Application of chemically induced dynamic nuclear polarization to a kinetic study of phenyl radical reactions

Abstract

The CIDNP enhancement coefficient of a stable product RX formed in a free radical reaction depends on nuclear relaxation in the paramagnetic precursor R' and decreases with an increase of its lifetime, τ R. This dependence was used for evaluation of the lifetime of phenyl radicals generated by thermolysis of N-nitrosoacetanilide (NAA). The value τ pb in CCl 4 solution at 60° was found to be 6.9 × 10 −5 sec which corresponds to a rate constant k = 1.4 × 10 3 8 mole −1 sec −1 for the reaction Ph + CCl 4 → PhCl + CCl 3 of which the activation energy equals 8.2 kcal mole −1. During the thermolysis of NAA CIDNP arises in a geminate radical pair (PhN 2 ON 2Ph) S from which it is transferred to aromatic products through PhNN' and the product of its destruction, Ph' radical. Comparison of the experimental CIDNP enhancement coefficient of the protons in PhNN' radical just leaving the radical pair, with those calculated for different lifetimes of the intermediate gives its mean lifetime τPhN 2 = 10 −7 sec.