Magnetic Field Dependence of the 31P CIDNP in the Photolysis of a Benzyl Phosphite. Evidence for a T-−S Mechanism

Abstract

31P chemically induced dynamic nuclear polarization (CIDNP) studies of the photochemistry of p-acetylbenzyl dimethyl phosphite (1) provide evidence for the operation of the relatively rare T-−S intersystem crossing mechanism for a photochemically generated triplet free radical pair. The mechanism of intersystem crossing of the triplet radical pair p-acetylbenzyl (2) and dimethoxyphosphonyl (3), generated from 1, is influenced by the large 31P hfcc of 3 and switches from the T-−S mechanism in a relatively low magnetic field (18.8 kG) to the conventional T0−S pathway at a higher field (58.8 kG). This change in mechanism is evidenced by the photolysis of 1 in the presence of radical scavengers (halocarbons, thiophenol) which yields the corresponding products from atom abstraction by 3 that has escaped the initial radical cage. These escape products are absorptively polarized at high field (T0−S) but are emissively polarized at lower field (T-−S) in a particularly well-defined display of the effect of magneti...