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

Abstract

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-C 6H 11Br) have been recorded. Variable temperature (−55 to −100 oC) studies of the infrared spectra (3200–400 cm −1) of bromocyclohexane ( c-C 6H 11Br) 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 r 0 structural parameters have been obtained. The determined heavy atom distances for the most stable chair-equatorial conformer in Å are r 0(C 1–C 7, 8) = 1.532(3); r 0(C 7, 8–C 13, 14) = 1.539(3); r 0(C 4–C 13, 14) = 1.524(3); and r 0(C 4–Br 6) = 1.966(5) and the angles in degrees: ∠C 1C 7, 8C 13, 14 = 111.3(5)°; ∠Br 6C 4C 13, 14 = 109.8(5)° with the two dihedral angles ∠C 8C 1C 7C 13 = 55.9(10)° and ∠C 14C 4C 13C 7 = 57.7(10)° .For the axial form the major differences in distances are obtained for the r 0(C 7,8–C 13,14) = 1.531(3) and r 0(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.