Investigations of the Structure and Dynamics of Novel Muoniated Radicals McKenzie, Iain
Muonium (Mu) can be considered as a light isotope of hydrogen in which the nucleus is a positive muon. Radicals labelled with Mu are used to probe the effect of isotopic substitution on the structure and dynamics of radicals. Novel muoniated radicals have been produced in which the muon is attached to the radical centre (muoniated radical) or the unpaired electron is in a (α orbital. The CH₂Mu and CD₂Mu isotopmers of the methl radical were produced by the reaction of Mu with ketene and deuterated ketene, respectively, and the hyperfine coupling constants (hfcs) were measured between approximately 80 and 230 K. The temperature dependence of the hfcs is due to interaction with the solvent which dominated by the spin-rotation mechanism. The reduced muon hfc (given by the muon hfc divided by the ratio of the muon and proton gyromagnetic ratios) is 3% larger than the proton hfc, consistent with the larger zero-point energy of the C-Mu bond compared with the C-H bond. An additional radical was observed just below the boiling point of ketene. Investigation revealed that this radical resulted from Mu addition to the ketene dimer, which forms at temperatures above 200 K. Alpha-muoniated radicals were produced by the reaction of Mu with stable singlet carbenes. Four stable carbenes (1,3-bis(isopropyl)-4,5-dimethylimidazol-2-ylidene, 1,3bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene, 1,3,4-triphenyl-4-5-dihydro-1H-1,2,4-triazol-5-ylidene and 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene) were studied, and in every case Me added exclusively to the carbeneic carbon. The radicals were identified by a comparison of the experimental hfcs with values obtained from density functional theory calculations. The temperature dependence of the muon hfcs cannot be accounted for by only considering vibrational motion at the radical centre. The N-muono-N-tert-butylcarbamoyl and N-muono-N-tert-butylthiocarbamoyl radicals were generated by the reaction of Mu with tert-butylisocyanate and tert-butylisothiocyanate. These radicals have the unpaired electron in a σ orbital and are the first acyl radicals to be observed directly by μSR (muon spin, rotation, resonance and relaxation spectroscopy).
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