Effect of the electron spin exchange relaxation on the high-field CIDNP polarization produced by radical pairs

Abstract

The effect of the electron spin relaxation of the radicals of a radical pair has been examined at the high-field approximation in a reaction whose products display chemically induced nuclear polarization. We have found that the polarization varies noticeably when the relaxation rates are comparable to the singlet-triplet mixing parameter Q, typically of the order of 10 7 rad/sec. Special emphasis has been placed on the relaxation process due to the Heisenberg electron spin exchange between a stable radical and the radicals of the pair since in this case it may be sufficiently fast to affect the extent of the polarization. In the photolysis of a solution of 9,10-phenanthrenequinone and fluorene performed in the presence of di- tert-butyl nitroxide radical, changes of the polarization were obtained for the polarized line of the adduct on varying the concentration of the nitroxide. The experimental and the calculated changes of the polarization were compared and the average value of the rate constants of the collisional dimers formed by the stable radical and the single radicals of the pair could be determined.