CH-Stretching Overtone Spectra of 3-Hexyne and Butane

Abstract

We have recorded vapor-phase CH-stretching overtone spectra of 3-hexyne in the ΔvCH = 2−8 regions and of 3-hexyne-d10 in the ΔvCD = 2−7 regions. We have recorded CH-stretching overtone spectra of butane in the ΔvCH = 2−7 regions and of propane in the ΔvCH = 7 and 8 regions to facilitate comparison with the 3-hexyne spectra. The spectra were recorded at room temperature with the use of conventional and intracavity laser photoacoustic spectroscopy. The spectra can be explained in terms of a harmonically coupled local-mode model (HCAO) with one oscillator for each of the nonequivalent CH bonds. Spectral assignments are made and correlated with ab initio geometry optimized structures. The intensities of the CH stretching transitions are calculated using the HCAO local-mode model and ab initio dipole moment functions. Parameters for the calculations come from a local-mode analysis of the spectra and from ab initio calculations. Our simple calculations are in good agreement with the observed absolute and relative intensities. The ethyl group in 3-hexyne is freely rotating, but there is little difference in the CH bond length with the ethyl group position. No peaks due to internal methyl or ethyl rotation are observed in the spectra of 3-hexyne. The frequency of the in-plane methyl bond increases significantly in 3-hexyne compared to those in propane and butane.