Disproportionation-Combination Reactions of Caged Geminate Radical Pairs Formed from the Photodecomposition of 2,2'-Azoisobutane. Anisotropic Reorientation of tert-Butyl Radicals in Viscous Media and Birth Effects in Very Viscous Media

Article abstract of DOI:10.1021/ja00365a041

The solvent viscosity dependence of the self-termination reactions of tert-butyl radical pairs generated from the photolysis of 2,2'-azoisobutane, in a number of inert solvents admixed with thiophenol, shows a linear relationship between the cage efficiency, 1/cage yield, and fluidity, 1/η, of the solvent.The viscosity dependence of the ratio of disproportionation/combination rate constants, k_d/k_c, for these cage reactions was compared to the values observed for the reactions of encounter pairs generated by the photolysis of di-tert-butyl ketone.At viscosities between 0.5 and 2.7 cP the k_d/k_c ratios obtained for geminate radical pairs were the same as those obtained from the reaction of the encounter pairs, while at higher viscosities of 12-808 cP the orientation of the geminately formed radicals at birth appeared to bias the radicals to undergo a higher proportion of combination.