Raman spectra of liquid cyclohexane, C 6 H 12 , and deuterated cyclohexane, C 6 D 12 , were recorded with both parallel and perpendicular polarizations. The observed vibrational wavenumbers, depolarization ratios, and their intensities were measured and compared with the corresponding predicted values as well as the experimental values previously reported. The conformational energetics were obtained with the Møller-Plesset perturbation method to the second order [MP2(full)] as well as with density functional theory by the B3LYP method utilizing a variety of basis sets. The average ab initio predicted difference in energy between the more stable chair form (D 3d ) and the less stable twisted-boat form (D 2 ) is 2213 cm −1 (26.47 kJ/mol), with a similar value of 2223 cm −1 (26.59 kJ/mol) from the density function theory calculations. By using two dihedral angles as variables, we calculated the chair-boat interconversion pathway for cyclohexane at the MP2(full)/6-31G(d) level. The harmonic force constants, Raman intensities, depolarization values, and the potential energy distribution were predicted from both MP2(full) and B3LYP calculations with the 6-31G(d) basis set and compared with the experimental values for the chair form when available. The 'adjusted' r 0 structural parameters were obtained from MP2/6-311+G(d,p) calculations and previously reported microwave rotational constants of five isotopomers of cyclohexane: (equatorial and axial). The determined distances in Å are: r(CC) = 1.536(3), r(CH) ax = 1.098(1); r(CH) eq = 1.095(1); and the angles in degrees: ∠CCH ax = 108.8(3); ∠CCH eq = 110.2(3); ∠CCC = 111.1(3); and ∠HCH = 107.6(3) with dihedral angle ∠CCCC = 55 .7(3). These values are compared with those previously reported and it is found that the difference in the r 0 distances (0.003 Å) between the two CH values is much smaller than the difference (0.008 Å) previously reported for the r s values.

The Raman spectra (3500-50 cm −1 ) of the liquid and solid methylcyclohexane and the infrared spectra of the gas and solid methylcyclohexane have been recorded. The Raman band at 754 cm −1 in the liquid has been confidently assigned to the less stable axial conformer and its intensity was recorded as a function of temperature from 25 to −95 • C. By the utilization of 15 different temperatures, the enthalpy difference between the more stable chair-equatorial conformer and the chair-axial form was determined to be 712 ± 71 cm −1 (8.50 ± 0.84 kJ/mol). The ab initio predicted value of 710 cm −1 (8.50 kJ/mol) from the MP2(full)/6-311G(2d,2p) calculations with and without diffuse functions is in excellent agreement. The harmonic force fields, infrared intensities, Raman activities, depolarization ratios, and vibrational wavenumbers have been obtained for both conformers from MP2(full)/6-31G(d) ab initio calculations. With two scaling factors of 0.88 for the C-H stretches and 0.9 for the remaining ones, the fundamental wavenumbers have been predicted and along with the depolarization values and infrared band contours (B-type for A modes) a complete vibrational assignment has been made for the chair-equatorial conformer. Predicted r 0 structural parameters have been provided from adjusted parameters from ab initio MP2(full)/6-311+G(d,p) calculations. The results are discussed and compared with the corresponding properties of some similar molecules. Copyright

Variable temperature (-55 to -105°C) studies of the infrared spectra (4000-400 cm -1 ) of chlorocyclohexane (c-C 6 H 11 Cl) dissolved in liquefied xenon have been carried out. The infrared spectra of the gas and solid have also been recorded from 4000-100 cm -1 . By analyzing six conformer pairs in the xenon solution, a standard enthalpy difference of 132 ± 13 cm -1 (1.58 ± 0.16 kJ/mol) was obtained with the equatorial conformer the more stable form. At ambient temperature, the abundance of the axial conformer is 34 ± 1%. The potential surface describing the conformational interchange has been determined and the Fourier coefficients were obtained. From MP2 ab initio calculations utilizing various basis sets with and without diffuse functions, the equatorial conformer is predicted to be more stable by 161 ± 18 cm -1 from the four largest basis set calculations, which is consistent with the experimental results. However, the average from the corresponding B3LYP density functional theory calculations is 274 ± 15 cm -1 which is certainly too large. By utilizing the previously reported microwave rotational constants for two isotopomers ( 35 Cl, 37 Cl) combined with the structural parameters predicted from the MP2(full)/6-311+G(d,p) calculations, adjusted r 0 structural parameters have been obtained. The determined heavy atom distances for the most stable chairequatorial conformer in Å are: r 0 (C 1 -C 7 , 8 ) = 1.532(3); r 0 (C 7 , 8 -C 13 , 14 ) = 1.536(3); r 0 (C 4 -C 13 , 14 ) = 1.524(3); and r 0 (C 4 -Cl 6 ) = 1.802(5) and the angles in degrees: \C 1 C 7 , 8 C 13 , 14 = 111.3(5)8; \Cl 6 C 4 C 13 , 14 = 109.7(5)8 with the two dihedral angles \C 8 C 1 C 7 C 13 = 56.3(10)8 and \C 14 C 4 C 13 C 7 = 56.7(10)8. These parameters are in good agreement with those reported earlier from microwave and electron diffraction studies where the CC and CH distances were all assumed to be equal. A few of the previously reported vibrational assignments have been corrected. The results of these spectroscopic and theoretical studies are discussed and compared to the corresponding results for some similar molecules.

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