The radical formed by muonium addition to diketene has been studied by transverse field muon spin rotation (TF-[small micro]SR) and muon avoided level-crossing resonance ([small micro]ALCR). The TF-[small micro]SR spectrum shows that muonium adds to only one site in diketene, and it is clear from the [small micro]ALCR spectrum that the radical product contains two inequivalent sets of protons. The muon and proton hyperfine coupling constants (hfcs) were determined at several temperatures between 280 and 362 K. The muon hfc falls with increasing temperature, one proton hfc increases, and the other remains constant. The magnitude and temperature dependence of the hfcs support assignment of the 4-muonomethyl-oxetan-2-on-4-yl radical. Density functional calculations were performed to model the temperature dependence of the hfcs. The results are consistent with a preferred conformation of the muoniated methyl group in which the C-Mu bond eclipses the orbital containing the unpaired electron.
