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

Abstract

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 r 0 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) C 1–C 7,8 = 1.533(3)[1.531(3)], C 7,8–C 13,14 = 1.543(3)[1.538(3)], C 4–C 13,14 = 1.518(3)[1.520(3)], C 4–F 6 = 1.404(3)[1.410(3)] and angles in degrees ∠C 1C 7,8C 13,14 = 111.0(5)[111.2(5)], ∠F 6C 4C 13,14 = 108.7(5)[107.9(5)] and τC 8C 1C 7C 13 = 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.