Micelle-induced change of mechanism in the reaction of muonium with acetone

Abstract

Muonium atoms add to the O atom of the carbonyl group of acetone to give the muonated free radical (CH3)2Ċ-O-Mu when the reaction takes place in water or hydrocarbons, but not when the acetone is localized in micelles. Micelles have no effect on the formation of muonated cyclohexadienyl radicals when muonium reacts with benzene under similar conditions. The addition reaction with acetone appears to have been subsumed by a faster alternative reaction in the micellar environment. Evidence is presented for this interpretation rather than for an inhibition of the radical or for a shift in the muon level-crossing resonance spectrum with hydrogen (muonium) bonding, though major shifts are seen for the spectrum of this radical in pure solvents of widely different dielectric constant. It is suggested that muonium's "abstraction" reaction takes over in micelles because significant micelle-induced enhancement effects were previously observed in that type of reaction. The data are consistent with a rate constant for the abstraction reaction of muonium with acetone in micelles of >6 × 108 M−1 s−1. Key words: muonium, kinetic isotope effects, micelle enhancement, H/Mu-addition, H/Mu abstraction.