Abstract
In the absence of radical scavengers, the products of biradicals generated from photolysis of 2-phenylcycloalkanones exhibit no CIDNP. Addition of a radical scavenger such as CCl 4 or CBrCl 3 gives rise to strong CIDNP derived from T 0 -S intersystem crossing. The CIDNP intensity depends on the scavenger concentration, the molecular chain length of the biradical, and the reaction temperature. The results are interpreted in terms of a simple kinetic scheme which implies a kinetic window within which T 0 -S CIDNP is efficiently generated in biradicals. Bimolecular scavenging is sufficient to provide the strong competition with intersystem crossing needed to produce T 0 -S CIDNP. From a knowledge of the dependence of CIDNP intensity on scavenger concentration and the previously measured biradical lifetimes, the rate constant of halogen atom abstraction by the acyl radical moiety is estimated to be 1 × 10 6 M −1 s −1 for Cl abstraction from CCl 4 and 1 × 10 9 M −1 s −1 for Br abstraction from CBrCl 3 . © 1991, American Chemical Society. All rights reserved.