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

A comparison of the CH vibrational overtone spectra of vapor phase neopentane-d0 (C(CH3)4), -d6 (C(CH3)2(CD3)2), and -d9 (C(CH3)(CD3)3) and tetramethylsilane (TMS) in the frequency range ΔvCH = 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.