The ultraviolet photochemistry of condensed-phase acetyl chloride

Ultraviolet (UV) irradiation of amorphous and crystalline samples of solid acetyl chloride produces metastable HCl · ketene complexes in a 1:1 ratio following S1 photoexcitation. The HCl · ketene complex is the only product observed following UV irradiation of either amorphous or crystalline samples. The condensed-phase reaction mechanism contrasts starkly with that of the gas phase mechanism that produces Cl and CH3CO radicals by prompt photolysis, followed by dissociation of internally excited CH3CO to form CO and CH3 in a non-concerted process.