Structure and hyperfine coupling constants of radicals in muon-irradiated single-crystal naphthalene

We have observed muonated cyclohexadienyl-type radicals in a naphthalene single crystal, using the standard transverse-fieldSR technique. Two types of radicals were found, corresponding to muonium addition at the and positions. Owing to the crystal field, the isotropic hyperfine coupling constants show large shifts from the solution values, and there is significant anisotropy in the hyperfine tensors. The results for the radicals are similar to those observed for protonated-hydronaphthyl radicals, but isotope effects are evident. The radicals, whose protonated analogues have not been fully studied, show a more pronounced localized character. Based on the hyperfine tensor directions and geometrical considerations, we have assigned each radical to a specific muon site within the crystal.