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The isomerization process between ionized phenol and ionized cyclohexadienone is studied by performing ion/molecule reactions with several alkyl nitrites in a hexapole collision cell inserted in a six-sector mass spectrometer. The distinction between both isomeric species is readily achieved on the basis of the completely different reactivity patterns observed for them in subsequent reactions. When reacting with alkyl nitrite, ionized phenol undergoes two competitive reactions corresponding to the formal radical substitution of the hydroxylic hydrogen atom by respectively (i) the nitrosyl radical (m/z 123) and (ii) an alkoxyl radical (m/z 138 if alkyl ϭ ethyl). Both reactions were theoretically demonstrated by density functional theory calculations p) ϩ ZPE] to involve hydrogen-bridged radical cations as key intermediates. The ion/molecule reaction products detected starting from ionized cyclohexadienone as the mass-selected ions arise from • OAlkyl, • OH, and NO 2 • radical additions. The occurrence of a spontaneous ring-opening of cyclohexadienone radical ion into a distonic species is suggested to account for the observed ion/molecule reaction products. We also demonstrated that ionized cyclohexadienone is partly isomerized during a proton-transfer catalysis process into ionized phenol inside the Hcell with ethyl nitrite as the base. The molecular ions of phenol generated in such conditions consecutively undergo reactions producing m/z 123 and 138 radical cations. The proposed mechanism is supported by results of quantum chemical calculations. . It is now well established that, in both condensed and gas phases, neutral carbonyl compounds are usually more stable than their enol tautomers. Exceptions to this rough rule are nevertheless observed when the carbon-carbon double bond of the enol becomes conjugated with another functional group such as a second carbonyl group or included in an aromatic moiety (see following text). The remarkable feature when dealing with the ionized species is that they show a reversed stability order. Indeed, as a rule, enol ions are always by far the most stable structures within a set of isomeric radical cations. As a consequence, enol ions often appear as reaction intermediates or as the ultimate ionic product of a fragmentation process. The higher stability of ionized enols with regard to the corresponding ionized carbonyl compounds is explained on the basis of the large difference between the ionization energies (IEs) of the neutral molecules, the IE of the neutral enol being always by far lower than the IE of the corresponding carbonyl compound [2]. The usual considerable difference between the IEs being much larger than the energy difference between both neutral molecules explains the inversion in the stability order after ionization [2].The thermochemical stability of an ionized enol is also associated with a large kinetic stability. In other words, enol ions are generally protected against dissociation or isomerization by significant energy barriers [3]. Consequently, ioni...

By using a combination of mass spectrometric methodologies and density functional theory calculations [DFT/B3LYP/6-311 ϩϩ G(d, p)], it is proposed that the decarboxylation of metastable methyl benzoate molecular ions occurs via distonic and ion-neutral complex (INC) intermediates. The same INC involving a complex between the benzyl radical and protonated carbon dioxide is also generated upon decarboxylation of metastable phenylacetic acid molecular ions. Internal proton transfer within the INC produces in fine a mixture of toluene and isotoluene radical cations. (J Am Soc Mass Spectrom 2006, 17, 807-814)