UV photodissociation of matrix-isolated propionyl chloride

We have investigated the photodecomposition of propionyl chloride, CH3CH2COCl, in an argon matrix at 10 K using FTIR absorption spectroscopy. The decomposition products formed following irradiation at 266, 254, or 248 nm are methyl ketene, CH3CH=CO, and HCl; no other photoproducts are observed. We have carried out FTIR polarization studies to determine the relative orientation of the photoproducts and found that the HCl molecule is situated perpendicular to the carbonyl group in the methyl ketene. The orientation in the photoproducts and kinetic studies of the acid chloride dissociation point to a direct elimination mechanism for propionyl chloride decomposition. This is consistent with the direct mechanism proposed for acetyl chloride, CH3COCl, photodissociation in both an argon matrix and the solid crystalline form. We have calculated detailed thermodynamics for CH3COCl and CH3CH2COCl decomposition and found them consistent with the proposed elimination mechanism. We also assign the fundamental vibrational frequencies for the methyl ketene-HCl complex on the basis of ab initio calculations and polarization studies.