Kinetics of recombination, dismutation, and disproportionation reactions involving neutral ketyl radicals and radical anions

Abstract

AbstractThe kinetics of fast elementary recombination of neutral ketyl radicals of benzophenone and its four derivatives (BPH⋅), equation image the dismutation of benzophenone radical anions, equation image the disproportionation between BPH⋅ and stable nitroxyl radicals, (magnified image), magnified image and the electron transfer magnified image have been investigated in both individual solvents and binary mixtures of different viscosities. Reaction (1) for unsubstituted BPH in water, water glycerol, and n‐hexane is controlled by diffusion with 2k1 ≃ kdiff. In aliphatic alcohols and toluene, which form solvation complexes with BPH⋅, reaction (1) is diffusion‐enhanced and activation‐controlled, respectively, with 2k1 < kdiff. In a viscous solvent such as 1‐propanol–glycerol mixture (100 ≲ η ≲ 450 cP) reaction (1) is diffusion‐controlled. Reaction (2) in alkaline 1‐propanol and alkaline 1‐propanol–glycerol mixture is activation controlled. The rates of reactions (3) and (4) for benzophenone radicals and nitroxyl radicals of the imidazoline series decrease as the viscosity of the water–glycerol and 1‐propanol–glycerol mixtures is increased. The reactions are molecular mobility limited; nevertheless, the numerical values of k3 (k4) are 2–6 times as small as the corresponding kdiff values due to the low steric factor of the reactions (therefore called pseudodiffusion‐controlled reactions). The theoretical estimates of k3 (k4) are in good agreement with the experimental results. The elimination of spin forbiddance in the process of radical recombination in viscous solvents is discussed.