Flash photolysis technique has been used to obtain the rate and thermodynamic parameters of the reversible dimerization reactions of a range of ten phenoxy radicals (I-X) in a toluene-dibutylphthalate mixture (0.6 CP < TJ < 18.4 cF):
The main reason for the difference in the k l values are the different steric hindrances in radicals. It has been found that the values of k l for 2,6-diphenyl-4-methoxy-(I), 2-phenyl-(1111, and 2-methoxyphenoxy (IV) radicals are 3-5 times smaller than the respective diffusion constants calculated according to the Debye formula with regard to the spin-statistical factor:
The resultant AH: values for these radicals in toluene and dibutylphthalate are close to the activation energies of the viscous flow of the solvents B . Linear relationships with a slope equal to unity are observed between log k l and log(T/q). The recombination of radicals I, 111, and IV is limited by translational diffusion. The k l values for 2,6-diphenyl-(VII), 2,6di-tert -butyl-(IX), and 2,6-di-tert-butyl-4-methylphenoxy (X) radicals are 10-60 times smaller than &iff and AHf < B. In the case of radical X in toluene AHf < 0. The recombination of these three radicals includes an intermediate step of complex formation: R + R s R.. . . R -D For 4-phenyl-(II), 2,6-dimethoxy-(V), 2,4-diphenyl-(VI), and radicals VII, IX, and X the linear relationships between log k l and log (T/TJ) have a slope of from 0.5 f 0.05 to 0.8 f 0.05. The k;' versus TJ relationships for these radicals are not straight lines. The recombination of these six radicals is limited by translational and rotational diffusion. With the aid of theoretical models, the k l versus TJ relationships have been used to derive the steric factor f in radical recombination and the angle 19 between the axis and the solid angle generatrix. The solid angle defines the reaction spot on the radical-sphere surface. The recombination of the 2,6-diphenyl-4-diphenylmethylphenoxy radical (VIII) takes place in the region intermediate between the diffusion and the kinetic ones, and the relationship between log k l and log (T/TJ) for this radical has a plateau portion. The log k-1 versus log ( T / T J ) relationships have precisely the same form as the corresponding kl relationships, which is quite in line with the theory of diffusion-controlled reversible recombination reactions.