Interradical interactions

The Hamiltonian for interradical interactions can be decomposed into two terms corresponding to the electron exchange interaction and the electron dipolar interaction 

FIGURE 8.2 (a) Variation of RP energy with interradical separation exemplified by a pair of 

The exchange interaction typically drops to magnitudes of the same order as hyperfine couplings within a single diffusive step. For neutral RPs, the exchange 

The dipolar interaction is a direct interaction between the magnetic dipoles of the electrons on the two radicals. 

MAGNETIC FIELD EFFECTS ON RADICAL PAIRS IN HOMOGENEOUS SOLUTION 

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The intraradical interactions provide the mechanism for coherent spin-state mixing and the interradical interactions act contrary to this process. 

RP reactions have been found to be well described by the application of a spin Hamiltonian, " a common approach used in the field of magnetic resonance, which reduces the full Hamiltonian to one that contains only spin-dependent terms. The interactions capable of influencing spin-state mixing processes in RPs are concisely introduced in the expression for the spin Hamiltonian of a RP, which can be written as a sum of interradical, intraradical, and external interactions. 

Within a time on the order of 10 11 s, diffusion will effect a separation of the radicals. While this does not influence the spins directly, the exchange interaction decreases with the interradical distance and vanishes once the... 

Figure 1. Typical plot of the energies E of singlet and triplet states of a radical pair as functions of the interradical distance r. The inset shows the additional splitting of the triplet levels caused by the Zeeman interaction. [Adapted from ref. [6m] with permission. Copyright 1995 John Wiley Sons, Inc.].

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