Temperature dependence of the rate constants for reaction of inorganic radicals with organic reductants

Rate constants for the reactions of several inorganic radicals with several organic 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 those of the radicals N₃^•, NO₂^•, Br₂^•-, I₂^•-, and (SCN)₂^•- reacting with several phenols and ascorbate. Rate constants were also measured for the reactions of Cl₂^•- with phenol and of ClO₂^• with p-methoxyphenolate. The rate constants measured were in the range of 10⁵ to nearly 10¹⁰ M^-1 s^-1 and the calculated Arrhenius activation energies ranged from 7 to 41 kJ mol^-1. The preexponential factors also varied considerably, with log A ranging from 9.2 to 13.9. The temperature dependences of these reactions do not seem to relate to their exothermicities. Variations in rate constants appear to be more strongly dependent on changes in preexponential factors rather than on changes in activation energy.