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

Abstract

High resolution infrared absorption spectra of cyclohexane (C 6 H 12 ) 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 ν 27 (e u ) and ν 14 (a 2u ) CH 2 scissor modes at 1452.9 cm −1 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 4 main strong allowed C-H stretching modes appear as two e u a 2u pairs near at 2862 cm −1 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.

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“…Figure 13 shows the FTIR spectrum of a pure lupine extract. The −OH stretching frequency appears at 3419 cm –1 , 69 the –CH stretching frequency at 2933 cm –1 , 70 the C=O stretching frequency at 1654 cm –1 , 71 the –C=C stretching frequency at (multiple bands) 1405–1547 cm –1 , 72 the −CN stretching frequency at 1246 cm –1 , 73 and the −CO stretching frequency at 1055 cm –1 . 74 According to the theoretical infrared (IR) spectra of lupanine, multiflorine, and sparteine, the multiflorine IR is more like that of an experimental lupine extract than it is for lupanine and sparteine.…”

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“…Generally, in the vibration spectrum CH 2 stretching occurs as asymmetric and symmetric stretching. The range of asymmetric CH 2 vibrations occurs at 3000–2900 cm −1 and for symmetric CH 2 vibration observed between 2900 and 2800 cm −1 [54, 55]. The bending vibrations appears at the range of 1470–1430 cm −1 [56].…”

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“…Recent analyses have also demonstrated considerable transferability of the couplings between local vibrational modes in C−H stretch models, for example. 50,75,76 As a demonstrative example of this locality for the present investigation, the two-mode couplings for hexatriene, H 2 C-(CH) 4 -CH 2 are depicted in Figure 1. For this analysis, the rootmean-squared coupling definition, ⟨00|ΔV (2) |00⟩ 1/2 from ref 67 is employed (where the integration is performed over the harmonic ground-state wavefunctions for the sake of transferability), although many other alternatives exist.…”

“…Even shorter-range cutoffs and multiple-order-of-magnitude accelerations were demonstrated for 3-MR potentials. Recent analyses have also demonstrated considerable transferability of the couplings between local vibrational modes in C–H stretch models, for example. ,, …”