Bimolecular self‐reactions of phenyl‐substituted phenoxyl radicals studied by flash photolysis

Rate, equilibrium, and thermodynamic data for reaction (1) of 2,6‐diphenyl‐4R‐phenoxyl radicals, where R==OCH3 (I), Ph (II), OC2H5 (III), O‐n‐C18H37 (IV), and 2,6‐dicyclohexyl‐4‐phenylphenoxyl radical (V), equation image in various solvents are obtained. The k1 values of radicals I to V are within (5.5 ± 1.0) × 107‐(1.4 ± 0.3) × 109M−1·sec−1 in propanol. The solvent effect on k1 for radicals I and II was studied. The dimerization of radical I is diffusion‐controlled in all solvent studies. The dimerization of radical II is viscosity‐dependent but not diffusion‐controlled. Plots of k1 against ET have a V shape. Specific solvent–solute interactions are seeming to be responsible for numerical k1 values of radicals I and II. The solvent effect is more pronounced for “slow” dimerization of radicals II than for “fast” dimerization of radicals I. The minimum k1 values correspond to pyridine and chloroform. The reaction (1) rate strongly depends upon the composition of a chloroform (S)–cosolvent binary mixture. Besides reaction (1) the following reactions proceed in binary mixture: equation image (radical I, S‐CCL4 mixture) (radical II, S‐C6H14 mixture) (radical II, S‐CCL4 mixture) In all cases k16 ≪ k15. Factors influencing dimerization rates in strongly nonideal mixtures CH3OH‐CCL4 and CH3OH‐CHCl3 are discussed.