Infrared absorption intensities of methyl acetylene

Abstract

The infrared absorption intensities of the fundamental bands of CH3CCH, CD3CCH and CH3CCD were measured in the gas phase by the pressure broadening technique. Out of 32 possible sign combinations of (∂p/∂Qi) for each of a1 and e species, only one combination was selected for both species, utilizing the isotopic data, Coriolis information, and the CNDO calculation. Thus, the polar tensor and the effective charges were obtained, as well as the dipole moment derivatives with respect to primitive coordinates. Comparison of these parameter values with those of other molecules suggests that the methyl hydrogens in methyl acetylene resemble those in methyl halides rather than those in saturated hydrocarbons, while the acetylenic hydrogen seems to retain the property in acetylene molecule itself. On the other hand, the CNDO method was not found very successful in this molecule as in propane, cyclopropane, or benzene. In particular, its capability to predict intensity parameter values concerned with the acetylenic group of this molecule was found fairly poor.