Beta cleavages of distonic ions, alpha cleavages of enol ions, isomerizations of distonic and enol ions and corresponding reactions of free radicals

This study reports transition state energies for the losses of X from CH2 CHXZ and XCH2CH Z ions and neutrals (X = H or an alkyl radical, Z = a functional group) and for 1,2-shifts by the same species. Ab initio methods were used to characterize methyl losses from CH3CH(CH2)C(OH)(2)(+.) and CH3CH2CHC(OH)(2)(+.). H shifts between and H losses from the alpha and beta carbons of C3H6O2+., and C3H4O+. isomers and the corresponding reactions of ethyl and C2H6N radicals were similarly characterized. Cleavage of bonds to the beta carbons of enolic and related species was found to be slightly more favorable energetically than dissociation from the alpha carbons of isomeric beta distonic ions and of beta radicals. However, dissociation from the alpha carbons of beta distonic radical cations probably also occurs at suprathreshold energies, but at a lower rate than the competing dissociation from the beta carbons of isomeric radical cation enols. Critical energies are similar for corresponding dissociations of the ions and neutrals. Finally. 1.2-shifts in radical cations. including shifts of functional groups, have substantially lower critical energies than do the corresponding reactions of neutral free radicals such that the ions but not the free radicals can isomerize below their dissociation thresholds. In summary, the presence of a charged group appears to facilitate the interconversion of enolic and beta distonic radical cations by 1,2-shifts, but not dissociation of those ions. (C) 2001 Elsevier Science B.V.