Free Radicals Formed by H Atom Addition to Allenes as Determined by Muon Spin Spectroscopy

Allyl and vinyl radicals are important intermediates in diverse areas of chemistry, ranging from combustion to synthesis. However, questions remain about the competitive formation of these radicals from allenes. Here, we present a study of prototypical allyl and vinyl radicals formed by H atom addition to allenes. They were studied by forming the analogous muonium adducts because muonium (Mu) behaves as a light isotope of hydrogen, and muoniated species can be characterized by muon spin spectroscopy (μSR). Two techniques were employed: transverse-field muon spin rotation (TF-μSR) and muon level-crossing resonance (μLCR), which allow for the measurement of muon hyperfine constants (hfcs) and other nuclear hfcs, respectively, and thus aid identification of the formed radicals. TF-μSR has already been used to determine that two radicals are formed by Mu addition to 1,1-dimethylallene, but μLCR techniques were undeveloped at the time of that study, so assignments were based on electron spin resonance (ESR) data of similar allyl and vinyl radicals. We report here the μSR of multiple radicals detected from positive muon irradiation of 1,1-dimethylallene and 1-methoxyallene in solution. The radicals were identified by comparison of muon and proton hfcs with ESR data and the results of density functional theory calculations. The conclusion is that muonium (and by extension, the H atom) can add to all three carbons of the allene system, albeit with preference for the central carbon.