Conformational stability of chlorocyclohexane from temperature-dependent FT-IR spectra of xenon solutions, r 0 structural parameters, and vibrational assignment

Abstract

Variable temperature (−55 to −105 °C) studies of the infrared spectra (4000–400 cm−1) of chlorocyclohexane (c-C6H11Cl) 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 (35Cl, 37Cl) 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 chair-equatorial conformer in A are: r 0(C1–C7,8) = 1.532(3); r 0(C7,8–C13,14) = 1.536(3); r 0(C4–C13,14) = 1.524(3); and r 0(C4–Cl6) = 1.802(5) and the angles in degrees: ∠C1C7,8C13,14 = 111.3(5)o; ∠Cl6C4C13,14 = 109.7(5)o with the two dihedral angles ∠C8C1C7C13 = 56.3(10)o and ∠C14C4C13C7 = 56.7(10)o. 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.