Muon spin rotation (μSR) for characterizing radical addition to C=S in xanthene-9-thione and thioxanthene-9-thione

Muonium (μ⁺e^-) can be produced by capture of an electron by a spin-polarized muon (μ⁺) and has been utilized as a light isotope of hydrogen. In this paper, we demonstrate muon spin rotation (μSR) experiments and preliminary computational analyses of the regioselective muonium addition to xanthene-9- thione and thioxanthene-9-thione providing the C-centered radicals. The transverse-field muon spin rotation (TF-μSR) experiments of the cyclic thioketones revealed considerable increase of muon hyperfine coupling constants (muon hfc, A_μ) compared with the muoniated thiobenzophenone, indicating crucial roles of the bridging chalcogen atom promoting the overlap between the S-Mu bond orbital and the radical p orbital. (author)