First Evidence of Vibrationally Driven Bimolecular Reactions in Solution: Reactions of Br Atoms with Dimethylsulfoxide and Methanol

We present evidence for vibrational enhancement of the rate of bimolecular reactions of Br atoms with dimethylsulfoxide (DMSO) and methanol (CH3OH) in the condensed phase. The abstraction of a hydrogen atom from either of these solvents by a Br atom is highly endoergic: 3269 cm–1 for DMSO and 1416 or 4414 cm–1 for CH3OH, depending on the hydrogen atom abstracted. Thus, there is no thermal abstraction reaction at room temperature. Broadband electronic transient absorption shows that following photolysis of bromine precursors Br atoms form van der Waals complexes with the solvent molecules in about 5 ps and this Br•–solvent complex undergoes recombination. To explore the influence of vibrational energy on the abstraction reactions, we introduce a near-infrared (NIR) pump pulse following the photolysis pulse to excite the first overtone of the C—H (or O—H) stretch of the solvent molecules. Using single-wavelength detection, we observe a loss of the Br•–solvent complex that requires the presence of both photo...