Formation of radical pairs by photochemical reduction of sterically hindered quinones with phenols in frozen solutions

Abstract

1. According to ESR data, the photochemical reduction of vitrified solutions of 3,6-di-t-butyl-o-benzoquinone and 2,4,6-tri-t-butylphenol results in the formation of two types of polyoriented radical pairs (RP) in which the distances between the unpaired electrons are 5.4±0.05 and 6.3±0.05 A. 2. A mechanism is proposed for RP formation, consisting in the phototransfer of a hydrogen atom from the phenol to the quinone, which forms a weak complex in the frozen solution. 3. Both types of RP are formed by intermolecular hydrogen transfer in the complex through one and the same channel. The formation of the different RP's is connected with the subsequent rapid exchange of a proton between the adjacent carbonyl groups. 4. The kinetics of dark destruction of the RP's has been studied. When 2,4,6-tri-t-butylphenol-d1 deuterated on the OH group is used, a kinetic isotope effect is observed. This confirms the participation of the hydrogen of the OH group in the formation and destruction of the RP. The stepwise character of the dark destruction indicates that the nonuniformity of the matrix affects the rate of hydrogen transfer.