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

Abstract

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 75C. The reactions studied were of the dihalide radicals, Cl{sub 2}{sup {sm bullet}{minus}}, Br{sub 2}{sup {sm bullet}{minus}}, and I{sub 2}{sup {sm bullet}{minus}}, the (SCN){sub 2}{sup {sm bullet}{minus}} radical, and the neutral radical N{sub 3}{sup {sm bullet}}, reacting with the substitution-inert metal complexes, Fe(CN){sub 6}{sup 4{minus}}, Mo(CN){sub 8}{sup 4{minus}} and W(CN){sub 8}{sup 4{minus}}, and with the anions SO{sub 3}{sup 2{minus}} HSO{sub 3}{sup {minus}}, NO{sub 2}{sup {minus}}, and ClO{sub 2}{sup {minus}}. The rate constants measured were in the range of 10{sup 6} to 5 {times} 10{sup 9} M{sup {minus}1}s{sup {minus}1} and the calculated Arrhenius activation energies ranged from 5 to 35 kJ mol{sup {minus}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 of 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.