CIDNP Determination of the Rate of In-Cage Deprotonation of the Triethylamine Radical Cation

Abstract

CIDNP (chemically induced dynamic nuclear polarization) effects in the sensitized photoreactions of triethylamine DH (sensitizers A, 9,10-anthraquinone and 2,7-dinitrofluorenone) are investigated in a series of aprotic solvents of widely varying relative permittivity ε, including mixtures of acetonitrile and chloroform. The dependence of the polarization pattern, that is, the relative polarization intensities of the olefinic α- and β-protons in the reaction product N,N-diethylvinylamine, on ε is studied quantitatively by evaluating the polarization ratio r of these protons. The factor responsible for the change of the polarization pattern is the rate, relative to the kinetic window of CIDNP, of in-cage deprotonation of the triethylamine radical cation by the sensitizer radical anion (rate constant kdep), which converts the initially formed radical ion pair into a neutral radical pair . A theory of pair substitution taking place solely during encounters of the radicals is presented. Based on this, an expression for the dependence of r on kdep is derived, which in turn allows the extraction of kdep from the experimentally observed polarization ratio. The accessible range of kdep in these systems is from about 108 to about 1010 M-1 s-1.