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.

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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a b s t r a c tVariable temperature (À55 to À145°C) studies of the infrared spectra (4000-50 cm À1 ) of allyl alcohol (3-hydroxypropene), CH 2 @CHCH 2 OH, dissolved in liquid krypton and/or liquid xenon have been carried out. The infrared spectra of the gas and solid have also been recorded. From these data four of the five possible stable conformers have been identified and their relative stabilities determined. The order of the conformers stabilities has been obtained experimentally where the first indicator is for the relative position of the OH group (C = cis or G = gauche) to the double bond (rotation around the CAC bond) and the second one (t = trans, g = gauche, g 0 = gauche 0 ) is the relative position of the hydroxyl rotor, i.e. rotation around the CAO bond. The enthalpy differences have been determined between the most stable Gg conformer and the second most stable rotamer, Cg, to be 135 ± 14 cm À1 (1.62 ± 0.1 kJ/mol), the third most stable conformer Ct, 260 ± 46 cm À1 (3.11 ± 0.6 kJ/mol), and the fourth most stable conformer Gt 337 ± 75 cm À1 (4.03 ± 0.9 kJ/mol). This experimentally determined order is consistent with the order of stability predicted by ab initio calculations Gg > Cg > Ct > Gt > Gg 0 . No evidence was obtained for the fifth conformer Gg 0 which is predicted by most of the ab initio calculations to be less stable by an enthalpy difference more than 500 cm À1 than the Gg form. The percentage of each conformer at ambient temperature is estimated to be Gg(54 ± 2%), Cg(28 ± 2%), Ct(8 ± 2%) and Gt(11 ± 3%). The conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios and vibrational frequencies have been obtained for all of the conformers from ab initio MP2(full)/6-31G(d) calculations. The optimized geometries and conformational stabilities have been obtained from ab initio calculations utilizing several different basis sets up to MP2(full)/aug-cc-pVTZ and from density functional theory calculations by the B3LYP method. By utilizing previously reported microwave rotational constants for the Gg and Cg conformers along with ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r 0 parameters have been obtained for these two conformers. The determined heavy atom structural parameters for the Gg [Cg] conformer are: the distances(Å) C@C = 1.343(3)[1.340(5)], CAC = 1.499(3)[1.504(5)], CAO = 1.428(3)[1.419(5)] and angles( o ) \CCC = 122.8(5)[124.7(5)], \ CCO = 112.1(5)[114.9(5)] and \ CCCO = 122.7(10)[5.63 (5)]. Several of the CH distances are significantly different from the values previously reported from the earlier microwave data. The results are discussed and compared to the corresponding properties of some similar molecules.

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