Time-resolved infrared absorption studies of the solvent-dependent photochemistry of ClNO

The photochemistry of nitrosyl chloride (ClNO) dissolved in cyclohexane is investigated using ultrafast time-resolved infrared (TRIR) spectroscopy. Following 266 nm photolysis, the photochemistry is measured by following changes in optical density at frequencies spanning the N═O stretch fundamental transition. A photoinduced depletion in optical density is observed consistent with the depletion of ground-state ClNO. The depletion in optical density remains constant out to ∼50 ps demonstrating that ClNO photodissociation is not followed by recombination of the Cl and NO photofragments. In addition, no evidence for the formation of the ClON photoisomer is observed. These results stand in contrast to previous studies in acetonitrile where ClNO photolysis is followed by geminate recombination of Cl and NO, and by the production of ClON. These differences in ClNO photochemistry are proposed to arise from the population of different excited-states caused by solvent dependence of the ground-state potential energy surface minimum along the Cl-N stretch coordinate. Solvent-dependent vibrational relaxation and differences in strength of the solvent cage are also proposed to contribute to the solvent-dependent photochemistry. Finally, these results are placed in the context of recent models of ClNO photochemistry and role of this compound in tropospheric ozone production.