Preamble

Free radicals are recognized to be important reactive intermediates in many branches of chemistry and biology, so all aspects of their formation, structure and reactivity are of widespread interest. Hitherto, most free radical studies have relied on magnetic resonance techniques utilizing the absorption of electromagnetic radiation; but in the muon spin rotation method, one observes the evolution of the muon's spin polarization – as modified by its hyperfine interaction with the unpaired electron in a free radical containing the muon. Already the ability of μSR to study μ+-containing free radicals can be regarded as one of its most valuable assets.

The radicals involved are generally those resulting from formal addition of Mu to a double bond, so the muon is attached to an atom at least one removed from the atom carrying the majority of the unpaired electron density. But because of its unique nuclear moment, the muon probes that electron density differently to protons around it in equivalent positions. There are certainly isotope effects of considerable interest and use – both in the hyperfine coupling constants and in reactivity of Mu compared with H – but the muon is primarily acting here as a distinguishable nuclear probe. Fortunately, the observational timescale of μSR coincides with typical free radical lifetimes in media of prime interest. This allows both structure and kinetics to be studied. In fact one can even draw inference about reaction rates back into the subnanosecond timescale, because of the requirement in μSR of initial phase coherence in the precession of the species observed at times > 10−7s.