Photochemical oxidation of halide ions by a nitratochromium(III) complex. Kinetics, mechanism, and intermediates

The 266 nm laser flash photolysis of the title complex in the presence of halide ions X(-) (X = I, Br, and Cl) generates halogen atoms on nanosecond time scales, followed by the known X(*)/X(-) reactions to yield dihalide radical anions, X2(*-). Plots of k(obs) against the concentration of X(-) were linear with zero intercepts, but the yields of X2(*-) increased with increasing concentrations of [X(-)]. This result suggests that a short-lived, strongly oxidizing intermediate reacts with X(-) to generate X(*) in parallel with the decomposition to Cr(aq)O(2+) and (*)NO2, both of which were identified in steady-state photolysis experiments in the presence of selective trapping agents. Bromide was oxidized quantitatively to bromine, and a combination of molecular oxygen and methanol channeled the reaction toward superoxochromium(III) ion, Cr(aq)OO(2+). In the absence of scavengers, nitrite and chromate were produced. The proposed reaction scheme draws additional support from the good agreement between the experimental product yields and those predicted by kinetic simulations.