Abstract
Irradiation of argon-purged acetonitrile solutions of 2-phenylcyclododecanone (1) in the probe of a 250-MHz NMR spectrometer does not produce significant 1 H or 13 C chemically induced dynamic nuclear polarization (CIDNP). However, irradiation of 1 in oxygen-purged acetonitrile, argon-purged acetonitrile containing small amounts of bromotrichloromethane, or argon-purged carbon tetrachloride solutions, in the probe of a 250-MHz NMR spectrometer, produces intense 1 H and 13 C CIDNP. Irradiation of argon-purged acetonitrile solutions of 1 in the earth's field does not lead to measurable 13 C enrichment of the recovered starting material or in the products of photolysis. However, irradiation of 1 in carbon tetrachloride or in acetonitrile solutions purged with oxygen or containing small amounts of bromotrichloromethane results in significant enrichment of an isomeric product, 2. The development of CIDNP and of isotopic enrichment is proposed to result from a common feature of the systems containing oxygen, carbon tetrachloride, or bromotrichloromethane: the opening of a competitive chemical scavenging pathway (oxygen, carbon tetrachloride, or bromotrichloromethane as scavengers) for the triplet biradical ( 3 BR) produced by homolytic cleavage of electronically excited 1. This competitive pathway, when its rate is within an appropriate "kinetic window", provides a mechanism for effective sorting of nuclear spins to develop CIDNP and the effective sorting of nuclear isotopes to develop isotopic separation. © 1991 American Chemical Society.