Through Space Coupling and Fermi Resonances in Neopentane-d₀, -d₆, -d₉, and Tetramethylsilane

Abstract

A comparison of the CH vibrational overtone spectra of vapor phase neopentane-d 0 (C(CH3)4), -d 6 (C(CH3)2(CD3)2), and -d 9 (C(CH3)(CD3)3) and tetramethylsilane (TMS) in the frequency range Δv CH = 4−8 (10 800−18 200 cm-1) has revealed pronounced differences between the spectra of TMS and the neopentanes, and subtle differences among the spectra of the neopentanes. These spectral differences are interpreted as a manifestation of geometry and vibrational frequency dependent differences in coupling efficiencies that facilitate the de-excitation of local modes of vibration via IVR. Fermi resonance plays a key role in this coupling. Some of the states are perturbed by through space coupling (a collision-like van der Waals interaction) that can facilitate IVR. The normal modes of vibration, which are implicated in Fermi resonance of the neopentanes and TMS, have been calculated ab initio using density functional theory and are shown to be affected by through space interactions.

Phase shift cavity ring down and Fourier transform infrared measurements of C–H vibrational transitions, energy levels, and intensities of (CH3)3Si–C≡C–H

“…Besides the main components corresponding to C-H a and C-H s , an absorption to the high frequency side of the C-H a overtone increases in intensity from υ = 5 to 6. A similar absorption has been reported by Petryk and Henry 28,29 in overtone studies of tetramethylsilane (TMS) and has been assigned to a Fermi resonance. In this case, a pure local mode (υ, 0, 0) mixes with a local-mode normalmode combination state with a local mode component (υ-1, 0, 0) and a normal mode component (2υ bend ) where υ bend is a bending mode.…”

Absolute configuration of chirally deuterated neopentane

“…The molecule's different rotamers therefore do not convert rapidly into each other. The amplitude of the hindered rotational motion of the methyl groups was estimated from model potentials to be about 15u at room temperature 22 . The C-C stretching force constant was found to depend on the torsional angle 23 .…”

CH Stretching Vibrational Overtone Spectra of tert-Butylbenzene, tert-Butyl Chloride, and tert-Butyl Iodide

“…This coupling is enhanced by steric hindrance. 21 At all overtones, the lowest frequency aryl transitions occur at lower frequencies than are typical of aryl C-H oscillators (see ref 45). In Figure 4 we note the alkyl regions (ca.…”

“…21 Dephasing is likewise unimportant as the local environment does not fluctuate rapidly compared to the lifetime of the excited state.…”

“…[18][19][20] Steric crowding has been reported to give rise to intramolecular coupling through space between proximal, nonbonded atoms in neopentane, C(CH 3 ) 4 . 21 This coupling through space can enhance Fermi resonances, thereby accelerating IVR and leading to complex spectral features.…”