Solvent dependent branching between C-I and C-Br bond cleavage following 266 nm excitation of CH2BrI

Abstract

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 CH2BrI 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 CH2BrI at 266 nm in acetonitrile, 2-butanol, and cyclohexane. Channels involving formation of CH2Br + I radical pairs, iso-CH2Br-I, and iso-CH2I-Br are identified. The solvent environment has a significant influence on the branching ratios, and on the formation and stability of iso-CH2Br-I. Both iso-CH2Br-I and iso-CH2I-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-CH2I-Br photoproduct as well as iso-CH2Br-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-CH2Br-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 CH2BrI. The concentration dependence highlights the role that intermolecular interactions can play in the quenching of unstable isomers of dihalomethanes.

Transient isomers in the photodissociation of bromoiodomethane

“…The rise in the concentration of CH 2 I-Br roughly correlates with a depletion of CH 2 Br-I, suggesting a conversion from CH 2 Br-I to CH 2 I-Br. This conversion was earlier found by Anderson et al 83 supporting the hypothesis of isomer conversion at our experimental conditions. This allows us to further incorporate the conversion rate into the fitting.…”

“…Such behavior is influenced by the different polarities of the solvents. 83 Results of six Gaussians' fitting (not reported here) clearly show no contribution of the B-band, neither in cyclohexane nor in methanol, at the wavelength of λ = 267 nm. It can be concluded that the formation of the CH 2 I-Br in the solvent cage just after the UV photodissociation is unlikely.…”

“…2, it can be observed that the solvent shifts the absorption peak at both the C-Br and C-I chromophores with respect to the gas phase. 61,83 In particular, the absorption in cyclohexane is red-shifted, whereas in methanol it is blue shifted. Such behavior is influenced by the different polarities of the solvents.…”

“…4(d)]. Anderson et al 83 reported a ratio of 2.8:1 between initially formed CH 2 Br-I and CH 2 I-Br within the 100 ps timedelay and 10% for intra-isomerization yields. Thus, our fitting outcome yielding a 4:1 ratio between CH 2 Br-I and CH 2 I-Br at time below 200 ps, and 30% for intra-isomerization between CH 2 Br-I and CH 2 I-Br correlate with the earlier observations of isomer formation and reorganisation.…”

“…Anderson et al 83 used ultrafast transient absorption spectroscopy of UV photoexcited CH 2 IBr in a series of four solvents (from apolar to polar), and captured the photochemical process involving CH 2 IBr in cyclohexane. In their analysis, both isomers are assumed at time zero.…”