Keto-Enol Tautomerism of Gas-Phase Ions. Structure of Reactive C6H6O Radical Cations

The keto-enol tautomerism of the gas-phase phenol and 1, 3-cyclohexadien-5-one radical cations is postulated to explain the ion chemistry of C6H6O radical cations generated from ethyl phenyl ether and bicyclo[2.2.2]oct-2-ene-5, 7-dione as well as direct ionization of phenol. The C6H6O radical cations generated from these sources were studied by measurement of metastable kinetic energy release for the reaction C6H6O+ →C5H6+ +CO. Contrary to previous reports, it is shown that both phenol and cyclohexadiene ions interconvert if they are sufficiently activated to decompose by undergoing CO loss. The phenol ions isomerize to a keto form by a high-energy sigmatropic [1, 3] hydrogen shift, which is the rate-determining step for CO loss. Because of a large kinetic barrier for the ketonization, a large fraction (~20%) of excess energy in the transition state is released as kinetic energy in the decarbonylation reaction of metastable ions.