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

High-resolution infrared absorption spectra of cyclohexane have been recorded from 1100 to 4000 cm at room temperature and 241 K. Cyclohexane is an oblate symmetric top with symmetry. A rotational analysis was obtained for the ν (e) and ν (a) CH scissor modes at 1452.9 and 1456.4 cm, respectively. Several combination modes were also assigned and rotationally analyzed. The C-H stretching modes are perturbed by overtone and combination modes of the CH 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 a e pairs near at 2862 and 2933 cm. 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.