Temperature dependence of the rate constants for reaction of dihalide and azide radicals with inorganic reductants

Rate constants for several reactions of inorganic radicals with inorganic reductants in aqueous solutions have been measured by pulse radiolysis as a function of temperature, generally between 5 and 75°C. The reactions studied were of the dihalide radicals, Cl₂^•-, Br₂^•-, and I₂^•-, the (SCN)₂^•- radical, and the neutral radical N3^•, reacting with the substitution-inert metal complexes, Fe(CN)₆^4-, Mo(CN)₈^4-, and W(CN)₈^4-, and with the anions SO₃^2-, HSO₃^-, NO₂^-, and ClO₂^-. The rate constants measured were in the range of 10⁶ to 5 × 10⁹ M^-1 s^-1 and the calculated Arrhenius activation energies ranged from 5 to 35 kJ mol^-1. The preexponential factors also varied considerably, with log A ranging from 8.9 to 13.1. The temperature dependence of the reaction rate constant is correlated to the reaction exothermicity for the metal complexes, which apparently react by outer-sphere electron transfer. The simple anions, however, have lower activation energies, which do not correlate well with the exothermicities, suggesting that these anions probably react by an inner-sphere mechanism.