Anomalous kinetic hydrogen isotope effect in the oxidation of formate by bromine: Tunneling via an intermediate

Abstract

Rates of oxidation of XCOO^− (X = H, D) by Br_2 in acid aqueous media were measured between 274 and 332 K. The derived Arrhenius parameters for both reactions log k_H(M^(-1) s^(-1) = (18.81 ± 0.10) - (14.33 ± 0.13)/ø log k_D(M^(-1) s^(-1) = (13.77 ± 0.13) - (17.62 ± 0.04)/ø where θ = 4.575T × 10^(−3) kcal/mol, with (k_H/k_D)_(298K) = 2.85, reveal a primary isotope effect, but the difference (E_D - E_H) = 3.29 kcal/mol and the ratio A_D/A_H = 91 fall beyond the limits imposed by semiclassical transition-state theory, suggesting tunneling or a multiple-stage mechanism. However, it can be shown that either tunneling in a single step or a three-step, internal return mechanism can be ruled out as alternative models, since both require unreasonable kinetic parameters to fit the data. The simplest scheme accounting for the present observations involves tunneling in the decomposition of a charge transfer complex in equilibrium with the reactants.

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