Time-resolved studies of bromine radicals in solution and the effects of solvent on charge-transfer transitions

We employ ultrafast pump-probe spectroscopy to monitor the dynamics of bromine radicals in solution. A 267-nm pulse dissociates either bromoform or dibromomethane to form Br and a partner radical and broadband transient electronic absorption follows their time evolution. In both neat bromoform and dibromomethane a species absorbing at 390 nm appears promptly and decays in less than 15 ps as another longer wavelength feature appears. The presence of an isosbestic point between the two spectra suggests that the initial feature is the precursor to the second absorber. The initial feature is consistent with the cage recombination of the radical fragments to form either the original molecule or an isomer where the bromine radical binds to a bromine atom on the bromomethyl radical. A large amount of internal energy, leftover from the photolysis, permits the release of the weakly bound Br atom to form a complex with other solvent molecules. The 390-nm iso-bromoalkane absorption does not change upon dilution, but the transition of the CHBr3-Br or CH2Br2-Br complex systematically increases with the addition of cyclohexane. This trend agrees with the predicted dependence of a charge-transfer transition on the dielectric constant of the medium. These complexes live for more than a nanosecond in solution.;These experiments provide the necessary ground work for studying vibrationally mediated bimolecular reactions. The barrier for hydrogen abstraction by a bromine atom is similar in magnitude to the amount of energy of a single quantum of C-H stretch, making it an ideal candidate for vibrational enhancement.