol) Energies of Conformers of DMAP Calculated at the B3LYP/ 6-311++G(d,p) level of theory s. no.
structure
1 2 3 4 5 6
G±(tc)G±G± G±(te±)G±G± G±(te∓)G±G± G±(g±e∓)G±G± G±(g±e±)G±G± G±(g±c)G±G±
7 8 9 10
T(tc)G±G± T(te∓)G±G± T(te±)G±G± T(g±e±)G±G±
a
absolute energya (Hartrees) G±(xy)G±G± Cluster −839.458227 −839.458162 −839.458090 −839.457652 −839.457494 −839.457195 T(xy)G±G± Cluster −839.457171 −839.456855 −839.456706 −839.455434
relative energya (kcal/mol) 0.000 0.041 0.086 0.361 0.460 0.647 0.663 0.861 0.954 1.753
Energies corrected for ZPE.
interconvert to the T(tc)G±G± structure during optimization. The elimination of these conformers is also likely due to the steric interaction between the allyl chain and the methyl group connected to the adjacent oxygen (inter chain interaction). The effective conformational minima for the T(xy)G±G± cluster then turns out to be four against six of the G±(xy)G±G± cluster of DMAP. The above exercise clearly helped us to eliminate certain conformers in DMAP (three out of nine in the G±(xy)G±G± cluster and f ive out of nine in the T(xy)G±G± cluster), which greatly simplified the conformational search of TAP. All optimized conformers of DMAP along with the energies calculated at B3LYP level of theory using 6-311++G(d,p) basis set are given in Table 1 and the structures of a few selected conformers are shown in Figure 5. The calculations were then performed on the conformers of DAMP, which had two allyl chains and a methyl group. These computations were accomplished to examine the variation of the conformational orientations, if any, due to the inter chain interaction. The number of conformers for the G±(xy)G±(xy)G± and the T(xy)G±(xy)G± clusters turn out to be 36 (6 × 6) and 24 (4 × 6), respectively. Out of 60 (36 for G±(xy)G±(xy)G± and 24 for T(xy)± G (xy)G±) conformers in total, 58 minima were obtained for DAMP, confirming that there is minimal interchain interaction between the two allyl chains. Of the 36 G±(xy)G±(xy)G± clusters, 15 conformers were found to be degenerate with the already existing 21 conformers, leading to an effective total of 45 unique conformations in the potential energy surface of DAMP. The conformers with the geometries T(te±)G±(g±c)G± and T(g±e±)G±(g±c)G± interconverted to T(te±)G±(tc)G± and T(g±e±)G±(tc)G± conformers, respectively, indicating the interconversion is likely due to the steric interaction between the two allyl chains. Calculated absolute and relative energies of the conformers of DAMP computed at the B3LYP level of theory using the 6-311++G(d,p) basis set are given in the Supporting Information. The conformational analysis was then performed on TAP whose conformations were calculated to be 216 (6 × 6 × 6) and 144 (4 × 6 × 6) for G±(xy)G±(xy)G±(xy) and T(xy)G±(xy)G±(xy) clusters, respectively. Clearly, the conformational analysis performed on DMAP and DAMP molecules eliminated the complexity in probing the conformations of TAP; otherwise, 729 (9 × 9 × 9) conformers should 4021
DOI: 10.1021/acs.jpca.5b00889 J. Phys. Chem. A 2015, 119, 4017−4031
Article
The Journal of Physical Chemistry A
Table 2. Structures, Computed and Predicted (Based on the “Additivity Rule”) Relative Energies (kcal/mol), Vibrational Wavenumbers, and Room Temperature Population of All 131 Conformers of TAPa calculated (PO stretch + CH2 twist) vibrational wavenumber (cm−1)
relative energy (kcal/mol) s. no.
structure
computedb ±
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53
G±(tc)G±(tc)G±(tc) G±(tc)G±(te±)G±(te∓) G±(te±)G±(te±)G±(te±) G±(tc)G±(te±)G±(te±) G±(te±)G±(te±)G±(te∓) G±(tc)G±(tc)G±(te±) G±(tc)G±(tc)G±(te∓) G±(te±)G±(te∓)G±(te∓) G±(te∓)G±(te∓)G±(te∓) G±(tc)G±(te∓)G±(te∓) G±(tc)G±(tc)G±(g±e∓) G±(tc)G±(te±)G±(g±e∓) G±(te±)G±(te±)G±(g±e∓) G±(tc)G±(te∓)G±(g±e∓) G±(te±)G±(te±)G±(g±e±) G±(te±)G±(te∓)G±(g±e∓) G±(te∓)G±(te∓)G±(g±e∓) G±(tc)G±(tc)G±(g±e±) G±(te±)G±(te∓)G±(g±e±) G±(tc)G±(te∓)G±(g±e±) G±(tc)G±(te±)G±(g±e±) G±(te∓)G±(te∓)G±(g±e±) G±(tc)G±(te±)G±(g±c) G±(te∓)G±(te∓)G±(g±c) G±(te±)G±(te±)G±(g±c) G±(te±)G±(g±e∓)G±(g±e±) G±(tc)G±(g±e∓)G±(g±e∓) G±(tc)G±(g±e∓)G±(g±e±) G±(te±)G±(g±e∓)G±(g±e∓) G±(te∓)G±(g±e∓)G±(g±e∓) G±(te±)G±(te∓)G±(g±c) G±(te∓)G±(g±e∓)G±(g±e±) G±(tc)G±(g±e∓)G±(g±c) G±(te±)G±(g±e±)G±(g±e±) G±(te∓)G±(g±e∓)G±(g±c) G±(te∓)G±(g±e±)G±(g±e±) G±(te±)G±(g±e∓)G±(g±c) G±(tc)G±(g±e±)G±(g±e±) G±(te∓)G±(g±e±)G±(g±c) G±(g±e∓)G±(g±e∓)G±(g±e±) G±(te±)G±(g±e±)G±(g±c) G±(g±e∓)G±(g±e∓)G±(g±e∓) G±(g±e∓)G±(g±e±)G±(g±e±) G±(te∓)G±(g±c)G±(g±c) G±(g±e±)G±(g±e±)G±(g±e±) G±(tc)G±(g±c)G±(g±c) G±(te±)G±(g±c)G±(g±c) G±(g±e∓)G±(g±e∓)G±(g±c) G±(g±e∓)G±(g±e±)G±(g±c) G±(g±e±)G±(g±e±)G±(g±c) G±(g±e∓)G±(g±c)G±(g±c) G±(g±e±)G±(g±c)G±(g±c) G±(g±c)G±(g±c)G±(g±c)
0.000 0.041 0.042 0.046 0.061 0.070 0.081 0.121 0.158 0.170 0.366 0.414 0.426 0.440 0.457 0.484 0.485 0.515 0.525 0.540 0.541 0.619 0.692 0.708 0.747 0.781 0.782 0.786 0.789 0.789 0.798 0.880 0.910 0.914 0.922 1.001 1.034 1.051 1.182 1.226 1.258 1.261 1.288 1.312 1.406 1.411 1.414 1.502 1.536 1.624 1.709 1.780 2.162
54 55 56
T(tc)G±(tc)G±(te∓) T(tc)G±(te∓)G±(te∓) T(tc)G±(te±)G±(te∓)
0.507 0.541 0.621
unscaledc
predicted ±
scaledd
population (%)
(165) (170) (142) (147) (168) (151) (181) (197) (234) (211) (160) (149) (146) (205) (168) (175) (207) (185) (197) (211) (176) (234) (156) (226) (159) (174) (155) (185) (149) (176) (176) (201) (183) (195) (196) (233) (163) (207) (225) (171) (185) (139) (202) (216) (230) (196) (179) (163) (192) (222) (185) (210) (203)
1285.5 1282.3 1280.6 1282.0 1280.6 1283.4 1284.2 1281.4 1281.6 1283.0 1283.0 1280.9 1279.7 1282.5 1279.3 1280.0 1280.1 1282.8 1279.7 1281.3 1280.5 1280.6 1283.4 1282.3 1281.2 1279.1 1280.4 1280.1 1278.6 1278.8 1281.0 1279.2 1283.2 1279.1 1280.9 1280.2 1280.5 1280.9 1282.1 1278.8 1280.9 1276.8 1278.6 1283.3 1279.5 1285.9 1282.9 1279.0 1280.9 1280.8 1281.3 1283.1 1282.8
0.89 4.98 0.83 2.49 2.43 2.37 2.34 2.19 0.69 2.01 1.44 2.64 1.32 2.58 1.23 1.60 1.17 1.11 2.22 2.16 2.16 0.93 1.68 0.81 0.75 1.44 0.72 1.44 0.72 0.72 1.38 1.20 1.14 0.57 1.14 0.48 0.96 0.90 0.72 0.33 0.55 0.11 0.30 0.30 0.08 0.24 0.24 0.21 0.42 0.18 0.15 0.12 0.02
(211) (186) (210)
1317.8 1315.3 1316.6
2.28 1.08 1.88
±
G (xy)G (xy)G (xy) Cluster 0.000 1257.8 0.127 1254.7 0.123 1253.0 0.082 1254.4 0.168 1253.0 0.041 1255.8 0.086 1256.6 0.213 1253.8 0.258 1254.0 0.172 1255.4 0.361 1255.4 0.402 1253.3 0.443 1252.2 0.447 1254.9 0.542 1251.8 0.488 1252.4 0.533 1252.5 0.460 1255.2 0.587 1252.2 0.546 1253.7 0.501 1252.9 0.632 1253.0 0.688 1255.8 0.819 1254.7 0.729 1253.6 0.862 1251.6 0.762 1252.8 0.821 1252.5 0.763 1251.1 0.808 1251.3 0.774 1253.4 0.907 1251.7 1.008 1255.6 0.961 1251.6 1.094 1253.3 1.006 1252.6 1.049 1252.9 0.920 1253.3 1.193 1254.5 1.182 1251.3 1.148 1253.3 1.083 1249.3 1.221 1251.1 1.380 1255.7 1.380 1252.0 1.294 1258.2 1.335 1255.3 1.369 1251.5 1.408 1253.3 1.567 1253.2 1.655 1253.7 1.754 1255.5 1.941 1255.2 T(xy)G±(xy)G±(xy) Cluster 0.749 1289.4 0.835 1287.0 0.790 1288.3 4022
DOI: 10.1021/acs.jpca.5b00889 J. Phys. Chem. A 2015, 119, 4017−4031
Article
The Journal of Physical Chemistry A Table 2. continued calculated (PO stretch + CH2 twist) vibrational wavenumber (cm−1)
relative energy (kcal/mol) s. no.
structure
computedb
±
57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74
T(te∓)G±(te∓)G±(te∓) T(te∓)G±(tc)G±(te∓) T(tc)G±(tc)G±(te±) T(te±)G±(tc)G±(te∓) T(tc)G±(tc)G±(tc) T(te±)G±(te∓)G±(te∓) T(tc)G±(te±)G±(te±) T(te∓)G±(tc)G±(te±) T(te∓)G±(te±)G±(te∓) T(te±)G±(te±)G±(te∓) T(te∓)G±(tc)G±(tc) T(te∓)G±(te±)G±(te±) T(te±)G±(tc)G±(te±) T(tc)G±(te∓)G±(g±e±) T(te±)G±(tc)G±(g±c) T(te±)G±(tc)G±(tc) T(tc)G±(te∓)G±(g±e∓) T(tc)G±(tc)G±(g±e±)
0.705 0.717 0.721 0.786 0.812 0.817 0.838 0.892 0.892 0.991 1.011 1.015 1.022 1.072 1.089 1.089 1.142 1.153
75 76 77 78
T(te±)G±(te±)G±(te±) T(tc)G±(te±)G±(g±e±) T(tc)G±(tc)G±(g±e∓) T(te∓)G±(te∓)G±(g±e±)
1.170 1.187 1.259 1.262
79
T(te∓)G±(tc)G±(g±e±)
1.329
80
T(te∓)G±(te±)G±(g±e±)
1.331
81 82 83 84
T(tc)G±(g±e±)G±(g±e±) T(te∓)G±(te∓)G±(g±e∓) T(tc)G±(g±e±)G±(g±e∓) T(te±)G±(te∓)G±(g±e±)
1.340 1.349 1.351 1.400
85 86 87 88 89 90 91 92 93
T(tc)G±(te∓)G±(g±c) T(te±)G±(tc)G±(g±e±) T(tc)G±(te±)G±(g±e∓) T(te∓)G±(g±e±)G±(g±e±) T(te±)G±(te∓)G±(g±e∓) T(te∓)G±(tc)G±(g±e∓) T(te±)G±(te±)G±(g±e±) T(te±)G±(tc)G±(g±e∓) T(g±e±)G±(tc)G±(te∓)
1.424 1.431 1.435 1.443 1.448 1.471 1.472 1.481 1.491
94 95 96 97 98 99 100 101 102 103 104 105 106 107
T(g±e±)G±(te∓)G±(te∓) T(tc)G±(g±e∓)G±(g±e∓) T(te∓)G±(g±e±)G±(g±e∓) T(te±)G±(g±e±)G±(g±e±) T(g±e±)G±(g±e±)G±(g±e∓) T(te±)G±(g±e±)G±(g±e∓) T(te∓)G±(te±)G±(g±e∓) T(te∓)G±(te∓)G±(g±c) T(g±e±)G±(tc)G±(te±) T(te±)G±(te±)G±(g±e∓) T(g±e±)G±(te±)G±(te∓) T(g±e±)G±(tc)G±(tc) T(te±)G±(g±e∓)G±(g±e∓) T(tc)G±(te±)G±(g±c)
1.535 1.541 1.594 1.603 1.660 1.662 1.676 1.700 1.708 1.734 1.736 1.752 1.765 1.766
unscaledc
predicted
scaledd
population (%)
1314.1 1317.1 1321.1 1317.1 1321.2 1314.2 1320.8 1320.0 1317.2 1317.1 1320.9 1320.0 1320.2 1318.3 1320.7 1320.7 1311.8 1313.7 1320.2 1320.4 1318.9 1314.2 1303.3 1310.8 1318.9 1313.2 1319.0 1312.1
0.82 1.60 1.58 1.42 0.68 0.68 0.65 1.20 1.20 1.00 0.49 0.48 0.96 0.88 0.86 0.43 0.78 0.76
±
T(xy)G (xy)G (xy) Cluster 1.033 1285.8 (219) 0.947 1288.7 (224) 0.704 1292.7 (211) 1.040 1288.7 (231) 0.663 1292.8 (217) 1.126 1285.9 (226) 0.745 1292.4 (198) 0.902 1291.6 (212) 0.988 1288.8 (212) 1.081 1288.7 (206) 0.861 1292.5 (228) 0.943 1291.6 (202) 0.995 1291.8 (219) 1.209 1289.9 (170) 1.601 1292.3 (237) 0.954 1292.3 (237) 1.110 1283.6 (171) 1.123 1285.4 (138) 1291.8 (102) 1.039 1292.0 (211) 1.164 1290.5 (177) 1.024 1285.9 (179) 1.493 1275.2 (84) 1282.6 (69) 1290.5 (132) 1.321 1284.9 (139) 1290.6 (112) 1.362 1283.9 (103) 1290.3 (136) 1.583 1279.0 (204) 1.404 1282.2 (180) 1.484 1278.1 (210) 1.500 1282.4 (120) 1290.6 (133) 1.396 1283.9 (186) 1.414 1285.4 (165) 1.065 1286.6 (164) 1.801 1278.1 (206) 1.401 1282.2 (182) 1.242 1285.9 (183) 1.455 1290.2 (160) 1.315 1285.2 (194) 1.839 1285.4 (118) 1292.9 (123) 1.925 1282.6 (153) 1.985 1278.7 (195) 1.702 1277.4 (222) 1.874 1278.2 (206) 2.574 1276.4 (229) 1.775 1277.2 (225) 1.263 1285.5 (178) 1.680 1282.9 (204) 1.794 1293.8 (180) 1.356 1285.0 (186) 1.880 1286.1 (136) 1.753 1294.4 (204) 1.676 1277.6 (212) 1.351 1287.0 (185) 4023
1307.1 1310.4 1306.2 1310.6 1319.0 1312.1 1313.7 1314.9 1306.2 1310.4 1314.2 1318.6 1313.5 1313.7 1321.3 1310.8 1306.8 1305.5 1306.3 1304.5 1305.3 1313.8 1311.1 1322.3 1313.3 1314.4 1322.9 1305.7 1315.3
0.37 0.72 0.64 0.64
0.56 0.56 0.28 0.56 0.54 0.50 0.48 0.48 0.48 0.23 0.46 0.44 0.44 0.44 0.44 0.20 0.20 0.36 0.18 0.32 0.32 0.32 0.30 0.30 0.28 0.28 0.14 0.14 0.28
DOI: 10.1021/acs.jpca.5b00889 J. Phys. Chem. A 2015, 119, 4017−4031
Article
The Journal of Physical Chemistry A Table 2. continued calculated (PO stretch + CH2 twist) vibrational wavenumber (cm−1)
relative energy (kcal/mol) s. no.
structure
computedb
±
108 109 110 111 112
T(te∓)G±(g±e∓)G±(g±e∓) T(tc)G±(g±e±)G±(g±c) T(te±)G±(te∓)G±(g±c) T(tc)G±(g±e∓)G±(g±c) T(g±e±)G±(te±)G±(te±)
1.784 1.785 1.785 1.924 1.928
113 114 115 116 117 118
T(te∓)G±(g±e±)G±(g±c) T(te∓)G±(te±)G±(g±c) T(te±)G±(g±e±)G±(g±c) T(g±e±)G±(g±e±)G±(g±c) T(te±)G±(te±)G±(g±c) T(g±e±)G±(tc)G±(g±e±)
1.981 2.016 2.049 2.053 2.059 2.093
119 120 121 122
T(g±e±)G±(te∓)G±(g±e±) T(te±)G±(g±e∓)G±(g±c) T(g±e±)G±(te∓)G±(g±e∓) T(g±e±)G±(te±)G±(g±e±)
2.098 2.150 2.173 2.228
123 124 125 126 127 128 129 130
T(te∓)G±(g±e∓)G±(g±c) T(g±e±)G±(tc)G±(g±e∓) T(g±e±)G±(g±e±)G±(g±e±) T(g±e±)G±(te∓)G±(g±c) T(g±e±)G±(te±)G±(g±e∓) T(g±e±)G±(g±e∓)G±(g±e∓) T(g±e±)G±(te±)G±(g±c) T(g±e±)G±(g±e∓)G±(g±c)
2.234 2.280 2.458 2.576 2.608 2.945 2.957 3.344
131
T(tc)G+(tc)G−(tc)
1.871
unscaledc
predicted
scaledd
population (%)
1305.6 1307.0 1312.2 1307.9 1316.7 1322.6 1306.4 1314.3 1306.1 1306.2 1314.3 1311.0 1318.7 1321.8 1308.0 1306.5 1308.6 1309.7 1317.8 1306.4 1310.9 1303.7 1309.6 1310.7 1303.3 1311.3 1303.0
0.13 0.26 0.26 0.20 0.10
±
T(xy)G (xy)G (xy) Cluster 1.583 1277.5 (211) 1.770 1278.9 (234) 1.687 1284.0 (194) 1.671 1279.7 (217) 1.835 1288.4 (94) 1294.1 (111) 1.988 1278.3 (247) 1.549 1286.0 (192) 2.061 1278.0 (247) 2.860 1278.1 (247) 1.642 1286.0 (196) 2.213 1282.8 (107) 1290.3 (70) 1293.3 (86) 2.299 1279.8 (114) 1.962 1278.4 (234) 2.200 1280.4 (143) 2.254 1281.5 (77) 1289.4 (81) 1.869 1278.3 (233) 2.114 1282.7 (131) 2.673 1275.6 (188) 2.486 1281.4 (156) 2.155 1282.5 (153) 2.475 1275.2 (179) 2.441 1283.1 (124) 2.761 1275.0 (211) T(xy)G+(xy)G−(xy) Cluster 1288.4 (191)
1316.7
0.18 0.18 0.16 0.16 0.16 0.16
0.16 0.14 0.14 0.12 0.12 0.12 0.04 0.06 0.06 0.02 0.04 0.02 0.06
The computations were performed at the B3LYP/6-311++G(d,p) level of theory. bEnergies corrected for ZPE. cIntensity in km mol−1 given in parentheses. dScaling factor = 1.022. a
were established to be 53. Similarly, for the T(xy)G(xy)G±(xy) cluster, of the 144 conformers, 60 conformers were found to be degenerate with the existing 84 conformers. Out of 84 conformers, 7 conformers with the “g±c” orientation interconvert to “tc” orientation, resulting in 77 conformational minima in the potential energy surface of TAP. Therefore, as a combined consequence of degeneracy and probable interconversion of a few conformers, the total number of conformations contributing to the room temperature population of TAP in the gas phase was found to be 130. Only one T(tc)G + (tc)G − (tc) conformer was considered from the T(xy)G +(xy)G−(xy) family whose energy was calculated to be 1.87 kcal mol−1 with respect to the ground state G±(tc)G±(tc)G±(tc) conformer of TAP. Figures 6 and 7 show a few representative G ± (xy)G±(xy)G±(xy) and T(xy)G±(xy)G±(xy) conformers of TAP. The cumulative population of the G±(xy)G±(xy)G±(xy) cluster was calculated to be 62.3%, and for the T(xy)G±(xy)G±(xy) cluster, it was 37.6%. The population calculated is a representative of all 360 + 1 conformers of TAP, which were essentially compressed to 130 + 1 conformers due to degeneracy and the probable interconversion of a few conformers during optimization. For a few conformers of TAP, the selected structural parameters computed at the B3LYP/6-311++(d,p) level of theory
the conformations is still a laborious task, and once they are resolved, the “additivity rule” just follows and simplifies the prediction of the relative energies of the conformers. The structures, relative energies in increasing order, vibrational wavenumbers, and population of the different TAP conformers calculated at B3LYP/6-311++G(d,p) level of theory are tabulated in Table 2. Predicted relative energies using the “additivity rule” are also given in Table 2 for comparison. The comparison of percentage errors between the calculated and the predicted relative energy values of all conformers of TAP are given in Supporting Information. The structures of all possible 216 conformers of G±(xy)G±(xy)G±(xy) and 144 conformers of T(xy)G±(xy)G±(xy) clusters of TAP (in total, 360) are presented in the Supporting Information. A closer examination revealed that in the G±(xy)G±(xy)G±(xy) cluster, of the 216 conformers, 160 conformers were found to be degenerate with the already existing 56 conformers. Consequently, 56 conformers corresponding to G±(xy)G±(xy)G±(xy) structures alone do exist in the potential energy surface of TAP. The G±(tc)G±(tc)G±(g±c), G±(tc)G±(te∓)G±(g±c), and G±(tc)G±(g±e±)G±(g±c) conformers respectively interconvert to G±(tc)G±(tc)G±(tc), G±(tc)G±(te∓)G±(tc), and G±(tc)G±(g±e±)G±(tc) presumably due to the interchain interaction. Therefore, the effective conformational minima for the G±(xy)G±(xy)G±(xy) cluster 4024
DOI: 10.1021/acs.jpca.5b00889 J. Phys. Chem. A 2015, 119, 4017−4031
Article
The Journal of Physical Chemistry A
Figure 6. Computed structures at B3LYP/6-311++G(d,p) level of theory of a few representative conformers of G±(xy)G±(xy)G±(xy) clusters of TAP. The ZPE corrected energy with respect to the ground state G±(tc)G±(tc)G±(tc) clusters of TAP is also given alongside of the structures.
Figure 7. Computed structures at B3LYP/6-311++G(d,p) level of theory of a few representative conformers of T(xy)G±(xy)G±(xy) clusters of TAP. The ZPE corrected energy with respect to the ground state G±(tc)G±(tc)G±(tc) clusters of TAP is also given alongside of the structures.
are presented in Table 3 and the computed vibrational wavenumbers in the different regions are given in Table 4. 4.4. Vibrational Assignments. When TAP was codeposited with the N2 matrix at 12 K (Figure 3), the spectral features were observed in the PO stretching region at 1303.7, 1280.6, and 1267.4 cm−1. The simulated spectrum of TAP in the PO stretching region was obtained by taking all 131 conformers into consideration and correcting the intensity of each of the conformers for their population at room temperature (Figure 3, trace a). As can be seen from the figure, the scaled computed wavenumbers of the G±(xy)G±(xy)G±(xy) and T(xy)G±(xy)G±(xy) clusters (Table 2) agreed well with the set of experimental wavenumbers observed at 1280.6/1267.4 and 1303.7 cm−1, respectively. After all, the wavenumbers in a given cluster are contiguous to each other, resulting in the broadening of spectral width. It should be noted that computed and experimental wavenumbers of G±(xy)G±(xy)G±(xy) and T(xy)G±(xy)G±(xy) clusters are well separated by ∼20 cm−1. One cannot miss noting that the corresponding vibrational wavenumbers are very broad and overlapping in the liquid spectrum, making the conformational analysis in the bulk phase practically impossible (Figure, 1, trace a). The features observed at 1291.7 and 1250.4 cm−1 are probably due to G±(xy)G±(xy)G±(xy) cluster−H2O and T(xy)G±(xy)G±(xy) cluster−H2O adducts, respectively. Interestingly, the relative
shifts of these water associates are in good agreement with the TMP (G±G±G±)−H2O and TG±G±−H2O adducts.70 The annealing experiments of TAP in the N2 matrix lead to the splitting of the 1280.6 cm−1 band characteristic of G±(xy)G±(xy)G±(xy) cluster to 1282.0, 1279.6, and 1276.3 cm−1. The splitting of the spectral band could probably be due to the matrix site effect. Because the experimental spectra for the vibrational modes in the C−C, P−O, and C−O stretching regions for the different conformations of TAP were not resolved, no attempt was made to firmly assign the features observed in these regions to the different cluster of conformations. It is apparent from the simulated spectra (Figure 2) that there is a significant overlapping of the features of both the clusters in this vibrational region. All the observed experimental features in C−C, C−O, and P−O stretching vibrational regions, therefore, can be cumulatively assigned to both the G±(xy)G±(xy)G±(xy) and T(xy)G±(xy)G±(xy) clusters of conformers of TAP. Table 5 lists the experimental vibrational wavenumbers in the different vibrational regions of TAP in N2, Ar, and Xe matrixes. 4.4.1. Conformational Behavior in Xe Matrix. Although the spectra recorded for TAP in the Ar matrix exhibit a behavior similar to that of the N2 matrix, Xe presents an interesting 4025
DOI: 10.1021/acs.jpca.5b00889 J. Phys. Chem. A 2015, 119, 4017−4031
Article
The Journal of Physical Chemistry A
Table 3. Selected Structural Parametersa of Representative Conformers from G±(xy)G±(xy)G±(xy), T(xy)G±(xy)G±(xy), and T(tc)G+(tc)G−(tc) Clusters of TAP Calculated at the B3LYP/6-311++G(d,p) Level of Theory G±(xy)G±(xy)G±(xy) cluster G±(tc)G±(tc)G±(tc) G±(te±)G±(te±)G±(te±) G±(te∓)G±(te∓)G±(te∓) G±(g±e∓)G±(g±e∓)G±(g±e∓) G±(g±e±)G±(g±e±)G±(g±e±) G±(g±c)G±(g±c)G±(g±c) G±(tc)G±(te±)G±(g±e±) G±(te∓)G±(g±e∓)G±(g±c) T(xy)G±(xy)G±(xy) cluster
P1O2 1.48 1.48 1.48 1.48 1.48 1.48 1.48 1.48 P1O2
P1O3 1.60 1.60 1.60 1.60 1.60 1.60 1.60 1.60 P1O4
∠O2P1O3 116.1 115.9 115.8 115.8 115.7 116.2 116.1 116.8 ∠O2P1O4
1.47 1.60 113.5 T(tc)G±(tc)G±(tc) T(te±)G±(te±)G±(te±) 1.47 1.60 113.4 T(tc)G±(tc)G±(te∓) 1.47 1.60 113.4 T(te∓)G±(te∓)G±(te∓) 1.47 1.60 113.5 T(g±e±)G±(g±e±)G±(g±e±) 1.47 1.59 113.6 T(xy)G+(xy)G−(xy) cluster P1O2 P1O4 ∠O2P1O4 T(tc)G+(tc)G−(tc)
1.47
1.61
111.7
∠C6O3P1O2
∠C7O4P1O2
∠C15C6O3P1
∠C17C15C6O3
dipole moment (Debye)
42.5 45.4 45.7 32.9 45.5 33.5 48.8 51.4 ∠C6O3P1O2
42.6 45.2 45.7 33.0 43.1 31.1 47.6 35.9 ∠C7O4P1O2
−171.3 −176.3 −158.1 81.6 104.2 110.1 −171.2 −158.8 ∠C20C7O4P1
−0.9 127.0 −123.9 −128.4 123.9 −3.2 −1.0 −124.6 ∠C22C20C7O4
0.76 1.26 1.15 0.46 1.27 0.27 1.14 0.91 dipole moment (Debye)
34.5 33.4 35.1 36.0 24.1 ∠C6O3P1O2
−179.6 176.3 177.8 176.2 173.0 ∠C7O4P1O2
175.5 170.3 173.9 176.4 101.3 ∠C20C7O4P1
0.9 125.6 0.8 −127.8 119.9 ∠C22C20C7O4
3.53 3.92 3.51 3.76 3.95 dipole moment (Debye)
23.6
−179.9
−178.7
−0.63
2.63
a
Bond distances in Å; bond angles and torsion angles in degrees. Torsion angles of the fragment ABCD denote the angle between ABC and BCD planes.
Table 4. Computed Vibrational Wavenumbers of Representative Conformers from G±(xy)G±(xy)G±(xy), T(xy)G±(xy)G±(xy), and T(tc)G+(tc)G−(tc) Clusters of TAP at the B3LYP/6-311++G(d,p) Level of Theorya vibrational wavenumbers (cm−1) conformer G±(tc)G±(tc)G±(tc)
870.9 (64)
936.6 (77)
G±(te±)G±(te±)G±(te±)
879.4 (95)
936.9 (44)
G±(te∓)G±(te∓)G±(te∓)
871.5 (99)
941.9 (26)
G±(g±e∓)G±(g±e∓)G±(g±e∓)
840.7 (89)
928.6 (41)
G±(g±e±)G±(g±e±)G±(g±e±)
843.2 (94)
937.7 (34)
G±(g±c)G±(g±c)G±(g±c)
833.1 (65)
920.0 (56)
G±(tc)G±(te±)G±(g±e±)
856.6 878.9 838.5 863.9 862.2 874.3 863.7 876.5 859.2 876.1 858.6 876.6 824.5 838.1 846.5 874.0
939.5 (40)
G±(te∓)G±(g±e∓)G±(g±c) T(tc)G±(tc)G±(tc) T(te±)G±(te±)G±(te±) T(tc)G±(tc)G±(te∓) T(te∓)G±(te∓)G±(te∓) T(g±e±)G±(g±e±)G±(g±e±) T(tc)G+(tc)G−(tc) a
PO stretch + CH2 rock + −CC stretch + in plane CH bend CH2 rock
(61) (114) (66) (120) (118) (101) (183) (125) (130) (104) (161) (124) (150) (132) (48) (174)
928.5 (32) 930.4 935.4 930.7 940.7 931.7 940.3 937.6 940.2 934.4 934.6 930.4 935.3
(57) (54) (16) (16) (33) (21) (20) (29) (19) (27) (36) (86)
CO stretch + CH2 rock + out of plane CH bend 1024.9 1025.4 1008.3 1009.2 1011.5 1012.3 1006.5 1007.4 1006.2 1006.8 1008.2 1008.9 1008.7 1018.3 1009.4 1010.6 1019.1 1030.4 1002.6 1014.5 1010.9 1015.3 1006.5 1014.6 1001.5 1015.4 1018.6 1029.0
(438) (430) (734) (737) (546) (531) (597) (598) (575) (563) (261) (253) (654) (443) (338) (572) (317) (356) (567) (492) (371) (122) (579) (579) (452) (584) (253) (396)
−CH2 rock + out of plane CH bend
PO stretch + −CH2 twist
1036.3 (100)
1257.8 (165)
1032.8 (64)
1253.0 (142)
1034.0 (120)
1254.0 (234)
1028.2 (13)
1249.3 (139)
1033.1 (53)
1252.0 (230)
1044.2 (106)
1255.2 (203)
1032.7 (74)
1252.9 (176)
1030.2 (40)
1253.3 (196)
1035.9 (49)
1292.8 (217)
1033.1 (175)
1292.0 (211)
1026.9 (369)
1289.4 (211)
1033.0 (102)
1285.8 (219)
1033.3 (31)
1275.6 (188)
1036.0 (110)
1288.4 (191)
The intensities (in km/mol) are given in parentheses.
wavenumbers observed at 1277.2/1272.9/1267.1 cm−1 and 1302.6/1299.9 cm−1, respectively, are due to G±(xy)G±(xy)G±(xy) and T(xy)G±(xy)G±(xy) clusters of TAP. As mentioned
variation (Figure 4). The main spectral features of TAP in the Xe matrix (in the PO stretching + CH2 twisting mode) occur at 1302.6, 1299.9, 1277.2, 1272.9, and 1267.1 cm−1. The set of 4026
DOI: 10.1021/acs.jpca.5b00889 J. Phys. Chem. A 2015, 119, 4017−4031
Article
The Journal of Physical Chemistry A Table 5. Experimental Vibrational Wavenumbers of TAP in Different N2, Ar, and Xe Matrixes experimental vibrational wavenumbers (cm−1) matrix
PO stretch + CH2 rock + in plane CH bend
−CC stretch + CH2 rock
CO stretch + CH2 rock + out of plane CH bend
−CH2 rock + out of plane CH bend
N2 Ar Xe
891.0, 925.8, 939.3 889.1, 923.8, 933.5 894.9, 919.0, 930.1, 935.9, 939.7
994.2 990.9 986.5
1027.5 1029.4 1026.0, 1031.8
1058.8 1059.8 1062.2
PO stretch + −CH2 twist 1267.4, 1280.6, 1303.7 1265.6,1280.6, 1305.7 1267.1, 1272.9, 1277.2, 1299.9, 1302.6
Table 6. NBO Analysis of Some of the Important Geminal and Vicinal Hyperconjugative Interactions, Showing the Donor and Acceptor Orbitals and the Second-Order Perturbation Energies, E2, in the Representative G±(tc)G±(tc)G±(tc), T(tc)G±(tc)G±(te∓), and T(tc)G+(tc)G−(tc) Conformers of TAP, Calculated at the B3LYP/6-311++G(d,p) Level of Theory second-order perturbation energies, E2 (kcal/mol)
second-order perturbation energies, E2 (kcal/mol) donor orbital
acceptor orbital
σP1O2 σP1O2 σP1O2 πP1O2 πP1O2 πP1O2 πP1O2 π1P1O2 π1P1O2 π1P1O2 σP1O3 σP1O3 σP1O3 σP1O3 σP1O4 σP1O4 σP1O4 σP1O5 σP1O5 σP1O5 σP1O5 σP1O5 π*P1O2 π*P1O2 π*P1O2 π1*P1O2 π1*P1O2 π1*P1O2 σ*P1O4 σ*P1O5
π*P1O2 σ*P1O3 σ*P1O5 σ*P1O2 σ*P1O3 σ*P1O4 σ*P1O5 σ*P1O3 σ*P1O4 σ*P1O5 π1*P1O2 π*P1O2 σ*P1O4 σ*P1O5 π*P1O2 σ*P1O3 σ*P1O5 σ*P1O2 π*P1O2 π1*P1O2 σ*P1O3 σ*P1O4 σ*P1O3 σ*P1O4 σ*P1O5 σ*P1O3 σ*P1O4 σ*P1O5 σ*P1O5 σ*P1O3
lp(2)O2 lp(2)O2 lp(2)O2 lp(1)O3
σ*P1O3 σ*P1O4 σ*P1O5 σ*P1O2
G±(tc)G±(tc) G±(tc)
T(tc)G±(tc) G±(te∓)
T(tc)G+(tc) G−(tc)
donor orbital
acceptor orbital
16.32 1.57 11.13 7.21
