FORMATION OF DESTABILISED CARBENIUM IONS BY CHARGE INVERSION OF NEGATIVE-IONS

The alpha-carbomethoxy carbenium ions CH3OC(= O)C+ R1R2 where R1 = R2 = H (1+), R1 = H, R2 = CH3 (2+), R1 = R2 = CH3 (3+) and R1 = H, R2 = C6H5 (4+) have been prepared by collisional charge inversion and by neutralisation-reionisation of the appropriate enolate ions. In addition, the alpha-benzoyl carbenium ions C6H5C(= O)C+ R1R2 where R1 = R2 = H (5+), R1 = H, R2 = CH3 (6+) and R1 = R2 = CH3 (7+) have been prepared by the same methods. In all cases the charge inversion and neutralisation-reionisation mass spectra are very similar. For the alpha-carbomethoxy carbenium ions the major fragmentation reaction involves elimination of CH3O. to form the appropriate ketene molecular ion. The spectra are quite different from those obtained by collision-induced dissociation of alpha-carbomethoxy carbenium ions prepared by electron ionisation. The ions formed by charge inversion fragment to a much lesser extent by rearrangement, presumably because they have a higher mean internal energy. For the alpha-benzoyl carbenium ions formation of the ketene molecular ion is negligible, a major reaction channel being elimination of CR1R2 to form the benzoyl ion. A recovered positive ion signal at the same m/z value as the enolate precursor is observed only for 3+ and 4+, indicating that only the most stable of the destablished carbenium ions will be observed as intact species when formed by charge inversion of negative ions.