Salt effects on the reactions of radical ion pairs formed by electron transfer quenching of triplet 2-methyl-1,4-naphthoquinone by amines. Optical flash photolysis and step-scan FTIR investigations

Abstract

Electron transfer quenching of triplet 2-methyl-1,4-naphthoquinone by amines in acetonitrile was investigated by means of nanosecond flash photolysis and step-scan infrared spectroscopy. With 2,5-di-tert-butylaniline as the electron donor, proton transfer following the primary electron transfer process forms neutral radicals, which were identified by comparing the resulting transient IR absorption spectra with those obtained in the presence of phenol as the quencher. Addition of lithium perchlorate promotes the formation of free radical ions. The salt effect on the reactions of the nascent radical ion pairs formed by electron transfer is discussed in terms of Eigen theory. The quinone radical anion strongly associates with lithium cations. The salt retards the diffusional recombination of the free radical ions, as predicted by the Debye–Smoluchowski equation. The observed, predictable salt effects provide a useful diagnostic tool for elucidation of the reaction mechanism.