| Abstract: | The energy dependence of the fragmentation of a selection of ester enolate ions has been studied by variable, low-energy collision-induced dissociation experiments in the quadrupole collision cell of a hybrid BEQQ mass spectrometer. The dominant fragmentation reactions observed are  where ΔH1 ? ΔH2=PA(RCCO]?) ? PA(?O]?) (PA=proton affinity). The anion of lowest proton affinity is formed preferentially at low internal energies with the yield of the anion of higher proton affinity increasing with increasing internal energy. The CH3OCOCOCH2]? anion derived from methyl pyruvate forms CH3OCO]? by reaction (2); this anion readily fragments to CH3G]?+ CO consistent with a structure represented by a dipole-stabilized cluster of CH3O]? and CO. Comparison of the 8-keV with the 50-eV collision-induced dissociation mass spectra indicated that the average internal energy of the fragmenting ions is considerably lower in the high-energy collisional experiments than it is in the low-energy collisional experiments. |
