An unusually small muon-electron coupling constant — muonium addition to biacetyl

The room temperature muon-electron coupling constant in the 2-muoxy-3-ketobut-2-yl radical formed by muonium addition to biacetyl (butane-2,3-dione) has been measured by two separate techniques (time-differential transverse field muon spin rotation and 500 MHz rf muon spin resonance spectroscopy) and corresponds to a 1H equivalent of 0.65 MHz, the smallest room temperature α-hydroxyl coupling so far measured in any radical. Its value is examined in the light of ab initio Hartree-Fock calculations and commented on in the context of the electronic structure of the radical and its relation to that of the parent molecule.