Chemically Induced Dynamic Nuclear Polarization in the Photochemical Reaction of Phenanthraquinone with Hydrogen Donors. II. A Consideration of Unusual Nuclear Spin Polarization

Abstract

Abstract Polarized quartet NMR signals due to methylene protons were observed in the hydrogen-abstraction reactions of phenanthraquinone with o-substituted toluenes under the irradiation of light. The relative intensities of these signals could be predicted only by the general treatment of the CKO theory, taking account of the electronic singlettriplet interactions (S–T0 and S–T1) of the radical pair. The scalar electronic-coupling constants, Je, were evaluated approximately by comparing the theory with the experimental results and were found to be in the order of 7×109 sec−1. The rather large value of Je was discussed in relation to the nature of the reaction system; consequently, the existance of electronic π–π interaction was suggested for the radical-pair intermediate of the reaction.