Kinetic and thermodynamic data for reaction (1) of certain C-centered aromatic radicals (referred to in this paper by the numbers I to X) in chlorobenzene:have been obtained. The k l values of radicals varied between (1.1 f 0.2) X 106M-I-sec-' (radical VIII) and (3.6 f 0.7) X 10gM-l-sec-l (radical VI) a t 2OOC. An investigation of the relationship between the recombination rates of radicals I-VIII and X and the solvent viscosity (mixture of toluene and dibutylphthalate, 0.6 < 9 < 18.4 cP) has shown that the recombination reactions involving radicals I-IV are limited by diffusion in solvents having a viscosity 9 > 10 CP and are activation reactions in solvents having a viscosity < 10 cP. The recombination of radicals VIII and IX is an activation reaction, while that of radicals V-VII is diffusioncontrolled in the entire viscosity range. The recombination of radical X is limited, in the viscosity range of 18.4 to 2 cP, by intrusion into the first coordination sphere of the partner, the effect of viscosity on the radical X recombination rate in the specified range being the same as its effect on diffusion-controlled reactions. The possible reasons of the discrepancies between the experimental fast recombination rate constants and the theoretical values calculated by the Debye-Smoluchowski theory are discussed. The equilibrium constant depends strongly on the nature of the substituent in the phenyl fragment: the substituents which increase unpaired electron delocalization in the radical intensify the dissociation of the respective dimer. Long-wave absorption hands have been recorded for radicals I-X and their extinction coefficients obtained. Dimers I-V are thermo-and photochromic compounds.

The kinetics of the reversible recombination of the 2-phenyl-(I), 2-p-methoxyphenyl-(111, and 2-p-nitrophenyl-3-oxo-2,3-dihydrobenzothiophene-2-y1 (111) radicals R + R ---' D

Rates and thermodynamic data have been obtained for the reversible self-termination reaction: In K , where K is the equilibrium constant of this reversible reaction. The transition state of Radical (I) dimer dissociation reaction looks more like a pair of radicals than the initial dimer. The role of specific solvation in radical self-termination reactions is discussed 2k-1

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