Multiple-photon chemistry in the benzophenone photoreduction during laser-jet photolysis: Effect of alcohol solvent on cross-coupling versus hydrogen abstraction of the electronically excited hydroxydiphenylmethyl radical

Abstract

The ground state of the hydroxydiphenylmethyl radical ( 1) leads to benzpinacol ( 2) through head-to-head coupling and the diols 3 as cross-coupling product. In contrast. under the high-intensity conditions of the laser-jet photolysis. the excited radical 1∗ couples in the para position to afford the benzophenone derivatives 4 ( head-to-tail coupling) (higher spin density at the para position / AM1 calculations). The major two-photon product is benzhydrol ( 5). The pronounced increase in hydrogen atom abstraction from MeOH to iPrOH by 1∗ is explained in terms of the greater electrophilic character of the electronically excited radical 1∗ versus its ground state.