Synthesis, microwave spectrum, quantum chemical calculations, and conformational composition of a novel primary phosphine, cyclopropylethynylphosphine, (C3H5C≡CPH2).

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Article

Synthesis, Microwave Spectrum, Quantum Chemical Calculations, and Conformational Composition of a Novel Primary Phosphine, Cyclopropylethynylphosphine, (CHC#CPH) 3

5

2

Harald Møllendal, Svein Samdal, Jürgen Gauss, and Jean-Claude Guillemin J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp506169g • Publication Date (Web): 29 Aug 2014 Downloaded from http://pubs.acs.org on August 31, 2014

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The Journal of Physical Chemistry

Synthesis, Microwave Spectrum, Quantum Chemical Calculations, and Conformational Composition of a Novel Primary Phosphine, Cyclopropylethynylphosphine, (C3H5C≡CPH2)

Harald Møllendal,*,ϯ Svein Samdal,ϯ Jürgen Gauss,‡ and Jean-Claude Guillemin*,§

ϯCentre

for Theoretical and Computational Chemistry (CTCC), Department of Chemistry, University of Oslo, P. O. Box 1033 Blindern, NO-0315 Oslo, Norway

‡Institut

für Physikalische Chemie, Universität Mainz, Duesbergweg 10–14, 55128 Mainz, Germany

§

Institut des Sciences Chimiques de Rennes, École Nationale Supérieure de Chimie de Rennes, CNRS, UMR 6226, 11 Allée de Beaulieu, CS 50837, 35708 Rennes Cedex 7, France

Received June 20, 2014

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Key words: synthesis, conformational composition, structures, microwave spectrum, energy difference, dipole moment, CCSD, CCSD(T), and MP2 calculations, centrifugal distortion constants, vibrational frequencies, vibration-rotation constants

ABSTRACT: The microwave spectrum of cyclopropylethynylphosphine, C3H5C≡CPH2, has been investigated in the 26 – 120 GHz spectral region. The spectrum is dominated by very rich and complex a-type R-branch pile-ups. There must be insignificant steric interaction between the phosphino group and the cyclopropyl ring due to the long distance between these two groups. However, the phosphino group does not undergo free or nearly free internal rotation. Instead, the spectra of two distinct conformers were assigned. Both these two forms have CS symmetry. The symmetry plane bisects the cyclopropyl ring and the phosphino group in both conformers and the lone electron pair of the phosphino group points in opposite directions in the two rotamers. The energy difference between the two forms was determined to be 1.9(6) kJ/mol. A simple model that takes into consideration the interaction of the lone electron pair of the phosphino group with the -electrons of the ethynyl group and the Walsh electrons of the cyclopropyl ring is able to give a qualitative explanation of the observation of two conformers and the nonexistence of free rotation of the phosphino group. The MW work was augmented by quantum chemical calculations using second-order Møller-Plesset perturbation and coupled cluster theory with results that are in good agreement with experiments.


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INTRODUCTION The conformational, structural and dynamical properties of many primary phosphines have been studied by microwave (MW) spectroscopy in the past. These studies include CH3PH2,1 (CH3)2PH,2,3 (CH3)3CPH2,4 (CH3)3P,5 (CH3)2CHPH2,6 CH3CH2PH2,7,8 cyclopropylphosphine (C3H5PH2),9 phenylphosphine (C6H5PH2),10 H2PCH2CH2CN,11 HCCPH2,12 H2C=CHPH2,13,14 H2PCH2CH2PH2,15 (C2H5P),19 (C5H5PH2),21

HCCCH2PH2,16

H2C=CHCH2PH2,17

cyclopropylmethylphosphine (chloromethyl)phosphine

H2C=C=CHPH2,18

(C3H5CH2PH2),20 (ClCH2PH2)22

and

phosphirane

cyclopentadienylphosphine (2-chloroethyl)phosphine,

ClCH2CH2PH2.23 While one conformer exists for most of these phosphines, the existence of two or more rotameric forms have been seen for each of CH3CH2PH2,7,8 H2PCH2CH2CN,11 H2PCH2CH2PH2,15 H2C=CHCH2PH2,17 H2C=C=CHPH2,24 C3H5CH2PH2,20 C5H5PH2,21 and (ClCH2PH2).22 A comparison of the structural and conformational properties of amines and phosphines has recently been investigated in a gas electron-diffraction and quantum chemical study,25 which concluded that these properties are often different for phosphines and their amine analogues.25 While many tertiary 1-alkynylphosphines have been synthesized26 and used as, for example, ligands in various reactions like the gold-catalyzed cyclization of acetylenic βketoesters,27 or acetylenic silyl enol ethers,28 or for many asymmetric transformations in Rhcatalyzed hydrogenation and Rh- or Pd-catalyzed C‒C bond-forming reactions,29 only few primary or secondary derivatives, which are kinetically unstable compounds, have been isolated by much more difficult syntheses. Thus, the simplest ethynyl derivative HC≡C‒PH2, was prepared for the first time in 1987 by low-pressure electric discharge of an acetylene-phosphine mixture.12 A few years later, the chemoselective reduction of 1-alkynylphosphonates was found 3 ACS Paragon Plus Environment


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to be a general approach for the low boiling derivatives.30,31 Interestingly, 1-alkynylphosphines were found to have much higher gas-phase acidities than the saturated alkylphosphines,32 and their base-induced rearrangement produced the corresponding phosphaalkynes.33 In this work, our studies of phosphines are extended to include the synthesis and microwave study augmented with quantum chemical calculations of the novel primary phosphine, cyclopropylethynylphosphine (C3H5C≡CPH2). Several interesting physical properties are associated with this compound. The distance between the phosphino group and the cyclopropyl ring is so long that steric interactions are hardly of importance for the dynamical and conformational behavior of its phosphino group. This group could either rotate freely, or a barrier caused by electronic interactions of the lone electron pair of the phosphorous atom with the ethynyl group -electrons and the Walsh electrons of the cyclopropyl ring could exist with rotational isomerism as a consequence. The unknown dynamic behavior of the phosphino group of C3H5C≡CPH2 was the primary motivation to carry out the present research. A successful investigation of the conformational and dynamical problems presented by cylcopropylethynylphosphine requires an experimental method with a superior accuracy and resolution. Fortunately, MW spectroscopy meets these requirements and has therefore been chosen. The MW work is augmented by high-level quantum chemical calculations, which were conducted both to obtain information useful for the assignment of the MW spectrum and for investigating the conformational preferences in the case of C3H5C≡CPH2. It was found in the course of this work that there is no free rotation of the phosphino group and that two rotamers, denoted I and II, exist instead. They are depicted in Fig. 1, with atom numbering indicated on I. The lone pair (lp) of the phosphorous atom and the C3‒H8 bond


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are synperiplanar in I, and antiperiplanar in II. Both forms have CS symmetry. The symmetry plane is formed by the H8‒C3‒C2≡C1‒P11 link of atoms. This plane bisects the cyclopropyl ring and the phosphine group.

EXPERIMENTAL Synthesis. The cyclopropylethynylphosphine was synthesized by chemoselective reduction

of

cyclopropylethynyldiethylphosphonate

(Scheme

1).

Cyclopropylethynyldiisopropylphosphonate had been prepared by Qu et al by a different approach.34

Scheme 1

Cyclopropylethynylphosphonic acid, diethyl ester. Ethylmagnesium bromide (0.1 mol, 1M) was prepared in THF under dry nitrogen. In a flask equipped with a stirring bar and a nitrogen inlet, cyclopropylethyne (6.6 g, 0.1 mol) in THF (50 mL) was cooled to ‒50°C and the ethylmagnesium bromide solution was added dropwise to form 2-cyclopropylethynyl magnesium bromide. The mixture was allowed to warm to room temperature and stirred for 15 min. In 5 ACS Paragon Plus Environment


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another flask equipped with a stirring bar and a nitrogen inlet, phosphorochloridic acid, diethyl ester (17.3 g, 0.1 mol) in THF (50 mL) was cooled to ‒50°C and the 2-cyclopropylethynyl magnesium bromide solution was added dropwise. The mixture was allowed to warm to room temperature and stirred for 15 min. The hydrolysis was then performed with a saturated solution of NH4Cl (30 mL) and the organic compounds were extracted with diethyl ether (3 × 50 mL). The solvents were removed in vacuo and the cyclopropylethynylphosphonic acid, diethyl ester was purified by distillation. bp0.1 = 105°C. Yield: 11 g, 55 %. 1H NMR (CDCl3, 400 MHz)  0.85 (m, 4H, CH2); 1.29 (t, 6H, 3JHH = 7.0 Hz, CH3); 1.33 (m, 1H, CH); 4.06 (quint, 3JHH = 3JPH = 7.0 Hz, 4H, OCH2). 13C NMR (CDCl3, 100 MHz)  -0.25 (1JCH = 170.9 Hz (d), 3JCP = 5.1 Hz (d), cCH); 9.1 (1JCH = 165.8 Hz (t), 4JCP = 1.4 Hz (d), c-CH2); 16.1 (1JCH = 127.6 Hz (q), 3JCP = 6.3 Hz (d), CH3); 62.8 (1JCH = 146.7 Hz (t), 2JCP = 5.8 Hz (d), OCH2); 68.9 (1JCP = 305.2 Hz (d), C-P); 105.9 (2JCP = 54.5 Hz (d), C≡CP).

31

P NMR (CDCl3, 160 MHz)  -6.1. IR (film, cm-1)  2986

(s), 2908 (m), 2201 (vs) (C≡C), 1261 (vs), 1025 (vs), 840 (m).

Caution! Alkynylphosphines are pyrophoric and nauseating compounds; preparation and handling must be carried out under a well-ventilated hood.

Cyclopropylethynylphosphine. (For similar experiments, see Guillemin et al.35) A vacuum line was equipped with two traps. The first trap was immersed in a ‒60°C cold bath and the second one was immersed in a ‒110°C cold bath. A three-necked flask equipped with a septum, a stirring bar and containing the reducing mixture (50 mmol of AlHCl2 in 50 mL tetraglyme) was attached to the vacuum line and degassed. The cyclopropylethynylphosphonic 6 ACS Paragon Plus Environment

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acid, diethyl ester (1.0 g, 5 mmol diluted in 10 mL of tetraglyme) was slowly added (10 min) at room temperature with a flex-needle through the septum. During and after the addition, the cyclopropylethynylphosphine was removed as it was formed from the reaction mixture. The high boiling impurities were condensed in the first trap and the phosphine was selectively trapped in the second one. At the end of the addition, the reaction mixture was still stirred at room temperature for 20 min and then the trap was disconnected from the vacuum line by stopcocks. Yield: 167 mg, 1.7 mmol, 34 %. 1H NMR (CDCl3, 400 MHz)  0.65 (m, 2H, 1 H of each c-CH2); 0.73 (m, 2H, 1H of each c-CH2); 1.24 (m, 1H, c-CH); 3.60 (dd, 1JPH = 214.1 Hz, 4JHH = 2.2 Hz, 2H, PH2). 13C NMR (CDCl3, 100 MHz)  0.41 (1JCH = 167.8 Hz (d), c-CH); 7.9 (1JCH = 166.2 Hz (t), c-CH2); 60.6 (1JCP = 8.0 Hz (d), C-P); 108.1 (2JCP = 2.9 Hz (d), C≡C‒P). 31P NMR (CDCl3, 160 MHz)  -177.1 (1JPH = 214.1 Hz (t)). IR (gas, cm-1)  2969 (w), 2863 (w), 2296 (s) (PH), 2198 (m) (C≡C), 1139 (s), 1077 (m), 830 (m). Spectroscopic Experiments. The MW spectrum was studied using the Stark microwave spectrometer of the University of Oslo. Details of the construction and operation of this device have been given elsewhere.36 Cyclopropylethynylphosphine is a colorless liquid with a vapor pressure of roughly 120 Pa at room temperature. Its spectrum was recorded in the 26 – 120 GHz frequency interval at a pressure of 5 – 10 Pa. A 2 m long Hewlett-Packard MW cell was employed in the experiments, which were carried out with the cell cooled by small portions of dry ice to about ‒30 ºC. Radio-frequency microwave double-resonance experiments (RFMWDR), similar to those of Wodarczyk and Wilson,37 were also conducted to unambiguously assign particular transitions. RESULTS Quantum Chemical Methods. The ab initio calculations were performed employing the 7 ACS Paragon Plus Environment


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Gaussian 09,38 Molpro,39 and CFour40 programs. Frozen-core second-order Møller-Plesset perturbation theory calculations41 (MP2), coupled cluster calculations with single and double excitations42 (CCSD), and CCSD calculations augmented by perturbative triples corrections CCSD(T)43 were carried out. Dunning’s44 correlation-consistent cc-pVTZ and cc-pVQZ basis sets were used in the calculations. Default convergence criteria were used in all calculations. Computational Results. A MP2/cc-pVTZ potential function for rotation about the C10‒ P11 bond was calculated using the scan option of Gaussian 09. The H8‒C3•••P11‒H12 dihedral angle, where the dots indicate that the phosphorous atom P11 and the carbon atom C3 are not directly bonded to one another, was scanned 360º in 10º intervals with no restrictions on the variation of the remaining structural parameters. The resulting function shown in Fig. 2 has two minima corresponding to conformers I and II separated by a barrier of about 2 kJ/mol. This is a strong indication that there is no free rotation of the phosphino group. The MP2/cc-pVTZ structures of conformers I and II were fully optimized and dipole moments, harmonic and anharmonic vibrational frequencies, quartic and sextic centrifugal distortion constants,45 vibration-rotation constants (the 's),46 and the re and r0 rotational constants46 were computed observing the precautions of McKean et al.47 These parameters are listed in Table 1S (conformer I) and 2S (rotamer II) of the Supporting Information. The H8‒ C3•••P11‒H12 dihedral angle is 132.00º in I and 311.95º in II. The lone electron pair of the phosphorous atom is synperiplanar with the C3‒H8 bond in I, and antiperiplanar in II. The MP2/cc-pVTZ electronic energy of conformer I is 0.071 kJ/mol less than that of II. Corrected for zero-point vibrational energies (ZPE) this difference becomes 0.029 kJ/mol. Moreover, the harmonic torsional frequencies of the phosphino group are 59 cm‒1 for I and 58 cm‒1 for II, far from free rotation. The transition state (TS), which has an electronic energy of 2.19 kJ/mol (1.59 8 ACS Paragon Plus Environment

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kJ/mol when corrected for ZPE) above the energy of I, occurs at a value of 219.61º for the H8‒ C3•••P11‒H12 dihedral angle. Its structure and vibrational frequencies are listed in the Supporting Information, Table 3S. The TS is depicted in Table 3S. The barrier height of 2.19 kJ/mol is low in comparison with 8.19 kJ/mol determined for the barrier to internal rotation in CH3PH2.1 A reduction of the barrier from 8.19 kJ/mol of methylphosphine was expected for cyclopropylethynylphosphine due to the fact that nonbonded interatomic distances are much longer in the latter compound with insignificant steric interactions and a lower barrier as a likely consequence. MP2/cc-pVQZ calculations were performed to investigate the dependency of the energy difference between I and II on the basis set. It was assumed in these computations that the two forms have a symmetry plane (CS symmetry). The results of these calculations are shown in the Supporting Information, Tables 4S and 5S. The electronic energy difference is 0.012 kJ/mol with I as the lower energy form compared to 0.071 kJ/mol found above in the MP2/cc-pVTZ result. Similar calculations were performed at the CCSD/cc-pVTZ (Tables 6S and 7S) and CCSD/cc-pVQZ (Tables 8S and 9S) levels of theory. Conformer I was found to be preferred by 0.042 kJ/mol in the former case, while II was computed to be the more stable conformer by 0.005kJ/mol in the latter case. CCSD(T)/cc-pVTZ calculations (Tables 10S and 11S) predict I to be 0.066 kJ/mol more stable than II. It is concluded that all five theoretical methods used in this work predict zero energy differences between I and II within methodological uncertainties. Our experience is that CCSD/cc-pVQZ structures are often more accurate than CCSD(T)/cc-pVTZ structures and they are therefore listed in Table 1 together with the dipole moments and the electronic energy difference. The CCSD/cc-pVQZ rotational constants 9 ACS Paragon Plus Environment


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calculated from these structures are shown in Table 5 (conformer I) and in Table 6 (II) together with their MP2 S-reduction45 centrifugal distortion constants and their experimental counterparts. The CCSD/cc-pVQZ structures warrant discussion. A partial r0-structure has been determined for HC≡CPH2. The value given in this work12 for the C‒P bond length is 177.4(5) pm, very similar to the CCSD result, 177.1 pm (Table 1). The P‒H bond lengths of 141.3 (same table) compare well with the reported 141.4(5) pm.12 A C≡C‒P angle of 173(2)° was required to obtain a consistent fit of the moments of inertia in the case of HC≡CPH2.12 The corresponding angle is 185.2° (‒174.8°) for conformer I and 174.8 for II (Table 1). The C9≡C10 bond length is 120.9 pm compared to the equilibrium bond length of acetylene, 120.2958(7) pm.48 Interestingly, the C‒C bond lengths of the ring are different. The C1‒C2 bond length is 149.5 pm, whereas the C1‒C3 and C2‒C3 bond lengths are 151.3 pm (Table 1). The CCSD/cc-pVQZ C‒C bond length in cyclopropane is 150.2 pm. The lengthening of 1.1 pm of the C‒C bonds adjacent to a substituent and a shortening of the C‒C bond opposite to the substituent by 0.7 pm are in accord with predictions.49 The equilibrium C‒C bond length in cyclopropane50 is 150.30(10) pm for comparison, which is the same within the uncertainty limit as the CCSD/cc-pVQZ bond length (150.2 pm). The Existence of Distinct Conformers of Bisubstituted Acetylenes. The existence of a barrier to internal rotation of the phosphino group in cyclopropylethynylphosphine warrants further investigation into the nature of this barrier. Acetylene exhibits a triple bond, consisting of a  bond and two equivalent  bonds, and has an electron-density distribution that is rotationally symmetric with respect to the CC bond axis. The rotational symmetry might be broken in the case of monosubstituted acetylenes, but this does not hamper the free rotation of the substituent. In other words, there exists no distinct conformers and there is no rotational barrier introduced. The


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substitution can only lift the degeneracy of the two  bonds, provided that the symmetry of the substituent is incompatible with the rotational symmetry of the triple bond. The effect is rather small in case of substituents such as CH3, but can be rather pronounced in the case of substituents that are either strong  donors or acceptors such as PH2, BH2, vinyl, cyclopropyl, etc. To explain the interactions in such monosubstituted acetylenes, we assume that the substituent (D for  donor and A for  acceptor) possesses just one orbital that can interact with the  orbitals of acetylene. In the case of a donor D, this orbital is occupied, in the case of an acceptor A, it is unoccupied. An orbital energy diagram for  interactions in monosubstituted acetylenes is shown in Fig. 3. Assuming that these orbitals of D and/or A have y symmetry, this means that the x and x* orbitals of the acetylene remain more or less unchanged, while there is a substantial change in the y and y* orbitals. Most important is probably here that a D substituent reduces the acceptor capabilities of the y* orbital, while at the same time this substituent enhances the donor capabilities of the y orbitals. For an A substituent, the situation is reversed, this means that the y orbital now has an increased donor and the y* a reduced acceptor capability. The rather pronounced interaction of the CC triple bond with these substituents results in a lengthening of the CC bond length. In the same way, rather short bond lengths to the substituents are observed. Substitution effects on CC triple bond lengths of monosubstituted acetylenes are demonstrated in Table 2, which is based on MP2 calculations. The  interactions between the CC triple bond and the substituent can furthermore be visualized by investigating the involved orbitals. The valence  orbitals are hence shown in Fig. 4 in the case of HC≡CPH2.



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In the case of the bisubstituted acetylenes, the best approach is to consider the interaction of the -orbitals of the monosubstituted acetylene with those of the second substituent. Two different possibilities now exist, namely, that the -orbital of the second substituents interacts with the x or the y orbitals, i.e. the same -orbitals as the first substituent or those which are orthogonal to them. If the interactions with both sets of -orbitals are comparable, no distinct conformer exist (apart from a rather small barrier) and the free rotation of the substituent is still possible. However, if the interactions are different, the free rotation is hindered and a specific orientation is favored. Considering now the possibility of -donors and acceptors as substituents, the following conclusions can be drawn: a) In the case of D-CC-D' (both substituents are donors), an orthogonal arrangement is preferred, as the first donor reduces the acceptor capabilities of the involved -orbitals. Therefore, the second donor, D', interacts with the second set of -orbitals. b) In the case of A-CC-A, everything is reversed and the situation is more or less the same. c) In the case of A-CC-D, one should expect a parallel (and if different, an antiparallel) arrangement, as both substituents interact with the same set of -orbitals. Calculations for the choices A = BH2 and D = PH2 confirm these expectations (see Table 3), as for both cases the parallel conformations (and also antiparallel conformation in the case of PH2) are no minima and only the orthogonal forms are the minima. The barriers are 19.8 kJ/mol for BH2 and 2.5 kJ/mol for PH2, sufficiently high to allow in principal an experimental detection. On the contrary, for H2B‒C≡C‒PH2 one finds the parallel arrangement to be the lowest in energy and the orthogonal form to be a transition state. Considering now our system of interest, namely, C3H5C≡CPH2 with PH2 and a cyclopropyl group as -donating substituents, the simple model predicts that the lone-pair orbital


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of PH2 and the three-membered ring, which defines the direction for the -donating Walsh orbital, are orthogonal with respect to each other. There exist actually two possibilities for realizing such an orthogonal orientation. In the first one, the lone-pair of PH2 points in the same direction as the three-membered ring, in the second, it points in the opposite direction. The transition state for interconversion between these two conformers shows an arrangement with the phosphorus lone pair parallel to the three-membered ring. The computations, as well as the experiments, confirm these expectations. Relevant results are listed in Table 4. Microwave Spectrum and Assignments. An a-c principal inertial axis plane bisecting the cyclopropyl ring and the phosphino group is the symmetry plane in both conformers. Both rotamers have their major dipole moment component along the a-inertial axis calculated to be 4.76 and 4.10 × 10‒30 C m, for I and II, respectively (Table 1). There is a smaller component along the c-inertial axis of 2.02 and 1.79 × 10‒30 C m for I and II (same table). b is zero for symmetry reasons in both cases. Ray’s asymmetry parameter51  is about ‒0.998 for both forms. The microwave spectra of both I and II were therefore predicted to consist of a comparatively strong and predominating series of a-type R-branch pile-ups regions separated by almost exactly the sum of the B and C rotational constants, as well as a much weaker c-type spectrum. The CCSD value of B + C is 2015.1 MHz in the case of I (Table 5), and 2038.8 MHz for II. The pile-ups belonging to conformers I and II were therefore expected to occur close to one another in the spectrum. Both conformers have several low-frequency vibrations (Tables 1S and 2S of the Supporting Information). In the case of I, the lowest MP2 harmonic frequencies are 59, 104, 112, 254, 254, and 453 cm‒1 (Table 1S). Further harmonic fundamentals have frequencies above 500 cm‒1. The lowest fundamental vibration (59 cm‒1) is the torsional vibration, whereas the remaining four vibrations below 500 cm‒1 are bending vibrations. Similar results were 13 ACS Paragon Plus Environment


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obtained for II (Table 2S). The Boltzmann populations of vibrationally excited states of these five low-frequency fundamentals are significant at the recording temperature of ‒30º C. This reduces the intensity of the spectrum and at the same time produces very complicated and dense patterns of vibrationally exited-state spectra. The intensity of the transitions increases with the square of the frequency and it was predicted that most assignments had to be made for high values of J in the upper part of the 26 – 120 GHz spectral region due to low intensities of the spectral lines. Centrifugal distortion becomes increasingly important as J and K‒1 increases. While the K‒1 < 3 or 4 transitions are split by asymmetry, the MP2 centrifugal distortion constants (Tables 5 and 6) predict that higher K‒1states are generally split by centrifugal distortion into pairs of lines having the same value of K‒1. These splittings, which also leads to a reduction of intensity, can be comparatively large and, moreover, were predicted to lead to frequent overlapping of spectral lines originating from other excited vibrational states, which is yet another complicating factor. Survey spectra revealed the expected patterns of extremely dense and relatively weak aRpile-ups. Fig. 5 shows a 140 MHz portion of the J = 31 ← 30 pile-up region with its characteristic spectral density. This region is actually about 1 GHz wide and encompasses the ground and many vibrationally excited states of both I and II. Most of the transitions of the band head of this region belong to the ground state of I. The pile-ups cover larger and larger frequency intervals as J increases. For J larger than about 40 a continuous spectrum was observed. The first assignments were obtained using the aR-pile-up transitions. The assignment of the J quantum numbers associated with each pile-up was obvious. The assignments of the K‒1 pairs were much less obvious. Fortunately, the MP2 centrifugal distortion constants were very helpful to obtain correct assignments of them. The fact that these K‒1-pairs are modulated at very low Stark fields was another useful property that was exploited for this purpose. 14 ACS Paragon Plus Environment

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The spectrum of the ground vibrational state of conformer I was first assigned. The strongest lines of this state are located in the band heads of the pile-ups (see Fig. 5). The assignments started with the K‒1 > 4 pairs and were gradually extended to higher and higher values of J and K‒1. The K‒1 = 1 and 0 were finally assigned by means of their Stark effects and spectral positions. The assignments of several K‒1 = 3 transitions were confirmed by RFMWDR experiments. The spectrum consisting of 1011 aR-transitions with J between 13 and 59 and with K‒1 up to 26 is listed in Table 12S in the Supporting Information. The transitions were leastsquares fitted to Watson's S-reduced Hamiltonian45 using Sørensen's program Rotfit52 and the spectroscopic constants shown in Table 5 were obtained. The c-type spectrum was predicted using these constants, but extensive searches for it were futile. MW intensities are proportional to the square of the dipole moment components. The dipole moment components of Table 1 indicate that the aR-transitions are much stronger, at least by a factor of at least 5 – 6 than the c-type transitions. The observed a-type spectrum is comparatively weak and the c-type spectrum would be much weaker and it was therefore not possible to assign its transitions. Most vibrationally excited state spectra of I are located at higher frequencies than their ground-state counterparts. The spectrum of a prominent vibrationally excited state of I was observed. Unfortunately, it was not possible to get detailed, unambiguous assignments of the K‒1 lines due to extensive overlapping by the spectra of other vibrationally excited states. The value of B + C is 2022.2 MHz for this state, which is possibly the first excited state of the phosphino group torsion. The changes in the CCSD rotational constants from I to II were used together with the experimental rotational constants of I (Table 5) to predict the approximate frequencies of aRtransitions belonging to conformer II. These transitions, which appear at higher frequencies than their conformer I counterparts, are overlapped by several vibrationally excited states presumably 15 ACS Paragon Plus Environment


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belonging both to I and II. This made the assignment of the aR-spectrum more difficult than in case of I. A total of 939 transitions with Jmax = 57 and K‒1max = 27 listed in Table 13S were ultimately used to determine the spectroscopic constants shown in Table 6. Searches for c-type lines were futile for the same reasons as those discussed above for conformer I. One vibrationally excited state, which is assumed to be the first excited state of the torsion about the C‒P bond or a low bending vibration was also assigned. The spectrum consisting of 598 transitions are shown in Table 14S, while the spectroscopic constants are displayed in Table 6. The spectroscopic constants in Tables 5 (conformer I) and 6 (II) merit discussion. The A rotational constants are poorly determined, which is due to the fact that only aR-type transitions have been assigned for these highly prolate compounds and these constants are therefore not discussed further. Very accurate values have been found for B and C. The MP2 calculations (Tables 1S and 2S) predict that the experimental effective B and C rotational constants should be approximately 3 MHz smaller than the equilibrium B and C constants. The CCSD/cc-pVQZ B and C rotational constants, which have been calculated from approximate equilibrium structures (Table 1) are smaller by 1 – 2 MHz than the experimental constants, whereas the opposite would presumably have been the case for rotational constants derived from true equilibrium structures. This indicates that the CCSD/cc-pVQZ structures are close to the equilibrium structures, but calculations at even higher methodological levels are needed to obtain the true equilibrium structures. Accurate values could be obtained only for two quartic centrifugal distortion constants, namely DJ and DJK, while the remaining three constants DK, d1 and d2 were preset at the MP2 values in the fitting of the spectra. Attempts to determined d1 and d2 were made, but only very uncertain results were obtained. DK could not be determined at all. The problems encountered for 16 ACS Paragon Plus Environment

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these three centrifugal distortion constants are natural since I and II are practically prolate rotors and only aR-transitions have been available for their eventual determination. Interestingly, the MP2 values of DJ and DJK are very close to their experimental equivalents in Tables 5 and 6. Two sextic centrifugal distortion constants, HJK and HKJ were determined with the remaining sextic constants preset at zero. The MP2 values of these two constants are listed in Tables 5 and 6 together with their experimental counterparts. The agreement between experiment and theory is poor in these cases. The reason could be that the derivation of the sextic constants are too demanding for computations at the MP2 level of theory. Internal Energy Difference. Comparison of intensities of carefully selected transitions was used to derive the internal energy differences between the ground vibrational states of I and II. A variant of Eq (3) of Esbitt and Wilson53 (see also Townes and Schawlow54) was employed to calculate the energy difference from the spectral intensities. According to Esbitt and Wilson,53 the energy difference E" – E' between two conformers is given by

E" – E' = RTlnL

(1)

where E' and E" are the internal energies of the two conformers in their ground vibrational states, R is the universal gas constant and T is the absolute temperature. L is given by

L=

S ' g " v"  " 2 l " v'  " (2 J '1) ( ) S " g ' v'  ' l ' v"  ' (2 J "1)

(2)

where S is the peak signal amplitude of the radiation-unsaturated line, g is the degeneracy other than the rotational degeneracy, which is 2J + 1. v is the frequency of the microwave transition, 



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Page 18 of 40

is the principal-axis dipole moment component, l is the radiation wavelength in the Stark cell, v is the line breadth at half height,

 is the line strength, and J is the principal rotational quantum

number. The radiation wave length (l) and the degeneracy (g) were assumed to be the same for the two transitions whose intensities were compared. The CCSD/cc-pVQZ a-axis dipole moment components () were employed in the calculations of the energy differences, because they have not been determined experimentally. It was especially difficult to measure accurately the peak signal amplitudes (S), and the line breadths (v) because the lines are relatively weak and the spectrum is very crowded resulting in frequent overlaps of spectral lines and Stark lobes. The four transitions pairs used to determine the energy difference are listed in the Supporting Information, Table 14S. All these transitions have relatively rapid Stark effects and are fully modulated at comparatively low Stark fields. They also appear to be well separated from other lines. It is seen from Table 14S that the energy differences derived from the different pairs vary between 2.2 and 1.5 kJ/mol. The average energy difference is 1.9 kJ/mol with II as the lowest-energy form. It is difficult to estimate one standard deviation to this number, but 0.6 kJ/mol seems reasonable considering the many factors that contribute to the uncertainty in this case. The experimental one standard deviation of 0.6 kJ/mol means that the 95% confidence interval is ±1.2 kJ/mol, which suggests that the two rotamers may be close to equal in energy. This is in accord with the theoretical energy differences that were zero within the methodological uncertainties (see above).However, II is found to be slightly lower in energy than I in the most advanced calculations (CCSD/cc-pVQZ). It should be pointed out that the energy difference (0.005kJ/mol) obtained in the CCSD/cc-pVQZ calculations is not the same as the experimental 18 ACS Paragon Plus Environment

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difference (1.9(6) kJ/mol). The theoretical energies refer to the approximate equilibrium structures of I and II, whereas the experimental values refer to the internal energies of the ground vibrational states of the two rotamers.

CONCLUSIONS The phosphino group is situated far away from the cyclopropyl ring in C3H5C≡CPH2. One might therefore suspect that this group would undergo practically free rotation. Quantum chemical calculations and the MW spectrum show that this is not the case. Instead, two rotameric forms separated by a barrier of a few kJ/mol exist. These two conformers have CS symmetry with opposite orientation of the phosphino group. The symmetry plane bisects the cyclopropyl ring as well as the phosphino group. One of the conformers, denoted II, is more stable than the other, called I, by 1.9(6) kJ/mol. A simple theoretical model which takes the interaction of the lone electron pair of the phosphino group, the  electrons of the triple bond, and the Walsh pseudo- electrons of the ring into consideration is capable of explaining qualitatively the observed conformational behavior of the title compound.

ASSOCIATED CONTENT Supporting Information Results of the theoretical calculations, including electronic energies; molecular structures; dipole moments; harmonic and anharmonic vibrational frequencies; rotational and centrifugal distortion constants; rotation-vibration constants; and differences between ground-state and equilibrium rotational constants. Microwave spectra of the ground and vibrationally excited states of two



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conformers. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Tel: +47 2285 5674; Fax: +47 2285 5441; E-mail: [email protected] Notes The authors declare no competing interests.

ACKNOWLEDGEMENTS We thank Anne Horn for her skillful assistance and Celine Levron for recording infrared spectra. This work has been supported by the Research Council of Norway through a Centre of Excellence Grant (Grant No. 179568/V30). It has also received support from the Norwegian Supercomputing Program (NOTUR) through a grant of computer time (Grant No. NN4654K). JC. G. thanks the Centre National d’Etudes Spatiales (CNES) for financial support. The work in Mainz was supported by the Deutsche Forschungsgemeinschaft (DFG GA 370/5-1). JG also thanks the Center of Theoretical and Computational Chemistry (CTCC) at the University of Oslo for the hospitality and financial support during a sabbatical.


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Figure 1. Models of conformers I and II whose MW spectra were assigned. Atom numbering is given on I. The phosphino group has different orientation in these two rotamers. Both conformers have CS symmetry. The plane formed by the H8‒C3‒C9‒C10‒P11 chain of atoms is the symmetry plane, which bisects the cyclopropyl ring and the phosphino group in each conformer.



2.5

-1

2.0

Relative energy / kJ mol

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1.5

1.0

0.5

0.0

-50

0

50

100

150

200

250

300

350

400

Dihedral angle / degree

Figure 2. Potential function of rotation about the C10‒P11 bond. Relative energies are given on the Y-axis and values of the H8‒C3•••P11‒H12 dihedral angle are shown on the X-axis (see text). The dots indicate that the phosphorous atom P11 and the carbon atom C3 are not directly bonded to one another. This curve has minima at 132.00 (conformer I) and at 311.95° (rotamer II). The lone electron pair of the phosphorous atom and the H8‒C3 bond is synperiplanar in I, and antiperiplanar in II. The energy difference is 0.042 kJ/mol with I as the lower-energy conformer. The transition state separating the two conformers occurs at 219.61°, 2.19 kJ/mol above the energy of I.


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Figure 3. Orbital energy diagram for  interactions in monosubstituted acetylenes.



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Figure 4. Valence  orbitals of HC≡CPH2.


Page 25 of 40

J = 31

30

Intensity

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62520

62560

62600

62640

Frequency / MHz

Figure 5. A 140 MHz portion of the MW spectrum taken at a Stark field strength of about 110 V/cm. The J = 31 ← 30 a-type transitions occur in this region. This spectral interval is actually about 1 GHz wide and includes a very large number of transitions of the ground and vibrationally excited states of both I and II. The strong transitions at the band head (left) belong to the ground vibrational state of conformer I. The intensity is given in arbitrary units.



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Page 26 of 40

Table 1. CCSD/cc-pVQZ Structures, Dipole Moments and Energy Difference of Conformers I and II of C3H5C≡CPH2 Conformer:

I

II

Bond distance (pm) C1–C2

149.5

149.5

C1–C3

151.1

151.1

C1–H4

107.9

107.9

C1–H5

107.9

107.9

C2–C3

151.1

151.1

C2–H6

107.9

107.9

C2–H7

107.9

107.9

C3–H8

108.0

108.0

C3–C9

144.0

144.0

C9–C10

120.9

120.9

C10–P11

177.1

177.1

P11–H12

141.3

141.3

P11–H13

141.3

141.3

Angle (deg) C2–C1–H4

118.4

118.4

C2–C1–H5

117.9

117.9

C3–C1–H4

116.9

116.9

(Table 1 continues next page) 26 ACS Paragon Plus Environment

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(Table 1 continued)

C3–C1–H5

116.7

116.7

H4–C1–H5

115.4

115.4

C1–C2–H6

118.4

118.4

C1–C2–H7

117.9

117.9

C3–C2–H6

116.9

116.9

C3–C2–H7

116.7

116.7

H6–C2–H7

115.4

115.4

C1–C3–H8

116.4

116.4

C1–C3–C9

119.1

119.1

C2–C3–H8

116.4

116.4

C2–C3–C9

119.1

119.1

H8–C3–C9

115.2

115.2

C10–P11–H12

97.7

97.8

C10–P11–H13

97.7

97.8

H12–P11–H13

94.1

94.1

C3–C9–C10

178.9

180.3

C9–C10–P11

185.2

175.0

Dihedral angle (deg) H4–C1–C2–H6

0.0

0.0

(Table 1 continues next page) 27 ACS Paragon Plus Environment


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(Table 1 continued)

H4–C1–C2–H7

–146.9

–146.9

H5–C1–C2–H6

146.9

146.9

H5–C1–C2–H7

0.0

0.0

H4–C1–C3–H8

–2.4

–2.5

H4–C1–C3–C9

142.7

142.7

H5–C1–C3–H8

–145.1

–145.1

H5–C1–C3–C9

0.0

0.0

H6–C2–C3–H8

2.4

2.4

H6–C2–C3–C9

–142.7

–142.7

H7–C2–C3–H8

145.1

145.1

H7–C2–C3–C9

0.0

0.0

C1–C3–P11–H12

–13.0

165.7

C1–C3–P11–H13

83.0

–98.2

C2–C3–P11–H12

–83.0

98.2

C2–C3–P11–H13

13.0

–165.9

H8–C3–P11–H12

132.0

–48.1

H8–C3–P11–H13

–132.0

48.1

(Table 1 continues next page)


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(Table 1 continued) Dipole Momentsa (10‒30 C m) a

4.76

4.10

b

0.0b

0.0b

c

2.02

1.79

tot

5.18

4.48

Relative electronic energyb (kJ/mol) 0.005

a

1 debye = 3.33564 × 10‒30 C m.

b

0.0

Relative to conformer II. Total electronic energy of II: ‒

1405168.489 kJ/mol.



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Table 2. MP2/cc-pVTZ CC-Triple Bond Lengths in Substituted Acetylenes rC≡C/pm HC≡CH

121.14

HC≡CNH2

121.27

HC≡CCH3

121.38

HC≡C‒cyclopropyl

121.62

HC≡C‒vinyl

121.73

HC≡CPH2

121.97

HC≡CBH2

122.22

C3H5C≡CPH2 Conformer I

122.49

C3H5C≡CPH2 Conformer II

122.48


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Table 3. Selected MP2/cc-pVTZ Results for Bisubstituted Acetylenes

H2B‒C≡C‒BH2

H2P‒C≡C‒PH2

H2P‒C≡C‒BH2

Type

Orientation

E/kJ mol‒1

AA

Parallel

19.8

123.45

i297.6

Orthogonal

0.0

123.23

153.4

Parallel

2.8

122.86

i85.4

Antiparallel

2.5

122.91

i87.8

Orthogonal

0.0

122.86

87.8

Parallel

0.0

123.08

144.1

Orthogonal

8.8

123.12

i184.8

DD

DA

rC≡C/pm

Lowest Frequency/cm‒1



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Table 4. Selected MP2/cc-pVTZ Results for C3H5‒C≡C‒PH2 Conformer

Orientation

E/kJ mol‒1

rC≡C/pm

Lowest Frequency/cm‒1

I

Orthogonal

0.0

122.49

58.6

II

Orthogonal

0.07

122.48

58.2

TS

Parallel

2.19

122.50

i59.2


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Table 5. Spectroscopic Constantsa of Conformer I of C3H5C≡CPH2 Experiment

Theoryb

A (MHz)

13069(23)

13205.0

B (MHz)

1014.4425(17)

1013.2

C (MHz)

1003.2655(17)

1001.9

DJ (kHz)

0.084213(47)

0.0816

DJK (kHz)

‒2.1604(11)

‒2.11

DK (kHz)

86.9c

86.9

d1 (kHz)

0.00475c

0.00475

d2 (kHz)

0.000025c

0.000025

HJ (Hz)

0.0c

0.000055

HJK (Hz)

‒0.00433(21)

‒0.0328

HKJ (Hz)d

0.1294(18)

1.81

Ne

1011

rmsf

0.954 (Table 5 continues next page)



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(Table 5 continued)

a

S-reduction, Ir-representation.45 Uncertainties represent one standard deviation. The spectrum is

listed in Table 12S of the Supporting Information.

b

CCSD/cc-pVQZ rotational and MP2/cc-

pVTZ centrifugal distortion constants. c Fixed. d Remaining sextic centrifugal distortion constants preset at zero in the least-squares fit. e Number of transitions used in the fit. f Root-mean-square deviation defined as rms2 = [(obs – calc)/u]2/(N – P), where obs and calc are the observed and calculated frequencies, u is the uncertainty of the observed frequency, N is the number of transitions used in the least-squares fit, and P is the number of spectroscopic constants used in the fit.


Page 35 of 40

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Table 6. Spectroscopic Constantsa of Conformer II of C3H5C≡CPH2 Theoryb

Experiment Vibrational state:

Ground

Vibrationally excited

A (MHz)

12661(39)

12110(100)

12714.9

B (MHz)

1022.6670(17)

1021.4242(21)

1020.6

C (MHz)

1014.8423(17)

1013.7293(21)

1012.2

DJ (kHz)

0.106976(45)

0.105989(65)

0.104

DJK (kHz)

‒3.33545(98)

‒3.2268(15)

‒3.33

DK (kHz)

97.4c

97.4c

97.4

d1 (kHz)

0.00749c

0.00749c

0.00749

d2 (kHz)

0.000040c

0.000040c

0.000040

HJ (Hz)

0.0c

0.0c

0.000086

HJK (Hz)

‒0.00516(23)

‒0.00193(34)

‒0.043

HKJ (Hz)d

0.0828(19)

0.0186(37)

2.18

Ne

939

598

rmsf

0.829

0.791

a

S-reduction, Ir-representation.45 Uncertainties represent one standard deviation. The spectra are

listed in Tables 13S and 14S of the Supporting Information. b-f Comments as for Table 5. 35 ACS Paragon Plus Environment


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Table of contents


Synthesis, microwave spectrum, quantum chemical calculations, and conformational composition of the novel compound cyclopropylethylidynephosphine (C₃H₅CH₂C≡P).

Conformational properties of cis- and trans-N-Cyclopropylformamide studied by microwave spectroscopy and quantum chemical calculations.

Design, synthesis, characterization, quantum-chemical calculations and anti-inflammatory activity of novel series of thiophene derivatives.

Ring planarity problem of 2-oxazoline revisited using microwave spectroscopy and quantum chemical calculations.

Molecular conformational stability and Spectroscopic analysis of Parared with experimental techniques and quantum chemical calculations.

Constant chemical potential approach for quantum chemical calculations in electrocatalysis.

Conformational study of L-methionine and L-cysteine derivatives through quantum chemical calculations and 3JHH coupling constant analyses.

Accurate quantum dynamics calculations of vibrational spectrum of dideuteromethane CH2D2.

Quantum chemical calculations on the geometrical, conformational, spectroscopic and nonlinear optical parameters of 5-(2-Chloroethyl)-2,4-dichloro-6-methylpyrimidine.

Spectroscopic analysis of cinnamic acid using quantum chemical calculations.

Machine Learning of Parameters for Accurate Semiempirical Quantum Chemical Calculations.

A hybrid bis(amino-styryl) substituted Bodipy dye and its conjugate diacid: synthesis, structure, spectroscopy and quantum chemical calculations.

Microwave, structural, conformational, vibrational studies and ab initio calculations of isocyanocyclopentane.

Quantum chemical calculations of (31)P NMR chemical shifts: scopes and limitations.

Microwave-assisted synthesis of chromenes: biological and chemical importance.

Porphyrin-cobaloxime complexes for hydrogen production, a photo- and electrochemical study, coupled with quantum chemical calculations.

Microwave spectrum and conformational properties of 4-isocyano-1-butene (H2C═CHCH2CH2N≡C).

In Vivo Anti-Leukemia, Quantum Chemical Calculations and ADMET Investigations of Some Quaternary and Isothiouronium Surfactants.

Regioselective synthesis and ab initio calculations of fused heterocycles thermally and under microwave irradiation.

Molecular structure and spectroscopic characterization of Carbamazepine with experimental techniques and DFT quantum chemical calculations.

Structure of saligenin: microwave, UV and IR spectroscopy studies in a supersonic jet combined with quantum chemistry calculations.

Skin sensitization prediction using quantum chemical calculations: a theoretical model for the S(N)Ar domain.

Elucidating triplet-sensitized photolysis mechanisms of sulfadiazine and metal ions effects by quantum chemical calculations.

Theoretical study of small sodium-potassium alloy clusters through genetic algorithm and quantum chemical calculations.