DOI: 10.1002/asia.201500013
Full Paper
Rotational Spectroscopy
Chloromethane–Water Adduct: Rotational Spectrum, Weak Hydrogen Bonds, and Internal Dynamics Qian Gou,[a] Lorenzo Spada,[a] Juan Carlos Lýpez,[b] Jens-Uwe Grabow,[c] and Walther Caminati*[a] Abstract: The rotational spectra of four isotopologues of the 1:1 complex between chloromethane and water revealed the presence of only one rotamer in a pulsed jet expansion. The two subunits are linked through two weak hydrogen bonds, O¢H···Cl (RH···Cl = 2.638(2) æ) and C¢H···O (RH···O = 2.501(2) æ), forming a five-membered ring. All transitions display the hyperfine structure due to the 35Cl (or 37Cl) nuclear quadrupole effects. Dynamical features in the spectrum are
Introduction Considerable attention has been drawn to freon molecules, due to their ozone-depletion potential and greenhouse effect. It has been pointed out that the complexation of freons with atmospheric water affects their reactivity and it seems to accelerate, for example, the decomposition rate of freons in the atmosphere.[1] Precise information on the shapes, stabilities, structures and internal dynamics of the water adducts has been obtained from rotational investigations in isolation, free from solvent effects or lattice strain. Intermolecular interactions and internal dynamics of water in weakly-bound complexes have been presented in a perspective paper.[2] Generally, freons act simultaneously as weak proton donors and weak proton acceptors, easily forming adducts with water, stabilized by weak hydrogen bonds (WHBs) such as O¢H···X (X = F, Cl, …) and C¢H···O. With freons containing two halogen [a] Dr. Q. Gou, L. Spada, Prof. W. Caminati Dipartimento di Chimica “G. Ciamician” Universit di Bologna Via Selmi 2, 40126 Bologna (Italy) Fax: (+ 39) 0512099456 E-mail: [email protected] [b] Prof. J. C. Lýpez Departamento de Qumica Fsica y Qumica Inorgnica Facultad de Ciencias, Universidad de Valladolid Paseo de Bel¦n, 7, 47011, Valladolid (Spain) [c] Prof. J.-U. Grabow Institut fìr Physikalische Chemie und Elektrochemie Gottfried-Wilhelm-Leibniz-Universtt Callinstr. 3 A, 30167 Hannover (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201500013. Chem. Asian J. 2015, 10, 1198 – 1203
caused by two large-amplitude motions. Each component line appears as an asymmetric doublet with a relative intensity ratio of 1:3. The splittings led to the determination of barrier to internal rotation of water around its symmetry axis, V2 = 320(10) cm¢1. Finally, an unexpected small value of the inertial defect (¢0.96 uæ2 rather than ¢3.22 uæ2) allowed the estimation of the barrier to the internal rotation of the CH3 group, V3 8 cm¢1.
atoms, such as CH2F2,[3] CH2FCl[4] and CH3CHFCl,[5] water forms a net of three WHBs. Besides one O¢H···X WHB, the two C¢H groups act as proton donors, enhanced by the electron withdrawal of the halogen atoms, interacting with the oxygen atom of water. When the freon molecule contains only one hydrogen atom, such as CHF2Cl, a five-membered cyclic structure with two WHBs is formed.[6] It is worth to mention that with CH2FCl and CHF2Cl, water forms a WHB O¢H···Cl rather than an O¢H···F bond,[4] but the contrary was found in the complex CH3CHFCl¢H2O.[5] Often, a doubling of the rotational transitions has been observed due to the internal rotation of water around its symmetry axis. The measured splittings allowed the determination of the corresponding potential energy function. In cases of aliphatic perhalogenated freons, water is linked to the freon molecule through H2O···X halogen bonds.[7] Often such a halogen bond is explained in terms of a “s-hole” formed at one of the halogen atoms.[8] Moreover, it has been observed that when a perhalogenated freon has a p electron system, a lone pair···p interaction (Bìrgi–Dunitz) is the linking interaction.[9] No rotational spectra have been reported, however, for complexes of water with a singly halogenated alkane, not even for the simplest one, CH3F. Similarly, the rotational spectrum has not been reported for CHF3-H2O. Rotational transitions have been measured for both complexes,[10] but the effects of the internal rotation of the symmetric top (CH3 or CF3), make the rotational assignment of the spectra quite complicated. For this reason, we decided to investigate the rotational spectrum of CH3Cl-H2O, since its spectral assignment, unlike CHF3-H2O, will be guided by 35Cl nuclear quadrupole hyperfine structure (hfs). CH3Cl-H2O has been investigated using FTIR spectroscopy in a solid neon matrix.[11] The authors discussed the cyclic struc-
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Full Paper ture constituted by a O¢H···Cl and a C¢H···O WHB, mainly based on their ab initio calculations. Precise experimental information on its structure, energetics, and internal dynamics will be supplied by the rotational spectrum. Below we report the obtained results.
Results and Discussion
We also performed calculations of the two isomers at the CCSD/6-311 + + G(d,p) level which provided predictions for the rotational constants of the observed isomer quite different from the experimental values, compared with the results from calculations at the MP2/6-311 + + G(d,p) level. Therefore we use the MP2/6-311 + + G(d,p) geometry throughout this work. The results obtained at the CCSD/6-311 + + G(d,p) level are available in the Supporting Information.
Theoretical Calculations A MP2/6-311 + + G(2d,2p) ab initio investigation of CH3Cl¢H2O has been reported.[11] The authors presented two isomers but did not report any spectroscopic constant useful for a rotational spectroscopy investigation. For this reason, we performed our own calculations at the MP2/6-311 + + G(d,p) level by using the program Gaussian 03,[12] giving qualitatively similar results. Vibrational frequency calculations at the same level confirmed these two to be minima and provide their zero-point relative energies (DE0). Their shapes are depicted in Table 1, giving the
Table 1. MP2/6-311 + + G(d,p) shapes and spectroscopic parameters of the two most stable forms of CH3Cl-H2O. II
0, 0, 0[a] 5.1 13 277, 3564, 2861 31.51, ¢102.68 ¢0.6, 0.0, 0.0 341, 90
6.9, 4.8, 3.8 2.8 10 7537, 1751, 1746 ¢68.49, 0.88 4.4, 0.0, 0.2
Following the predictions from the computations, we started to search for the ma-type R-band spectrum of isomer I. After several disappointing frequency scans, we observed some transitions exhibiting 35Cl nuclear quadrupole hfs matching the pattern expected for mb-type transitions of isomer I, despite the fact that the theoretical value of mb is almost zero. We assigned and measured 5 mb transitions falling in our frequency range, from which we determined quite precise rotational constants. We were then able to measure 5 much weaker ma-type transitions. Thus, the experimental ratio of the ma and mb values is reciprocal to the ab initio ratio. Each hfs component further splits into an asymmetric doublet at an intensity ratio 1:3 (see, e.g., three hfs components of the rotational transition 211 202, in Figure 1). This kind of splitting is generally due to the inter!
DE, DE0, DEBSSE [kJ mol¢1] ED0(BSSE) [kJ mol¢1] A, B, C [MHz] caa, cbb¢ccc [MHz] ma, mb, mc [D] V2, V3 [cm¢1]
I
Rotational Spectra
[a] Absolute values of the Cs configurations are ¢575.709998 Eh, ¢575.648176 Eh and ¢575.707855 Eh, respectively.
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Figure 1. Part of the recorded 211 202 rotational transition of the parent species of the observed isomer of CH3Cl-H2O, showing a hyperfine structure originating from the 35Cl nucleus and the internal rotation of H2O. Each component line exhibits the instrumental Doppler doubling. !
relative energies, electric dipole moment components, rotational constants, and nuclear quadrupole coupling parameters. Isomer I is linked by one O¢H···Cl and one C¢H···O WHB, where water has a dual role as proton donor and proton acceptor. Isomer II, with a trifurcated C¢H3···O WHB where water acts just a proton acceptor, is higher in energy. In order to obtain a better estimation of the energy differences, the intermolecular binding energy values were counterpoise corrected (DEBSSE) for basis set superposition error (BSSE).[13] We also report the zero-point dissociation energies, inclusive of the BSSE corrections, ED0(BSSE). Isomer I was calculated to be slightly distorted with respect to the Cs configuration. However, it is well known that in similar cases the vibrational ground state wavefunction is symmetric with respect to the “near”-symmetry plane. Herein we imposed water to be in the plane of symmetry. This is consistent, as shown below, with the experimental evidence. The configuration with one O¢H···Cl and two C¢H···O WHBs turned out to be the transition state in relation to the internal rotation of the CH3 top, giving the V3 barrier to be 90 cm¢1.
nal rotation of water around its C2v axis, which inverts the positions of a pair of fermions with spin I = 1=2 while giving rise to a symmetric and an anti-symmetric tunnelling state. Following spin statistical weights arguments, the weaker component belongs to the ground state. The transition frequencies were fitted, with Pickett’s SPFIT program,[14] using Watson’s “S” reduced semirigid-rotor Hamiltonian[15] in the Ir representation, given here in a simple form: H ¼ HR ð0Þ þ HR ð1Þ þ HCD þ HQ
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Full Paper HR(0) and HR(1) represent the rigid-rotor Hamiltonian for the v = 0 and v = 1 tunnelling states, the centrifugal distortion contributions are represented by HCD, while HQ is the operator associated with the interaction of the 35Cl (or 37Cl) nuclear electric quadrupole moment with the electric field gradient at the Cl nucleus. Centrifugal distortion and quadrupole coupling constants have been set to the same values for both states. The obtained spectroscopic constants are reported in the two left columns of Table 2.
Table 3. Spectroscopic parameters of CH335Cl-H218O and CH335Cl-D2O. CH335Cl-H218O v=0 A[MHz] B[MHz] C[MHz] DJ[kHz] DJK[kHz] d1[kHz] caa(Cl)[MHz] cbb-ccc(Cl)[MHz] caa(D)[b][MHz] cbb-ccc(D)[MHz] caa(D’)[b][MHz] cbb-ccc(D’)[MHz] Dc[uæ2] N[c] s[d][kHz]
Table 2. Spectroscopic parameters of CH335Cl-H2O and CH337Cl-H2O. CH335Cl-H2O v=0 A[MHz] B[MHz] C[MHz] DJ[kHz] DJK[kHz] d1[kHz] caa[MHz] cbb-ccc[MHz] Dc[uæ2] N[c] s[d][kHz]
CH337Cl-H2O v=0
v=1
13 224.920(1)[a] 13 225.179(1) 3555.040(1) 3554.612(1) 2816.8740(2) 2817.0105(6) 14.08(4) 130.3(1) ¢4.32(3) 33.272(3) ¢107.896(4) ¢0.96 75 3.6
v=1
13 044.259(1) 13 044.522(1) 3514.3838(9) 3513.9686(9) 2783.0725(6) 2783.2054(7) [14.08][b] [130.3] [¢4.32] 26.057(5) ¢84.872(8) ¢0.96 40 3.9
After empirical scaling to the rotational constants of the parent species, the rotational spectrum of the CH337Cl-H2O isotopologue was calculated and easily assigned in natural abundance. The intensity of transitions was found to be 1/3 of the parent species. Because of their lower intensity, a smaller number of transitions were recorded for this isotopologue, such that we needed to fix the centrifugal distortion constants at the corresponding values of the parent species. The splittings due to the internal rotation of water are similar to those of the parent species. The obtained spectroscopic parameters are shown in the two right columns of Table 2. Later on, the rotational spectra of the H218O and D2O isotopologues have been assigned and measured. The torsional splittings of the H218O species were slightly smaller than those of the parent species, while those of the D2O species were not resolved, due to the larger reduced mass of the motion. For this last isotopologue, we could resolve the component lines due to the quadrupole coupling effects of the two nuclei of deuterium (I = 1) and the quadrupole coupling constants of the two D nuclei were also determined. The spectroscopic parameters of these isotopologues are given in Table 3. All measured transition frequencies of the four isotopologues are given in the Supporting Information. No mc-type transitions have been observed in accordance with the Cs symmetry of the isomer. We reported in Table 2 and 3 also the values of the inertial defects, Dc, for the four isotopologues. They are all much smaller than the values within the rigid approximation (Dc = ¢3.22 uæ2). Such a discrepancy www.chemasianj.org
13 224.011(1) 3299.649(1) 2654.5346(7)
CH335Cl-D2O 13 003.203(4) 3295.320(7) 2643.3373(8) 80.3(3) 97(1) -3.20(2) 33.567(5) -108.320(8) 0.051(5) 0.25(1) 0.140(7) 0.10(1) ¢1.03 40 3.9
[a] Uncertainties (in parentheses) are expressed in units of the last digit. [b] D indicates the deuterium atom involved in the WHB, while D’ is used to label the free one. [c] Number of transitions in the fit. [d] Standard deviation of the fit.
[a] Uncertainties (in parentheses) are expressed in units of the last digit. [b] Values in brackets are fixed at the corresponding ones of parent species. [c] Number of transitions in the fit. [d] Standard deviation of the fit.
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13 223.756(1)[a] 3299.984(1) 2654.3985(7) 12.24(5) 114.8(2) ¢3.37(3) 33.273(3) -107.904(4) – – – – ¢0.98 75 3.8
v=1
can be interpreted in terms of a low barrier to internal rotation of the CH3 group, as discussed in the following section on the internal dynamics. On the other hand, the fact that all isotopologues have the same value of the inertial defect indicates that all substituted atoms lie in the plane of symmetry of the complex, which has then a Cs symmetry. We could not observe any lines belonging to the E state of the internal rotation of the CH3 top. This is related to the very small V3 barrier, which locates the E state transitions out of our frequency range. The search for the second isomer was unsuccessful. This could be due to the conformational relaxation to the most stable isomer upon supersonic expansion. It has, indeed, been shown that this kind of relaxation takes place easily when the interconversion barrier is smaller than 2kT.[16]
Structural Information The observed isomer of CH3Cl-H2O, is shown in Figure 2, where also the atom numbering and principal axes of the complex are drawn. It is stabilized by an O¢H···Cl and a C¢H···O WHB.
Figure 2. The observed isomer of CH3Cl-H2O with indicated atom numbering and the positions of the principal axes. X4 is a dummy “atom” lying on the symmetry axis of water.
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Full Paper The full ab initio equilibrium structure is available in the Supporting Information.
Table 4. The experimental coordinates of the Cl and O atoms in CH3ClH2O.
rs r0[a] rs r0[a]
a [æ] b [æ]
Cl
O
0.920(2) 0.926 0.526(3) 0.527
2.375(1) -2.377 0.04(3) 0.006
Internal Dynamics 1. The internal rotation of CH3 The almost free internal rotation of CH3 changes the “rigid” rotational constants to the effective ground state (v = 0, s = 0) values, according to:[19]
[a] Calculated with the r0 structure in Table 5. The coordinates c are set to be zero by symmetry.
A00 ¼ Ar þ W 00 ð2Þ F1a 2 From the rotational constants of the two monosubstituted isotopologues, it is possible to calculate the substitution coordinates, rs,[17] of the Cl and O atoms in the principal axes of the parent species. The obtained values are shown in Table 4 and are compared with the values of a partial r0 structure. The low-barrier to internal rotation of the CH3 top modifies the rotational constants of the A-state with respect to the “rigid” value and, consequently, the inertial defects of all isotopologues. The partial r0 structure was then obtained to reproduce the corrected rotational constants with the same V3 barrier deduced in the next section. By adjusting three structural parameters (RCl2O3, aC1Cl2O3 and aCl2O3X4), while keeping the remaining parameters fixed to their ab initio values, we could reproduce the experimental rotational constants of the CH335Cl-H2O, CH337Cl-H2O, and CH335Cl-H218O isotopologues within discrepancies of ~ 2 MHz. Such an adjustment was more problematic when including in the fit also the rotational constants of CH335Cl-D2O. This is not surprising because the H!D substitution of the hydrogen atoms involved in hydrogen bonds causes a shortening of the hydrogen bond distances, according to the reverse Ubbelohde effect.[18] The obtained parameters are reported in Table 5 and there compared to the ab initio values (re). From this partial r0 structure, the WHB parameters have been derived, and are also reported in Table 5. The “bond lengths” of the two WHBs (C¢H···O and Cl···H¢O) are quite similar to each other, RH7O3 = 2.501(2) and RH5Cl = 2.638(2) æ, respectively.
Table 5. Partial r0 and re structures of CH3Cl-H2O. Fitted parameters r0 re
RCl2O3 [æ] 3.344(2)[a] 3.321
aC1Cl2O3 [8] 74.2(1) 73.1
aCl2O3X4[b] [8] 89(2) 90.3
aC1Cl2H5 [8] 87(2) 86.1 aC1H7O3 [8] 132.5(1) 133.2
aCl2H5O3 [8] 131(2) 128.9
Derived parameters r0 re r0 re
RCl2H5 [æ] 2.638(2) 2.632 RH7O3 [æ] 2.501(2) 2.439
[a] Uncertainties (in parentheses) are expressed in units of the last digit. [b] X4 is a dummy “atom” lying on the symmetry axis of water (see Figure 2). Chem. Asian J. 2015, 10, 1198 – 1203
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ð2Þ
B00 ¼ Br þ W 00 ð2Þ F1b 2 C 00 ¼ C r þ W 00
ð2Þ
F1c
2
where Ar, Br and Cr are the “rigid” rotational constants in the limit of the infinite barrier. The W00(2) are the Herschbach’s barrier-dependent perturbation sums relative to the sublevels of the A-symmetry (s = 0) species of the torsional ground state (v = 0). In Equation (2), 1g = lgIa/Ig (g = a, b, c), where lg are the direction cosines between the internal rotation axis a and the principal axis g. F = h /[2·(1¢SglgIa/Ig)Ia] is the reduced rotational constant of CH3. Therefore, the increases of the effective rotational constants depend on the V3 barrier and on the angles between the principal axes and the internal rotation axis of a particular rotor, as discussed in several cases, like CF3Cl-CH3F[20] and CF3Cl-CH2O.[21] In Figure 2, the c-axis of the complex is perpendicular to the internal rotation axis of CH3Cl. Then lc, 1c and F1c2 are zero, and the rotational constant C is not affected by the internal rotations. Instead, the b-axis is almost parallel to the internal rotation axis of CH3Cl which alters the effective rotational constant B. Assuming the decrease (absolute value) of Dc, from the “rigid” value (¢3.22 uæ2) to the experimental value (¢0.96 uæ2) as due only to the internal rotation of CH3, we determined W00(2) 0.7 for all four isotopologues. The corresponding barrier is V3 8 cm¢1, smaller than the ab initio value (90 cm¢1). 2. The internal rotation of H2O The differences of the planar moments of inertia DMaa, DMbb and DMcc between the v = 0 and v = 1 states as shown, for example, in the case of CH2F2···H2O,[5] allow us to picture the potential pathway for the tunnelling motion of H2O by using Meyer’s one-dimensional flexible model.[22] Assuming that the effects of the internal rotation of CH3 top are the same for both v = 0 and v = 1 states, we can apply the following potential energy function, suitable for two equivalent periodic minima, VðtÞ ¼ 1=2 V 2 ½1 - cosð2tÞ¤
ð3Þ
where V2 and t are the potential energy barrier and the dihedral angle, t = aH5O3X4Cl2. t = 0 corresponds to the equilibrium value with H2O lying in the Cs plane of symmetry. The structural relaxations of the rO3···Cl2 distance and of the aO3Cl2C1 and aX4O3Cl2 angles (abbreviated below as a and b) as functions of t were also required for the model, according to the following relations: 1201
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Full Paper rðtÞ=æ ¼ 3:132 þ 1=2 ¡ Dr ½1¢cosð2tÞ¤ aðtÞ= ¼ 73:6 þ 1=2 ¡ Da ½1¢cosð2tÞ¤
Table 7. Binding energies of the investigated water adducts of freons stabilized by WHB linkages.
ð4Þ
bðtÞ= ¼ 85:0 þ ¡1=2 ¡ Db ½1¢cosð2tÞ Due to the lack of experimental data, Dr and Da have been fixed to the theoretical values (0.147 æ and ¢2.98, respectively). Eight vibrational states were calculated from the model potential optimization on a grid of 67 mesh points in the range 0 t 2p.[20] The flexible model results are reported in Table 6. The experimental value of the barrier to the internal rotation of H2O, V2 320(10) cm¢1, is in accordance with the ab initio predicted value (341 cm¢1).
ED [kJ mol¢1]
Ref.
CH2FCl···H2O
8.5
[6]
CH2F2···H2O
7.5
[5]
CH3CHFCl···H2O
5.4
[7]
CHF2Cl···H2O
5.5
[8]
CH3Cl···H2O
6.6
This work
Table 6. Flexible model results and potential energy parameters for the internal rotation of H2O in CH3Cl-H2O. Exptl.
Calc.
DMaa [uæ2] 0.004 ¢0.013 DMbb [uæ2] DMcc [uæ2] 0.012 – DE0 + 0¢ [GHz] Determined parameter: Db = ¢9(3)8, V2 = 320(10) cm¢1[a]
0.005 ¢0.013 0.012 2.2
Conclusions
[a] Uncertainties (in parentheses) are given in units of the last digit.
Dissociation energy The intermolecular stretching motion which leads to the dissociation appears to be almost parallel to the a-axis of the complex. By assuming that such a motion is separated from the other molecular vibrations, it is possible, within a pseudo diatomic approximation, to estimate the stretching force constant through the following equation:[23]
ks ¼ 16p4 ðmRCM Þ2 ½4B4 þ 4C 4 ¢ðB¢CÞ2 ðB þ CÞ2 ¤=ðhDJ Þ
ð5Þ
where m is the pseudo-diatomic reduced mass and RCM (3.257 æ) is the distance between the centers of mass of the two subunits. B, C and DJ are the spectroscopic parameters reported in Table 2. If it is then assumed that the intermolecular separation for this kind of complex can be described by a Lennard–Jones type potential approximation, the dissociation energy is evaluated through:[24] E D ¼ 1=72 ks RCM 2
ð6Þ
leading to ED = 6.6 kJ mol¢1, very close to the ab initio value (ED0(BSSE) = 5.1 kJ mol¢1). In Table 7, we compare the dissociation energy of CH3Cl-H2O to those of water adducts of freon molecules, obtained by using the same approximation. All of them have quite similar dissociation energies. Chem. Asian J. 2015, 10, 1198 – 1203
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The Fourier transform microwave investigation of CH3Cl-H2O pointed out that the observed isomer has a cyclic structure with one O¢H···Cl and one C¢H···O WHB resulting in the dissociation energy of 6.6 kJ mol¢1. The rotational spectrum displays the features of the quadrupole coupling effect of the 35Cl (or 37 Cl) nucleus, of the near-free internal rotation of CH3 top and of the internal rotation of water around its C2v axis. We interpreted the effects of the two large amplitude motions of the two constituent subunits. The V3 barrier to the internal rotation of CH3 has been estimated from the unexpected low values of the inertial defects, while the differences of the planar moments of inertia between the v = 0 and v = 1 states allowed an evaluation of the potential hypersurface pathway for the tunnelling motion of H2O. This study supplies the first rotational spectroscopic data on the structure and internal dynamics of an adduct of a monohalomethane with water.
Experimental Section Molecular clusters were generated in a supersonic expansion, under conditions optimized for the formation of CH3Cl-H2O. Details of the Fourier transform microwave spectrometer[25] (COBRAtype[26]), which covers the range 6.5–18 GHz, have been described previously.[27] A gas mixture of ca. 2 % of CH3Cl (commercial samples used without any further purification) in helium at a stagnation pressure of ~ 0.5 MPa was expanded over water (or H218O or D2O) through a solenoid valve (General Valve, Series 9, nozzle diameter 0.5 mm) into the Fabry–P¦rot cavity. The spectral line positions were determined after Fourier transformation of the time-domain signal with 8k data points, recorded with 100 ns sample intervals. Each rotational transition appears as a doublet due to the Doppler effect. The line position is calculated as the arithmetic mean of the frequencies of the Doppler components. The estimated accuracy of the frequency
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Full Paper measurements is better than 3 kHz. Lines separated by more than 7 kHz are resolvable.
Acknowledgements We acknowledge Italian MIUR (PRIN project 2010ERFKXL_001) and the University of Bologna (RFO) for financial support. Q. G. thanks the China Scholarships Council (CSC) and J.-U.G. also the Deutsche Forschungsgemeinschafft (DFG) and the Land Niedersachsen for funding. Keywords: atmospheric chemistry · hyperfine structure · internal dynamics · molecular complex · rotational spectroscopy · weak hydrogen bond [1] V. Vaida, H. G. Kjaergaard, K. J. Feierabend, Int. Rev. Phys. Chem. 2003, 22, 203 – 219. [2] L. Evangelisti, W. Caminati, Phys. Chem. Chem. Phys. 2010, 12, 14433 – 14441. [3] a) W. Caminati, S. Melandri, I. Rossi, P. G. Favero, J. Am. Chem. Soc. 1999, 121, 10098 – 10101; b) W. Caminati, S. Melandri, M. Schnell, D. Banser, J. Grabow, J. L. Alonso, J. Mol. Struct. 2005, 742, 87 – 90. [4] W. Caminati, S. Melandri, A. Maris, P. Ottaviani, Angew. Chem. Int. Ed. 2006, 45, 2438 – 2442; Angew. Chem. 2006, 118, 2498 – 2502. [5] G. Feng, L. Evagelisti, L. B. Favero, J.-U. Grabow, Z. Xia, W. Caminati, Phys. Chem. Chem. Phys. 2011, 13, 14092 – 14096. [6] B. J. Bills, L. F. Elmuti, A. J. Sanders, A. L. Steber, R. A. Peebles, S. A. Peebles, P. Groner, J. L. Neill, M. T. Muckle, B. H. Pate, J. Mol. Spectrosc. 2011, 268, 7 – 15. [7] a) W. Caminati, A. Maris, A. Dell’Erba, P. G. Favero, Angew. Chem. Int. Ed. 2006, 45, 6711 – 6714; Angew. Chem. 2006, 118, 6863 – 6866; b) L. Evangelisti, G. Feng, P. Êcija, E. J. Cocinero, F. CastaÇo, W. Caminati, Angew. Chem. Int. Ed. 2011, 50, 7807 – 7810; Angew. Chem. 2011, 123, 7953 – 7956. [8] T. Clark, M. Henneman, J. S. Murray, P. Politzer, J. Mol. Model. 2007, 13, 305 – 311. [9] Q. Gou, G. Feng, L. Evangelisti, W. Caminati, Angew. Chem. Int. Ed. 2013, 52, 11888 – 11891; Angew. Chem. 2013, 125, 12104 – 12107. [10] W. Caminati, unpublished work. [11] N. Dozova, L. Krim, M. E. Alikhani, N. Lacome, J. Phys. Chem. A 2005, 109, 10273 – 10279. [12] Gaussian 03, Revision B.01, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone,
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[19] [20] [21]
[22] [23] [24] [25] [26] [27]
B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez, J. A. Pople, Gaussian, Inc., Pittsburgh PA, 2003. S. F. Boys, F. Bernardi, Mol. Phys. 1970, 19, 553 – 566. M. H. Pickett, J. Mol. Spectrosc. 1991, 148, 371 – 377. J. K. G. Watson, in Vibrational Spectra and structure, Vol. 6 (Ed: J. R. Durig), Elsevier, New York/Amsterdam, 1977, pp. 1 – 89. See for example: R. S. Ruoff, T. D. Klots, T. Emilson, H. S. Gutowski, J. Chem. Phys. 1990, 93, 3142 – 3150. J. Kraitchman, Am. J. Phys. 1953, 21, 17 – 25. a) A. R. Ubbelohde, K. J. Gallagher, Acta Crystallogr. 1955, 8, 71 – 83; See also, for example, b) Q. Gou, G. Feng, L. Evangelisti, D. Loru, J. L. Alonso, J. C. Lûpez, W. Caminati, J. Phys. Chem. A 2013, 117, 13531 – 13534; c) G. Feng, Q. Gou, L. Evangelisti, W. Caminati, Angew. Chem. Int. Ed. 2014, 53, 530 – 534; Angew. Chem. 2014, 126, 540 – 544. D. R. Herschbach, J. Chem. Phys. 1959, 31, 91 – 108. Q. Gou, L. Spada, E. J. Cocinero, W. Caminati, J. Phys. Chem. Lett. 2014, 5, 1591 – 1595. Q. Gou, G. Feng, L. Evangelisti, M. Vallejo Lûpez, L. Spada, A. Lesarri, E. J. Cocinero, W. Caminati, Chem. Eur. J. 2015, in press, DOI: 10.1002/ chem.201406122. a) R. Meyer, J. Mol. Spectrosc. 1979, 76, 266 – 300; b) R. Meyer, W. Caminati, J. Mol. Spectrosc. 1991, 150, 229 – 237. a) D. J. Millen, Can. J. Chem. 1985, 63, 1477 – 1479; b) W. G. Read, E. J. Campbell, G. Henderson, J. Chem. Phys. 1983, 78, 3501 – 3508. S. E. Novick, S. J. Harris, K. C. Janda, W. Klemperer, Can. J. Phys. 1975, 53, 2007 – 2015. T. J. Balle, W. H. Flygare, Rev. Sci. Instrum. 1981, 52, 33 – 45. J.-U. Grabow, W. Stahl, H. Dreizler, Rev. Sci. Instrum. 1996, 67, 4072 – 4084. W. Caminati, A. Millemaggi, J. L. Alonso, A. Lesarri, J. C. Lopez, S. Mata, Chem. Phys. Lett. 2004, 392, 1 – 6.
Received: January 5, 2015 Revised: February 2, 2015 Published online on February 20, 2015
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Full Paper
Rotational Spectroscopy
Chloromethane–Water Adduct: Rotational Spectrum, Weak Hydrogen Bonds, and Internal Dynamics Qian Gou,[a] Lorenzo Spada,[a] Juan Carlos Lýpez,[b] Jens-Uwe Grabow,[c] and Walther Caminati*[a] Abstract: The rotational spectra of four isotopologues of the 1:1 complex between chloromethane and water revealed the presence of only one rotamer in a pulsed jet expansion. The two subunits are linked through two weak hydrogen bonds, O¢H···Cl (RH···Cl = 2.638(2) æ) and C¢H···O (RH···O = 2.501(2) æ), forming a five-membered ring. All transitions display the hyperfine structure due to the 35Cl (or 37Cl) nuclear quadrupole effects. Dynamical features in the spectrum are
Introduction Considerable attention has been drawn to freon molecules, due to their ozone-depletion potential and greenhouse effect. It has been pointed out that the complexation of freons with atmospheric water affects their reactivity and it seems to accelerate, for example, the decomposition rate of freons in the atmosphere.[1] Precise information on the shapes, stabilities, structures and internal dynamics of the water adducts has been obtained from rotational investigations in isolation, free from solvent effects or lattice strain. Intermolecular interactions and internal dynamics of water in weakly-bound complexes have been presented in a perspective paper.[2] Generally, freons act simultaneously as weak proton donors and weak proton acceptors, easily forming adducts with water, stabilized by weak hydrogen bonds (WHBs) such as O¢H···X (X = F, Cl, …) and C¢H···O. With freons containing two halogen [a] Dr. Q. Gou, L. Spada, Prof. W. Caminati Dipartimento di Chimica “G. Ciamician” Universit di Bologna Via Selmi 2, 40126 Bologna (Italy) Fax: (+ 39) 0512099456 E-mail: [email protected] [b] Prof. J. C. Lýpez Departamento de Qumica Fsica y Qumica Inorgnica Facultad de Ciencias, Universidad de Valladolid Paseo de Bel¦n, 7, 47011, Valladolid (Spain) [c] Prof. J.-U. Grabow Institut fìr Physikalische Chemie und Elektrochemie Gottfried-Wilhelm-Leibniz-Universtt Callinstr. 3 A, 30167 Hannover (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201500013. Chem. Asian J. 2015, 10, 1198 – 1203
caused by two large-amplitude motions. Each component line appears as an asymmetric doublet with a relative intensity ratio of 1:3. The splittings led to the determination of barrier to internal rotation of water around its symmetry axis, V2 = 320(10) cm¢1. Finally, an unexpected small value of the inertial defect (¢0.96 uæ2 rather than ¢3.22 uæ2) allowed the estimation of the barrier to the internal rotation of the CH3 group, V3 8 cm¢1.
atoms, such as CH2F2,[3] CH2FCl[4] and CH3CHFCl,[5] water forms a net of three WHBs. Besides one O¢H···X WHB, the two C¢H groups act as proton donors, enhanced by the electron withdrawal of the halogen atoms, interacting with the oxygen atom of water. When the freon molecule contains only one hydrogen atom, such as CHF2Cl, a five-membered cyclic structure with two WHBs is formed.[6] It is worth to mention that with CH2FCl and CHF2Cl, water forms a WHB O¢H···Cl rather than an O¢H···F bond,[4] but the contrary was found in the complex CH3CHFCl¢H2O.[5] Often, a doubling of the rotational transitions has been observed due to the internal rotation of water around its symmetry axis. The measured splittings allowed the determination of the corresponding potential energy function. In cases of aliphatic perhalogenated freons, water is linked to the freon molecule through H2O···X halogen bonds.[7] Often such a halogen bond is explained in terms of a “s-hole” formed at one of the halogen atoms.[8] Moreover, it has been observed that when a perhalogenated freon has a p electron system, a lone pair···p interaction (Bìrgi–Dunitz) is the linking interaction.[9] No rotational spectra have been reported, however, for complexes of water with a singly halogenated alkane, not even for the simplest one, CH3F. Similarly, the rotational spectrum has not been reported for CHF3-H2O. Rotational transitions have been measured for both complexes,[10] but the effects of the internal rotation of the symmetric top (CH3 or CF3), make the rotational assignment of the spectra quite complicated. For this reason, we decided to investigate the rotational spectrum of CH3Cl-H2O, since its spectral assignment, unlike CHF3-H2O, will be guided by 35Cl nuclear quadrupole hyperfine structure (hfs). CH3Cl-H2O has been investigated using FTIR spectroscopy in a solid neon matrix.[11] The authors discussed the cyclic struc-
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Full Paper ture constituted by a O¢H···Cl and a C¢H···O WHB, mainly based on their ab initio calculations. Precise experimental information on its structure, energetics, and internal dynamics will be supplied by the rotational spectrum. Below we report the obtained results.
Results and Discussion
We also performed calculations of the two isomers at the CCSD/6-311 + + G(d,p) level which provided predictions for the rotational constants of the observed isomer quite different from the experimental values, compared with the results from calculations at the MP2/6-311 + + G(d,p) level. Therefore we use the MP2/6-311 + + G(d,p) geometry throughout this work. The results obtained at the CCSD/6-311 + + G(d,p) level are available in the Supporting Information.
Theoretical Calculations A MP2/6-311 + + G(2d,2p) ab initio investigation of CH3Cl¢H2O has been reported.[11] The authors presented two isomers but did not report any spectroscopic constant useful for a rotational spectroscopy investigation. For this reason, we performed our own calculations at the MP2/6-311 + + G(d,p) level by using the program Gaussian 03,[12] giving qualitatively similar results. Vibrational frequency calculations at the same level confirmed these two to be minima and provide their zero-point relative energies (DE0). Their shapes are depicted in Table 1, giving the
Table 1. MP2/6-311 + + G(d,p) shapes and spectroscopic parameters of the two most stable forms of CH3Cl-H2O. II
0, 0, 0[a] 5.1 13 277, 3564, 2861 31.51, ¢102.68 ¢0.6, 0.0, 0.0 341, 90
6.9, 4.8, 3.8 2.8 10 7537, 1751, 1746 ¢68.49, 0.88 4.4, 0.0, 0.2
Following the predictions from the computations, we started to search for the ma-type R-band spectrum of isomer I. After several disappointing frequency scans, we observed some transitions exhibiting 35Cl nuclear quadrupole hfs matching the pattern expected for mb-type transitions of isomer I, despite the fact that the theoretical value of mb is almost zero. We assigned and measured 5 mb transitions falling in our frequency range, from which we determined quite precise rotational constants. We were then able to measure 5 much weaker ma-type transitions. Thus, the experimental ratio of the ma and mb values is reciprocal to the ab initio ratio. Each hfs component further splits into an asymmetric doublet at an intensity ratio 1:3 (see, e.g., three hfs components of the rotational transition 211 202, in Figure 1). This kind of splitting is generally due to the inter!
DE, DE0, DEBSSE [kJ mol¢1] ED0(BSSE) [kJ mol¢1] A, B, C [MHz] caa, cbb¢ccc [MHz] ma, mb, mc [D] V2, V3 [cm¢1]
I
Rotational Spectra
[a] Absolute values of the Cs configurations are ¢575.709998 Eh, ¢575.648176 Eh and ¢575.707855 Eh, respectively.
Chem. Asian J. 2015, 10, 1198 – 1203
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Figure 1. Part of the recorded 211 202 rotational transition of the parent species of the observed isomer of CH3Cl-H2O, showing a hyperfine structure originating from the 35Cl nucleus and the internal rotation of H2O. Each component line exhibits the instrumental Doppler doubling. !
relative energies, electric dipole moment components, rotational constants, and nuclear quadrupole coupling parameters. Isomer I is linked by one O¢H···Cl and one C¢H···O WHB, where water has a dual role as proton donor and proton acceptor. Isomer II, with a trifurcated C¢H3···O WHB where water acts just a proton acceptor, is higher in energy. In order to obtain a better estimation of the energy differences, the intermolecular binding energy values were counterpoise corrected (DEBSSE) for basis set superposition error (BSSE).[13] We also report the zero-point dissociation energies, inclusive of the BSSE corrections, ED0(BSSE). Isomer I was calculated to be slightly distorted with respect to the Cs configuration. However, it is well known that in similar cases the vibrational ground state wavefunction is symmetric with respect to the “near”-symmetry plane. Herein we imposed water to be in the plane of symmetry. This is consistent, as shown below, with the experimental evidence. The configuration with one O¢H···Cl and two C¢H···O WHBs turned out to be the transition state in relation to the internal rotation of the CH3 top, giving the V3 barrier to be 90 cm¢1.
nal rotation of water around its C2v axis, which inverts the positions of a pair of fermions with spin I = 1=2 while giving rise to a symmetric and an anti-symmetric tunnelling state. Following spin statistical weights arguments, the weaker component belongs to the ground state. The transition frequencies were fitted, with Pickett’s SPFIT program,[14] using Watson’s “S” reduced semirigid-rotor Hamiltonian[15] in the Ir representation, given here in a simple form: H ¼ HR ð0Þ þ HR ð1Þ þ HCD þ HQ
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Full Paper HR(0) and HR(1) represent the rigid-rotor Hamiltonian for the v = 0 and v = 1 tunnelling states, the centrifugal distortion contributions are represented by HCD, while HQ is the operator associated with the interaction of the 35Cl (or 37Cl) nuclear electric quadrupole moment with the electric field gradient at the Cl nucleus. Centrifugal distortion and quadrupole coupling constants have been set to the same values for both states. The obtained spectroscopic constants are reported in the two left columns of Table 2.
Table 3. Spectroscopic parameters of CH335Cl-H218O and CH335Cl-D2O. CH335Cl-H218O v=0 A[MHz] B[MHz] C[MHz] DJ[kHz] DJK[kHz] d1[kHz] caa(Cl)[MHz] cbb-ccc(Cl)[MHz] caa(D)[b][MHz] cbb-ccc(D)[MHz] caa(D’)[b][MHz] cbb-ccc(D’)[MHz] Dc[uæ2] N[c] s[d][kHz]
Table 2. Spectroscopic parameters of CH335Cl-H2O and CH337Cl-H2O. CH335Cl-H2O v=0 A[MHz] B[MHz] C[MHz] DJ[kHz] DJK[kHz] d1[kHz] caa[MHz] cbb-ccc[MHz] Dc[uæ2] N[c] s[d][kHz]
CH337Cl-H2O v=0
v=1
13 224.920(1)[a] 13 225.179(1) 3555.040(1) 3554.612(1) 2816.8740(2) 2817.0105(6) 14.08(4) 130.3(1) ¢4.32(3) 33.272(3) ¢107.896(4) ¢0.96 75 3.6
v=1
13 044.259(1) 13 044.522(1) 3514.3838(9) 3513.9686(9) 2783.0725(6) 2783.2054(7) [14.08][b] [130.3] [¢4.32] 26.057(5) ¢84.872(8) ¢0.96 40 3.9
After empirical scaling to the rotational constants of the parent species, the rotational spectrum of the CH337Cl-H2O isotopologue was calculated and easily assigned in natural abundance. The intensity of transitions was found to be 1/3 of the parent species. Because of their lower intensity, a smaller number of transitions were recorded for this isotopologue, such that we needed to fix the centrifugal distortion constants at the corresponding values of the parent species. The splittings due to the internal rotation of water are similar to those of the parent species. The obtained spectroscopic parameters are shown in the two right columns of Table 2. Later on, the rotational spectra of the H218O and D2O isotopologues have been assigned and measured. The torsional splittings of the H218O species were slightly smaller than those of the parent species, while those of the D2O species were not resolved, due to the larger reduced mass of the motion. For this last isotopologue, we could resolve the component lines due to the quadrupole coupling effects of the two nuclei of deuterium (I = 1) and the quadrupole coupling constants of the two D nuclei were also determined. The spectroscopic parameters of these isotopologues are given in Table 3. All measured transition frequencies of the four isotopologues are given in the Supporting Information. No mc-type transitions have been observed in accordance with the Cs symmetry of the isomer. We reported in Table 2 and 3 also the values of the inertial defects, Dc, for the four isotopologues. They are all much smaller than the values within the rigid approximation (Dc = ¢3.22 uæ2). Such a discrepancy www.chemasianj.org
13 224.011(1) 3299.649(1) 2654.5346(7)
CH335Cl-D2O 13 003.203(4) 3295.320(7) 2643.3373(8) 80.3(3) 97(1) -3.20(2) 33.567(5) -108.320(8) 0.051(5) 0.25(1) 0.140(7) 0.10(1) ¢1.03 40 3.9
[a] Uncertainties (in parentheses) are expressed in units of the last digit. [b] D indicates the deuterium atom involved in the WHB, while D’ is used to label the free one. [c] Number of transitions in the fit. [d] Standard deviation of the fit.
[a] Uncertainties (in parentheses) are expressed in units of the last digit. [b] Values in brackets are fixed at the corresponding ones of parent species. [c] Number of transitions in the fit. [d] Standard deviation of the fit.
Chem. Asian J. 2015, 10, 1198 – 1203
13 223.756(1)[a] 3299.984(1) 2654.3985(7) 12.24(5) 114.8(2) ¢3.37(3) 33.273(3) -107.904(4) – – – – ¢0.98 75 3.8
v=1
can be interpreted in terms of a low barrier to internal rotation of the CH3 group, as discussed in the following section on the internal dynamics. On the other hand, the fact that all isotopologues have the same value of the inertial defect indicates that all substituted atoms lie in the plane of symmetry of the complex, which has then a Cs symmetry. We could not observe any lines belonging to the E state of the internal rotation of the CH3 top. This is related to the very small V3 barrier, which locates the E state transitions out of our frequency range. The search for the second isomer was unsuccessful. This could be due to the conformational relaxation to the most stable isomer upon supersonic expansion. It has, indeed, been shown that this kind of relaxation takes place easily when the interconversion barrier is smaller than 2kT.[16]
Structural Information The observed isomer of CH3Cl-H2O, is shown in Figure 2, where also the atom numbering and principal axes of the complex are drawn. It is stabilized by an O¢H···Cl and a C¢H···O WHB.
Figure 2. The observed isomer of CH3Cl-H2O with indicated atom numbering and the positions of the principal axes. X4 is a dummy “atom” lying on the symmetry axis of water.
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Full Paper The full ab initio equilibrium structure is available in the Supporting Information.
Table 4. The experimental coordinates of the Cl and O atoms in CH3ClH2O.
rs r0[a] rs r0[a]
a [æ] b [æ]
Cl
O
0.920(2) 0.926 0.526(3) 0.527
2.375(1) -2.377 0.04(3) 0.006
Internal Dynamics 1. The internal rotation of CH3 The almost free internal rotation of CH3 changes the “rigid” rotational constants to the effective ground state (v = 0, s = 0) values, according to:[19]
[a] Calculated with the r0 structure in Table 5. The coordinates c are set to be zero by symmetry.
A00 ¼ Ar þ W 00 ð2Þ F1a 2 From the rotational constants of the two monosubstituted isotopologues, it is possible to calculate the substitution coordinates, rs,[17] of the Cl and O atoms in the principal axes of the parent species. The obtained values are shown in Table 4 and are compared with the values of a partial r0 structure. The low-barrier to internal rotation of the CH3 top modifies the rotational constants of the A-state with respect to the “rigid” value and, consequently, the inertial defects of all isotopologues. The partial r0 structure was then obtained to reproduce the corrected rotational constants with the same V3 barrier deduced in the next section. By adjusting three structural parameters (RCl2O3, aC1Cl2O3 and aCl2O3X4), while keeping the remaining parameters fixed to their ab initio values, we could reproduce the experimental rotational constants of the CH335Cl-H2O, CH337Cl-H2O, and CH335Cl-H218O isotopologues within discrepancies of ~ 2 MHz. Such an adjustment was more problematic when including in the fit also the rotational constants of CH335Cl-D2O. This is not surprising because the H!D substitution of the hydrogen atoms involved in hydrogen bonds causes a shortening of the hydrogen bond distances, according to the reverse Ubbelohde effect.[18] The obtained parameters are reported in Table 5 and there compared to the ab initio values (re). From this partial r0 structure, the WHB parameters have been derived, and are also reported in Table 5. The “bond lengths” of the two WHBs (C¢H···O and Cl···H¢O) are quite similar to each other, RH7O3 = 2.501(2) and RH5Cl = 2.638(2) æ, respectively.
Table 5. Partial r0 and re structures of CH3Cl-H2O. Fitted parameters r0 re
RCl2O3 [æ] 3.344(2)[a] 3.321
aC1Cl2O3 [8] 74.2(1) 73.1
aCl2O3X4[b] [8] 89(2) 90.3
aC1Cl2H5 [8] 87(2) 86.1 aC1H7O3 [8] 132.5(1) 133.2
aCl2H5O3 [8] 131(2) 128.9
Derived parameters r0 re r0 re
RCl2H5 [æ] 2.638(2) 2.632 RH7O3 [æ] 2.501(2) 2.439
[a] Uncertainties (in parentheses) are expressed in units of the last digit. [b] X4 is a dummy “atom” lying on the symmetry axis of water (see Figure 2). Chem. Asian J. 2015, 10, 1198 – 1203
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ð2Þ
B00 ¼ Br þ W 00 ð2Þ F1b 2 C 00 ¼ C r þ W 00
ð2Þ
F1c
2
where Ar, Br and Cr are the “rigid” rotational constants in the limit of the infinite barrier. The W00(2) are the Herschbach’s barrier-dependent perturbation sums relative to the sublevels of the A-symmetry (s = 0) species of the torsional ground state (v = 0). In Equation (2), 1g = lgIa/Ig (g = a, b, c), where lg are the direction cosines between the internal rotation axis a and the principal axis g. F = h /[2·(1¢SglgIa/Ig)Ia] is the reduced rotational constant of CH3. Therefore, the increases of the effective rotational constants depend on the V3 barrier and on the angles between the principal axes and the internal rotation axis of a particular rotor, as discussed in several cases, like CF3Cl-CH3F[20] and CF3Cl-CH2O.[21] In Figure 2, the c-axis of the complex is perpendicular to the internal rotation axis of CH3Cl. Then lc, 1c and F1c2 are zero, and the rotational constant C is not affected by the internal rotations. Instead, the b-axis is almost parallel to the internal rotation axis of CH3Cl which alters the effective rotational constant B. Assuming the decrease (absolute value) of Dc, from the “rigid” value (¢3.22 uæ2) to the experimental value (¢0.96 uæ2) as due only to the internal rotation of CH3, we determined W00(2) 0.7 for all four isotopologues. The corresponding barrier is V3 8 cm¢1, smaller than the ab initio value (90 cm¢1). 2. The internal rotation of H2O The differences of the planar moments of inertia DMaa, DMbb and DMcc between the v = 0 and v = 1 states as shown, for example, in the case of CH2F2···H2O,[5] allow us to picture the potential pathway for the tunnelling motion of H2O by using Meyer’s one-dimensional flexible model.[22] Assuming that the effects of the internal rotation of CH3 top are the same for both v = 0 and v = 1 states, we can apply the following potential energy function, suitable for two equivalent periodic minima, VðtÞ ¼ 1=2 V 2 ½1 - cosð2tÞ¤
ð3Þ
where V2 and t are the potential energy barrier and the dihedral angle, t = aH5O3X4Cl2. t = 0 corresponds to the equilibrium value with H2O lying in the Cs plane of symmetry. The structural relaxations of the rO3···Cl2 distance and of the aO3Cl2C1 and aX4O3Cl2 angles (abbreviated below as a and b) as functions of t were also required for the model, according to the following relations: 1201
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Full Paper rðtÞ=æ ¼ 3:132 þ 1=2 ¡ Dr ½1¢cosð2tÞ¤ aðtÞ= ¼ 73:6 þ 1=2 ¡ Da ½1¢cosð2tÞ¤
Table 7. Binding energies of the investigated water adducts of freons stabilized by WHB linkages.
ð4Þ
bðtÞ= ¼ 85:0 þ ¡1=2 ¡ Db ½1¢cosð2tÞ Due to the lack of experimental data, Dr and Da have been fixed to the theoretical values (0.147 æ and ¢2.98, respectively). Eight vibrational states were calculated from the model potential optimization on a grid of 67 mesh points in the range 0 t 2p.[20] The flexible model results are reported in Table 6. The experimental value of the barrier to the internal rotation of H2O, V2 320(10) cm¢1, is in accordance with the ab initio predicted value (341 cm¢1).
ED [kJ mol¢1]
Ref.
CH2FCl···H2O
8.5
[6]
CH2F2···H2O
7.5
[5]
CH3CHFCl···H2O
5.4
[7]
CHF2Cl···H2O
5.5
[8]
CH3Cl···H2O
6.6
This work
Table 6. Flexible model results and potential energy parameters for the internal rotation of H2O in CH3Cl-H2O. Exptl.
Calc.
DMaa [uæ2] 0.004 ¢0.013 DMbb [uæ2] DMcc [uæ2] 0.012 – DE0 + 0¢ [GHz] Determined parameter: Db = ¢9(3)8, V2 = 320(10) cm¢1[a]
0.005 ¢0.013 0.012 2.2
Conclusions
[a] Uncertainties (in parentheses) are given in units of the last digit.
Dissociation energy The intermolecular stretching motion which leads to the dissociation appears to be almost parallel to the a-axis of the complex. By assuming that such a motion is separated from the other molecular vibrations, it is possible, within a pseudo diatomic approximation, to estimate the stretching force constant through the following equation:[23]
ks ¼ 16p4 ðmRCM Þ2 ½4B4 þ 4C 4 ¢ðB¢CÞ2 ðB þ CÞ2 ¤=ðhDJ Þ
ð5Þ
where m is the pseudo-diatomic reduced mass and RCM (3.257 æ) is the distance between the centers of mass of the two subunits. B, C and DJ are the spectroscopic parameters reported in Table 2. If it is then assumed that the intermolecular separation for this kind of complex can be described by a Lennard–Jones type potential approximation, the dissociation energy is evaluated through:[24] E D ¼ 1=72 ks RCM 2
ð6Þ
leading to ED = 6.6 kJ mol¢1, very close to the ab initio value (ED0(BSSE) = 5.1 kJ mol¢1). In Table 7, we compare the dissociation energy of CH3Cl-H2O to those of water adducts of freon molecules, obtained by using the same approximation. All of them have quite similar dissociation energies. Chem. Asian J. 2015, 10, 1198 – 1203
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The Fourier transform microwave investigation of CH3Cl-H2O pointed out that the observed isomer has a cyclic structure with one O¢H···Cl and one C¢H···O WHB resulting in the dissociation energy of 6.6 kJ mol¢1. The rotational spectrum displays the features of the quadrupole coupling effect of the 35Cl (or 37 Cl) nucleus, of the near-free internal rotation of CH3 top and of the internal rotation of water around its C2v axis. We interpreted the effects of the two large amplitude motions of the two constituent subunits. The V3 barrier to the internal rotation of CH3 has been estimated from the unexpected low values of the inertial defects, while the differences of the planar moments of inertia between the v = 0 and v = 1 states allowed an evaluation of the potential hypersurface pathway for the tunnelling motion of H2O. This study supplies the first rotational spectroscopic data on the structure and internal dynamics of an adduct of a monohalomethane with water.
Experimental Section Molecular clusters were generated in a supersonic expansion, under conditions optimized for the formation of CH3Cl-H2O. Details of the Fourier transform microwave spectrometer[25] (COBRAtype[26]), which covers the range 6.5–18 GHz, have been described previously.[27] A gas mixture of ca. 2 % of CH3Cl (commercial samples used without any further purification) in helium at a stagnation pressure of ~ 0.5 MPa was expanded over water (or H218O or D2O) through a solenoid valve (General Valve, Series 9, nozzle diameter 0.5 mm) into the Fabry–P¦rot cavity. The spectral line positions were determined after Fourier transformation of the time-domain signal with 8k data points, recorded with 100 ns sample intervals. Each rotational transition appears as a doublet due to the Doppler effect. The line position is calculated as the arithmetic mean of the frequencies of the Doppler components. The estimated accuracy of the frequency
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Full Paper measurements is better than 3 kHz. Lines separated by more than 7 kHz are resolvable.
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Received: January 5, 2015 Revised: February 2, 2015 Published online on February 20, 2015
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