Faster dissociation: Measured rates and computed effects on barriers in aryl halide radical anions

Carbon-halogen bond dissociation rates for a series of aryl halide radical anions (ArX-.: X = Cl, Br) in NMP were measured at room temperature by pulse radiolysis with 10(-11) s time resolution. To obtain accurate dissociation rates, care was taken to measure and correct for competing decay channels. The observed rates correlated well with activation energies computed in the gas phase by density functional (DFT) calculations. The rates did not correlate well with electron affinities or dissociation energies obtained by the same computational methods, although such correlations are reported in the literature and are expected on the basis of simple models. The calculations also found that the transition state structures had bent carbon-halogen bonds. Bending enables large reductions of the activation energies by an electronic effect involving mixing of pi* and sigma* states. This bending-induced mixing is computed to increase the dissociation rates by a few orders of magnitude and is thus essential to understanding these reactions.