UV PHOTODISSOCIATION OF MATRIX-ISOLATED PROPIONYL CHLORIDE

We have investigated the photodecomposition of propionyl chloride, CH3 CH2COCl, 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 photoproduct s are observed. We have carried out FTIR polarization studies to deter mine the relative orientation of the photoproducts and found that the HCl molecule is situated perpendicular to the carbonyl group in the me thyl ketene. The orientation in the photoproducts and kinetic studies of the acid chloride dissociation point to a direct elimination mechan ism for propionyl chloride decomposition. This is consistent with the direct mechanism proposed for acetyl chloride, CH3COCl, photodissociat ion in both an argon matrix and the solid crystalline form. We have ca lculated detailed thermodynamics for CH3COCl and CH3CH2COCl decomposit ion 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 polariz ation studies.