Relation between CIDNP formed upon geminate and bulk recombination of radical pairs.

Abstract

A theoretical approach to time-resolved Chemically Induced Dynamic Nuclear Polarization (CIDNP) is proposed, which allows one to obtain the general relation between polarization formed upon recombination of geminate spin-correlated radical pairs, the so-called G-pairs, and upon recombination of radical pairs formed by encounters of free radicals in solution, the so-called F-pairs. This relation is described by a universal parameter denoted as γ. In this work, the γ value is computed for the arbitrary spin multiplicity, singlet or triplet, of the precursor of the G-pairs as well as for arbitrary recombination rate constants of radical pairs in singlet and triplet states, kS and kT, respectively. Furthermore, the treatment is extended to the situation where radicals undergo transformation resulting in different reactivity or magnetic parameters for F-pairs and G-pairs. The proposed theory enables modeling of time-resolved CIDNP data in cases where (i) both recombination channels are active and (ii) fast protonation/deprotonation of radicals changes the effective γ value.