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

Peter F Bernath; Edwin L Sibert Iii

doi:10.1021/acs.jpca.0c09185

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

J Phys Chem A. 2020 Dec 3;124(48):9991-10000. doi: 10.1021/acs.jpca.0c09185. Epub 2020 Nov 19.

Authors

Peter F Bernath¹, Edwin L Sibert Iii²

Affiliations

¹ Department of Chemistry and Biochemistry, Old Dominion University, Norfolk, Virginia 23529, United States.
² Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States.

PMID: 33210908
DOI: 10.1021/acs.jpca.0c09185

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 D_3d symmetry. A rotational analysis was obtained for the ν₂₇ (e_u) and ν₁₄ (a_2u) 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 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.