SOLVENT EFFECTS IN ELECTRONEUTRAL REACTIONS .2. DIFFUSION-CONTROLLED REACTIONS OF ETHYL AND TRIFLUOROMETHYL RADICALS

Ethyl and trifluoromethyl radicals were generated photochemically in a variety of solvents under controlled conditions which permitted the unambiguous identification of the primary cage reaction and allowed a quantitative measurement of the fractions of radicals undergoing the cage reaction and diffusion. The temperature dependence of the cage-diffusion reaction system for ethyl radicals was studied in isooctane, toluene, and decalin from -80 to +85° and was found to be the same in all three solvents. An energy of activation defined as Ea = Ecage - Ediff was thus calculated to be equal to -1.3 kcal. The cage-diffusion reaction system for trifluoromethyl radicals was investigated in 27 solvents at 65° and in six solvents at 30°. The data suggest that Ea is constant for these radicals also in nearly all the solvents examined; discrepancies were apparent in hydroxylic solvents. The logarithms of the probability ratios Fcage/Fdiff are linearly related to the reciprocal of the rate of change of the boiling point of the solvent with pressure. This is taken to mean that solvation interactions are similar in all solvents. An equation is developed for diffusion-controlled reactions relating the specific reaction rate to the viscosity coefficient of the solvent. By means of this equation the value of Ea for trifluoromethyl radicals in nonhydroxylic solvents is estimated to be -2.4 to -2.8 kcal. On the basis of the present data and of the data in related previous communications, it is suggested that the constancy of Ea in both radical systems is probably accidental and stems from a compensating effect between the transfer and immediate local effects of an excess energy in the radicals which controls the extent of the cage reaction and the self-cohesion of the solvent which controls the size and relaxation of the cage.