Multiquantum resonances in low-field optically detected ESR of radical-ion pairs

Spin-locking in low-frequency reaction yield detected magnetic resonance

The purported effects of weak magnetic fields on various biological systems from animal magnetoreception to human health have generated widespread interest and sparked much controversy in the past decade. To date the only well established mechanism by which the rates and yields of chemical reactions are known to be influenced by magnetic fields is the radical pair mechanism, based on the spin-dependent reactivity of radical pairs. A diagnostic test for the operation of the radical pair mechanism was proposed by Henbest et al. [J. Am. Chem. Soc., 2004, 126, 8102] based on the combined effects of weak static magnetic fields and radiofrequency oscillating fields in a reaction yield detected magnetic resonance experiment. Here we investigate the effects on radical pair reactions of applying relatively strong oscillating fields, both parallel and perpendicular to the static field. We demonstrate the importance of understanding the effect of the strength of the radiofrequency oscillating field; our experiments demonstrate that there is an optimal oscillating field strength above which the observed signal decreases in intensity and eventually inverts. We establish the correlation between the onset of this effect and the hyperfine structure of the radicals involved, and identify the existence of 'overtone' type features appearing at multiples of the expected resonance field position.

Magnetic field effects in chemical systems

Chemical reactions that involve radical intermediates can be influenced by magnetic fields, which act to alter their rate, yield, or product distribution. These effects have been studied extensively in liquids, solids, and constrained media such as micelles. They may be interpreted using the radical pair mechanism (RPM). Such effects are central to the field of spin chemistry of which there have been several detailed and extensive reviews. This review instead presents an introductory account of the field of spin chemistry, suitable for use by graduate students or researchers who are new to the area. It proceeds by giving a brief historical overview of the development of spin chemistry, before introducing the essential theory. This is then illustrated by application to a series of recent developments in solution-phase magnetic field effects (MFEs). The closing pages of this review describe the role played by spin chemistry in the remarkable magnetic compass sense of birds and other animals.

A study of spin chemistry in weak magnetic fields

This paper reviews the latest developments in the field of spin chemistry with a particular focus on the effects of weak static and/or oscillating magnetic fields (typically smaller than the average hyperfine coupling) on radical recombination reactions. Anisotropic magnetic field effects and their significance in the debate about potential mechanisms controlling magnetoreception in birds are discussed.