ABSTRACT
Chemically-induced spin polarization was one of the last truly new physical phenomena in chemistry to be discovered and explained during this century. So unusual were the observations and so ground-breaking the theoretical descriptions that, over a very short time period, the chemist’s way of thinking about free radical reactions and how to study them was fundamentally changed. After the earliest experimental reports of unusual phases of electron paramagnetic resonance (EPR) (1963) [1] and nuclear magnetic resonance (NMR) (1967) [2-4] transitions in thermal, photolytic and radiolytic reactions involving free radical intermediates, it took several years of theoretical development before the idea of the radical pair mechanism (RPM) was put forward to explain the results [5-9]. Gradually, the theory was tested and improved, and additional polarization mechanisms were discovered. The overall physical picture has stood the test of time and now both chemically-induced dynamic nuclear polarization (CIDNP) and its electron analogue (CIDEP) are well understood. The phenomena are exploited by many researchers who are trying to understand the kinetic and magnetic properties (and the links between them) of free radicals, biradicals and radical ion pairs in organic photochemistry, as well as photosynthetic reaction centres and other biologically relevant systems. The high structural resolution of NMR and EPR spectroscopies, combined with recent advances in fast data collection instrumentation and high powered pulsed lasers, has made time-resolved CIDNP and CIDEP experiments some of the most informative in the modern physical chemistry arsenal.
