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 -1 at room temperature and 241 K. Cyclohexane is an oblate symmetric top with D 3 d symmetry. A rotational analysis was obtained for the ν 27 (e u ) and ν 14 (a 2u ) CH 2 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 CH 2 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 2u e u 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.