Solvent dependent branching between C-I and C-Br bond cleavage following 266 nm excitation of CH_2BrI

摘要

It is well known that ultraviolet photoexcitation of halomethanes results in halogen-carbon bond cleavage. Each halogen-carbon bond has a dominant ultraviolet (UV) absorption that promotes an electron from a nonbonding halogen orbital (nX) to a carbon-halogen antibonding orbital (σ*C-X). UV absorption into specific transitions in the gas phase results primarily in selective cleavage of the corresponding carbon-halogen bond. In the present work, broadband ultrafast UV-visible transient absorption studies of CH_2BrI reveal a more complex photochemistry in solution. Transient absorption spectra are reported spanning the range from 275 nm to 750 nm and 300 fs to 3 ns following excitation of CH_2BrI at 266 nm in acetonitrile, 2-butanol, and cyclohexane. Channels involving formation of CH_2Br + I radical pairs, iso-CH _2Br-I, and iso-CH_2I-Br are identified. The solvent environment has a significant influence on the branching ratios, and on the formation and stability of iso-CH_2Br-I. Both iso-CH2Br-I and iso-CH_2I-Br are observed in cyclohexane with a ratio of ～2.8:1. In acetonitrile this ratio is 7:1 or larger. The observation of formation of iso-CH_2I-Br photoproduct as well as iso-CH _2Br-I following 266 nm excitation is a novel result that suggests complexity in the dissociation mechanism. We also report a solvent and concentration dependent lifetime of iso-CH_2Br-I. At low concentrations the lifetime is >4 ns in acetonitrile, 1.9 ns in 2-butanol and ～1.4 ns in cyclohexane. These lifetimes decrease with higher initial concentrations of CH_2BrI. The concentration dependence highlights the role that intermolecular interactions can play in the quenching of unstable isomers of dihalomethanes.