Conformational stability of bromocyclohexane from temperature dependent FT-IR spectra of xenon solutions, r0 structural parameters and vibrational assignment

The infrared spectra (3200–30 cm−1) of the gas, amorphous and annealed solid and Raman spectra (3200–30 cm−1) of liquid and solid bromocyclohexane (c-C6H11Br) have been recorded. Variable temperature (−55 to −100 oC) studies of the infrared spectra (3200–400 cm−1) of bromocyclohexane (c-C6H11Br) dissolved in liquefied xenon have been carried out. From these data, the enthalpy difference has been determined to be 239 ± 24 cm−1 (2.86 ± 0.29 kJ mol−1), with the chair-equatorial conformer the most stable form. At ambient temperature, the abundance of the chair-axial conformer is 24% ± 2. From MP2 ab initio calculations utilizing various basis sets with and without diffuse functions, the equatorial conformer is predicted to be more stable by 168 ± 22 cm−1 from the four largest basis set calculations. However, the average from the corresponding B3LYP density functional theory calculations is 300 ± 16 cm−1 which, considering the error, is consistent with the experimental value. By utilizing the previously reported microwave rotational constants for two isotopomers (79Br, 81Br) combined with the structural parameters predicted from the MP2(full)/6-311+G(d,p) calculations, adjusted r0 structural parameters have been obtained. The determined heavy atom distances for the most stable chair-equatorial conformer in Å are r0(C1–C7,8) = 1.532(3); r0(C7,8–C13,14) = 1.539(3); r0(C4–C13,14) = 1.524(3); and r0(C4–Br6) = 1.966(5) and the angles in degrees: ∠C1C7,8C13,14 = 111.3(5)°; ∠Br6C4C13,14 = 109.8(5)° with the two dihedral angles ∠C8C1C7C13 = 55.9(10)° and ∠C14C4C13C7 = 57.7(10)° .For the axial form the major differences in distances are obtained for the r0(C7,8–C13,14) = 1.531(3) and r0(C–Br) = 1.975(3) Å. 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.