Solvent and isotope effects on addition of atomic hydrogen to benzene in aqueous solution

Pulsed ESR has been used to measure rate constants for the addition of hydrogen atoms to benzene as a function of temperature in aqueous solution. At 298 K we obtain kH(aq) = (1.1 ± 0.1) · 109 M−1 s−1. The average Arrhenius activation energy, Ea = 19.1(6) kJ · mol−1, is essentially the same in solution as in the gas phase, but the frequency factor, represented by log(A/M−1 s−1) = 12.34(9), is 54 times higher than in the gas. This may be explained in terms of solvation of reactants and transition state, with most of the enhancement attributed to H atom solvation. In a 90% D2O/10% H2O mix, H atoms react at the same rate, but deuterium atoms react more slowly than H atoms by a factor of 1.4. The much smaller solvent enhancement previously demonstrated for the light muonium isotope is tentatively attributed to incomplete hydration of the transition state in that case.