Ionization of the three isomeric hydroxybenzoates by free electron transfer: product distribution depends on the mobility of the phenoxyl group

Abstract

Ionization of 2-, 3- and 4-hydroxy-methylbenzoate by free electron transfer (FET) to n-butyl chloride parent radical cations results in different product patterns. Whereas 3- and 4-hydroxybenzoate directly form comparable amounts of solute radical cations and phenoxyl radicals, from 2-hydroxybenzoate arise only radical cations. We interpret this finding as a molecule dynamic effect connected with the femtosecond rotation of the phenoxyl group which leads to a marked n-electron density shift. For 3- and 4-hydroxybenzoate, the diversity of all solute rotamers transforms in two different kinds of radical cations, unstable ones which promptly dissociate and longer living ones. Because of the H bridge bonding to the ester carbonyl, in 2-hydroxymethylbenzoate the phenoxyl group rotation is practically excluded and this should be the reason that exclusively stable radical cations derived from the planar molecule structure are formed.