The overtone transitions of neopentane-D0, -D6, -D9, and tetramethylsilane

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Petryk, Michael W. P.
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University of Guelph

Using a combination of conventional and intracavity laser photoacoustic spectroscopy (ICL-PAS), the vibrational overtone spectrum of gaseous neopentane-d\sb0 was studied in the energy range Δv\sbCH = 3-9, while the overtone spectra of gaseous neopentane-d\sb6, -d\sb9, and tetramethylsilane (TMS) were studied in the energy range Δv\sbCH = 3-8. A Fermi resonance is observed in the TMS transitions above Δv\sbCH = 3. The C-H stretching transitions observed in the overtone spectra of the neopentanes and TMS reveal that, in both the neopentanes and TMS, the vibrational motion of the C-H oscillators can be approximated well as uncoupled Morse oscillators. The pure local mode frequencies, ω, and anharmonicity, ωχ, of the gases are determined. The unusually broad pure local mode transitions of the neopentanes are attributed in part to through space coupling in which the C-H oscillators on different methyl groups in the same molecule of neopentane to couple. A through space coupling is thought to be much less efficient for TMS. The Lorentzian overtone transitions of neopentane-d\sb0 and TMS are modeled as rotational transitions which are homogeneously broadened through lifetime uncertainty. The absolute absorbance intensities of neopentane-d\sb0 were obtained via ICL-PAS spectroscopy employing methane as an internal standard in the energy range Δv\sbCH = 4-7. Good agreement was obtained between the oscillator strengths arrived at through conventional absorbance experiments and those arrived at using ICL-PAS techniques in the region Δv\sbCH = 5, 6. The absolute overtone intensities of neopentane-d\sb0 and TMS were accurately predicted by the HCAO model coupled with ab initio calculated dipole moment functions and the experimentally obtained parameters ω, ωχ, and γ\sp′.

overtone transitons, neopentane-D0, neopentane-D6, neopentane-D9, tetramethylsilane