Cyclohexane Vibrations: High-Resolution Spectra and Anharmonic Local Mode Calculations.

Abstract

High-resolution infrared absorption spectra of cyclohexane have been recorded from 1100 to 4000 cm-1 at room temperature and 241 K. Cyclohexane is an oblate symmetric top with D3d symmetry. A rotational analysis was obtained for the ν27 (eu) and ν14 (a2u) CH2 scissor modes at 1452.9 and 1456.4 cm-1, respectively. Several combination modes were also assigned and rotationally analyzed. The C-H stretching modes are perturbed by overtone and combination modes of the CH2 scissor vibrations, and an anharmonic local mode calculation was needed to interpret the spectra. The four main strong allowed C-H stretching modes appear as two a2u eu pairs near at 2862 and 2933 cm-1. The Fermi-resonance local mode model coupling terms give physical insight into the effects that organize the cyclohexane vibrational energy levels. The unstrained cyclohexane molecule is a useful paradigm for six-membered rings in larger chemical and biological systems.