When a crystalline 1,3-diphosphacyclobutane-2,4-diyl bearing a 3,5-dichloro-2,4,6-triazine substituent was irradiated with muon beam, two paramagnetic species showing muon hyperfine coupling constants (hfcs) of 4.5 MHz and 6.5 MHz were produced via addition of muonium. These observed paramagnetic species are consistent with the cis and trans isomers of the Cmuoniated 1,3-diphosphacyclobutane-2,4-diyl.

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, Aμ) 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.