GAS-PHASE OXIRANE ADDITION TO ACYLIUM IONS ON REACTION WITH 1,3-DIOXOLANES ELUCIDATED BY TANDEM AND TRIPLE STAGE MASS-SPECTROMETRIC EXPERIMENTS

Acylium ions containing a variety of substituents all undergo an unpre cedented reaction with 1,3-dioxolanes which gives rise to a cyclic, re sonance-stabilized oxonium ion, formally the product of oxirane (C2H4O ) addition to the reagent ion. The structure for the ion-molecule prod uct is supported by multiple-stage mass spectrometric experiments, per formed in a pentaquadrupole mass spectrometer, which show the expected fragmentation by C2H4O loss to yield the original reactant acylium io ns. The oxonium ions are formed in relatively high abundance in many c ases and are observed even when proton-transfer reactions would be exp ected to occur competitively owing to the acidity of some of the acyli um ions studied. This ion-molecule reaction is proposed to serve as a general method for identification and/or trapping of ions of the whole acylium ion class and also for the gas-phase generation of the oxoniu m ions. The reaction with 1,3-dioxolane is also useful in distinguishi ng the most stable C2H3O+ ion, the acetyl cation, from its two stable isomers, O-protonated ketene and the oxiranyl cation. The thioacetyl c ation, the only sulfur analog investigated, also reacts with dioxolane to form the corresponding oxirane addition product, indicating that t he C2H4O addition reaction occurs and that it may be useful for identi fication of the thioacylium class and for the gas-phase generation of sulfur analogs of oxonium ions.