Rate constants for some reactions of inorganic radicals with inorganic ions. Temperature and solvent dependence

Rate constants for several reactions of inorganic radicals with inorganic anions in aqueous and aqueous/acetonitrile solutions have been measured as a function of temperature by laser flash photolysis. The reactions studied were (1) Cl2− + N3−, (2) Br2− + N3−, (3) Cl2− + SCN−, (4) Br2− + SCN−, (5) SO4− + Cl−, (6) SO4− + CO32−, and (7) N3− + I−. The rate constants were corrected for ionic strength and ranged from 106 to 109 L mol−1 s−1. The Arrhenius activation energies varied from 2 to 12 kJ mol−1 for the first 4 reactions, were higher for reaction 6, and negative for reaction 5. The pre‐exponential factors also varied considerably with log A ranging from 5 to 14. The values of k298 decreased in most cases by more than an order of magnitude upon increasing the acetonitrile (ACN) fraction from 0 to 70%. For most reactions, this decrease in k298 was due to changes in log A with little regularity in the small changes observed in Ea. For reaction 7, k298 was practically unchanged due to compensating effects of the changes in Ea and log A with ACN mol fraction, giving an isokinetic relationship. An isokinetic relationship was also observed in the case of reaction 6; Ea and log A change in parallel while changing ACN mol fraction. Reaction 3 (Cl2− + SCN−) was also studied in water/t‐butanol and water/acetic acid mixtures. Linear correlation was found between log k and the dielectric constant of the medium for water/ACN and water/t‐BuOH but the lines for the two solvent mixtures had different slopes, suggesting specific solvation effects in addition to the primary solvent polarity effects. With water/acetic acid, k decreased and then increased upon addition of acetic acid.