Overtone intensities in neopentane and tetramethylsilane

Abstract

The CH vibrational overtone transitions of vapour phase neopentane-d0, -d6, -d9, and tetramethylsilane (TMS) are investigated via conventional absorbance spectroscopy and intracavity laser photoacoustic spectroscopy (ICL-PAS). The CH stretching overtones of neopentane-d0 are observed in the energy range Δ VCH = 3–9 (8 400 to 22 000 cm–1) and in the range Δ VCH = 3–8 for the remaining neopentanes and TMS. While we have extended the range of previous studies, our main focus in this work has been on overtone intensities. The oscillator strengths of the neopentane-d0 overtone transitions are determined directly by conventional absorbance spectroscopy for Δ VCH = 3–6 and via ICL-PAS with methane as an internal standard for Δ VCH = 4–7. The correspondence of measured intensities for the two techniques demonstrates the usefulness of the latter for higher overtone ICL-PAS spectra. The experimentally determined Morse parameters ω˜ and ω˜x as well as the harmonically coupled anharmonic oscillator (HCAO) model are used in conjunction with dipole moment functions that are derived from ab initio computations to calculate overtone transition intensities for the neopentanes and TMS. Key words: local mode, overtone transition, absolute intensity, photoacoustic spectroscopy.