SummaryTermination rate constants 2Kt for the self-reaction of t-butyl radicals in twelve different solvents are determined as a function of temperature by kinetic electron spin resonance. In the entire range measured, 7 . 107<2k,<2 . 10'O dm3 mol-' s-I, they are well described by the von Smoluchowski equation for diffusion controlled reactions, using a spin statistical factor, a temperature and solvent independent, isotropic reaction diameter, and diffusion coefficients for t-butyl estimated following empirical or semiempirical prescriptions. Methods for the prediction of diffusion coefficients are critically discussed, and rules for the estimation of termination rate constants of steric unhindered alkyl radicals are suggested.

Rate constants for the self-reaction of tert-butyl radicals in six n-alkanes are determined as functions of temperature by kinetic electron-spin resonance spectroscopy and from rates of product formation. They are well described by a Smoluchowski-Stokes-Einstein treatment including effects of microfriction and spin statistical factors, indicating complete diffusion control. The ratio of disproportionation to combination products is found to depend on temperature and solvent. This is tentatively ascribed to reorientational effects in the encounter pairs.

T h e rate constant for the bimolecular combination of benzyl radicals in cyclohexane and toluene is determined as a function of temperature. Further, it is studied in cyclohexanetoluene mixtures of different compositions. In the entire range covered, 9.8 X lo8 5 2/?, d 9.0 X 10YM-l.sec-', the data are very well described hy the Smoluchowski equation lor B diffusion-controlled reaction to ground-state products using a spin statistical t'actor of I!:$. a temperature-and solvent-independent reaction distance, and the known diffusion coell'icienl of toluene.

SummaryDisproportionation/combination ratios Pd/Pc of self-reacting t-butyl radicals are determined as a function of solvent and temperature. The observed large solvent and temperature dependences are ascribed to anisotropic reorientational motions of the radicals during their encounter in the solvent cage. Results for other alkyl radicals are compatible with this concept.

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