Invited article

Abstract

The concept of the spin-correlated radical pair has been used for some time in the interpretation of the results of magnetic polarization experiments in NMR and in ESR (CIDNP and CIDEP), and of the effects of magnetic fields on chemical reactions. It has, however, a much wider general significance as a reaction intermediate in all radical reactions, including photochemistry. Here the nature of the pair is introduced, and the evidence for it reviewed, and it is further shown how it can be treated theoretically, before some of the consequences of its existence are pointed out. Its recognition, and in particular the understanding of the processes which occur within it, notably the interplay of spin-mixing and diffusion, allow the design of new experiments in the absence or presence of an external magnetic field, and the optimization of reaction yields.

A Brief History of Carbon Radicals

“…Theoretical work to explain CIDNP and CIDEP phenomena was able to link, for the first time, the spin physics of radical pairs to their diffusion, molecular tumbling, confinement (solvent cages versus supramolecular environments 52 ), and the effects of externally applied magnetic fields. [53][54][55][56] Several chapters of this book show how magnetic field effects, as well as CIDEP and CIDNP spectral patterns, can be used to solve chemical problems. It should be noted that the study of how applied magnetic fields perturb chemical reactivity is a topic that is highly relevant to biological processes involving radical pairs, for example, photosynthesis.…”

Magnetic field effect on the photooxidation efficiency of ferrocene

“…Magnetic field effects on chemical reaction kinetics are due in most cases to some variant of the so-called radical pair mechanism [1] wherein diffusion, spin selective reactions and spin motion are combined into an intriguing reaction scenario [2] with spin correlated radical pairs (RPs) functioning as reaction intermediates [3]. The principle of spin conservation during an elementary chemical reaction step provides the basis of spin selectivity for a particular (usually the singlet) radical pair substate.…”

Magnetic field effect as a probe of distance-dependent electron transfer in systems undergoing free diffusion

“…[43] and references therein). Typical spin chemical e ects like chemically induced magnetic polarizatio n of nuclei and electrons, magnetic isotope e ects, and magnetic ® eld e ects on chemical reaction rates and yields of reactions involvin g radical pairs as intermediates [44] are based largely on the so-called radical pair mechanism, which associates the principles of spin conservation with chemical reaction, so that the R P' s electron spin alignment, triplet or singlet, determines the chemical reactivity towards recombination. For example, the recombination product being usually diamagnetic means that R Ps with singlet spin are reactive whereas R Ps with triplet spin are not, and triplet± singlet spin conversion, e.g., driven by hyper® ne interaction, will cause a change of reactivity.…”