Theory of chemically-induced dynamic spin polarization. IV. Low-field effects

Abstract

The Pedersen–Freed theory of chemically-induced dynamic spin polarization is extended to low field cases where the T±→S transitions must be included. The solutions of the stochastic-Liouville equation are again obtained by numerical finite-difference (FD) methods, but order of magnitude reductions in the size of the matrices to be inverted are realized by introduction of a simple modification of the FD expressions. The simplest case of a single nuclear spin of I=1/2 on one radical is treated in detail. The results show unequivocally the significant dependence of both CIDNP and CIDEP signals on both the magnitude and the range of the exchange interaction. Also, significant polarizations which should be observable are predicted for both types of experiment under appropriate low field conditions. All the computed results are consistent with a simple qualitative model involving two primary regions in r space: in the outer region the hyperfine terms induce T±→S transitions and J (r) =0, while in the inner region J (r) modulates the effectiveness of these transitions. Consideration is also given to magnetic-field effects on chemical reactivity as well as T±→S contributions in high field but viscous CIDEP.