Oxidation of Bromide Ions by Hydroxyl Radicals: Spectral Characterization of the Intermediate BrOH^•–

The reaction of ^•OH with Br^– has been reinvestigated by picosecond pulse radiolysis combined with streak camera absorption detection and the obtained spectro-kinetics data have been globally analyzed using Bayesian data analysis. For the first time, the absorption spectrum of the intermediate species BrOH^•– has been determined. This species absorbs in the same spectral domain as Br₂^•–: the band maximum is roughly at the same wavelength (λ_max = 352 nm instead of 354 nm) but the extinction coefficient is smaller (ε_max = 7800 ± 400 dm³ mol^–1 cm^–1 compared with 9600 ± 300 dm³ mol^–1 cm^–1) and the band is broader (88 nm versus 76 nm). Quantum chemical calculations have also been performed and corroborate the experimental results. In contrast to Br₂^•–, the existence of several water–BrOH^•– configurations leading to different transition energies may account for the broadening of the absorption spectrum in addition to the higher number of degrees of freedom