Conformational stability from variable temperature infrared spectra of xenon solution, r0 structural parameters, and vibrational assignment of fluorocyclohexane

The infrared spectra (3500-300 cm−1) of the gas and xenon solutions of fluorocyclohexane have been recorded. By the utilization of nine different temperatures of nine conformer pairs the enthalpy difference between the more stable chair-equatorial conformer and the chair-axial form was determined to be 48 ± 5 cm−1 (137 ± 14 cal/mol). The ab initio predicted values ranged from 121 cm−1 with the chair-axial conformer more stable from the MP2(full)/6-311G(d,p) calculations to 24 cm−1 with the chair-equatorial form more stable with diffuse functions. The r0 structural parameters have been obtained from predicted parameters from ab initio MP2(full)/6-311+G(d,p) calculations adjusted to fit the previously reported microwave determined rotational constants. The determined heavy atom structural parameters for the equatorial[axial] conformer are: the distances (Å) C1-C7,8 = 1.533(3)[1.531(3)], C7,8-C13,14 = 1.543(3)[1.538(3)], C4-C13,14 = 1.518(3)[1.520(3)], C4-F6 = 1.404(3)[1.410(3)] and angles in degrees ∠C1C7,8C13,14 = 111.0(5)[111.2(5)], ∠F6C4C13,14 = 108.7(5)[107.9(5)] and τC8C1C7C13 = 55.9(10)[55.9(10)]. The harmonic force fields, infrared intensities, Raman activities, depolarization ratios, and vibrational frequencies 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 both conformers. The results are discussed and compared to the corresponding properties of some similar molecules.