Muon Spin Rotation/Resonance (μSR) for Studying Radical Reactivity of Unsaturated Organophosphorus Compounds

The positive muon (μ+) can be regarded as a light isotope of proton and has been an important tool to study radical reactions of organic compounds. Recently, muons have been applied to produce short‐lived paramagnetic species from the heavier unsaturated organic molecules including the p‐block elements. This article overviews recent muon spin rotation/resonance (μSR) studies on the phosphorus analogs of alkenes, anthracenes, and cyclobutane‐1,3‐diyls together with the fundamentals of μSR. The acyclic phosphaalkene of P=C and phosphasilenes of P=Si can accept muonium (Mu=[μ+e−]) at the heavier double bonds, and the corresponding radicals have been characterized. The phosphorus atom in 9‐phosphaanthracene, whose P=C double bond is stabilized by the peri‐substituted CF3 groups, predominantly captures muonium to provide the corresponding paramagnetic fused heterocyclic system. The peri‐trifluoromethyl groups are functional to promote the unprecedented light isotope effect of muon providing the planar three‐cyclic molecular structure to consume the increased zero‐point energy. The formally open‐shell singlet 1,3‐diphosphacyclobutane‐2,4‐diyl unit can accept muonium at the (ylidic) phosphorus or the skeletal radicalic carbon, and the corresponding paramagnetic phosphorus heterocycles can be characterized by μSR. The findings on these muoniation processes to the unsaturated phosphorus‐containing compounds will contribute not only to development of novel paramagnetic functional species but also to progress on muon science. The positive muon (μ+) can be regarded as a light isotope of proton and has been useful for study of radical reactions of organic compounds through formation of muonium (Mu=[μ+e–]). This article overviews the Muon Spin Rotation/Resonance (μSR) studies of the phosphorus congeners of alkenes, anthracenes, and cyclobutane‐1,3‐diyls. The hydrogen surrogate enabled characterization of the regioselective radical additions producing the particular paramagnetic organophosphorus molecules.