Role of Diffusion-Controlled Reaction in Chemically Induced Nuclear-Spin Polarization II. General Theory and Comparison with Experiment

A more detailed study is made of an earlier proposal that the nuclear-spin polarizations observed in nuclear magnetic resonance studies of the products of free radical reactions are produced when a pair of radicals fail to react the first time they are together but instead diffuse apart and subsequently come together again. In this model the magnetic interactions within the radicals have ample time to produce a nuclear-spin dependent growth or decay of the singlet character of the radical pair during the interval between separation and return. Since the singlet character of the radical pair determines its recombination probability the products formed at the second encounter will be nuclear-spin polarized. This process is analyzed for the cases where the initial state of the radical pair is: (1) a triplet; (2) a singlet; and (3) uncorrelated electron spins. The predictions that sign of the nuclear-spin polarization is the same for the cases of a triplet and uncorrelated spins but is reversed for the singlet state are in agreement with experiment. The intensities of the polarized nuclear magnetic resonance lines are calculated for some typical reactions and the results are in good agreement with experiment.