The first observation of a muonium-carbonyl adduct with a negative muon coupling constant

Muonium atom addition to cyclopent-4-ene-1,3-dione I leads to the adduct ra dical II; in contrast with all other muonium adducts of carbonyl (C=O) comp ounds so far studied, in which the isotropic muon coupling is of positive s ign, the temperature dependence of the muon coupling in II reveals that its sign is negative; this may be explained as a consequence of the electronic 'push-pull' interaction within the MuO-C-C=C-C=O system, which reduces the out-of-plane vibrational amplitude of the muon by increasing the MuO-C/MuO =C partial pi-bond character.