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 unprecedented reaction with 1,3-dioxolanes which gives rise to a cyclic, resonance-stabilized oxonium ion, formally the product of oxirane (C2H4O) addition to the reagent ion. The structure for the ion-molecule product is supported by multiple-stage mass spectrometric experiments, performed in a pentaquadrupole mass spectrometer, which show the expected fragmentation by C2H4O loss to yield the original reactant acylium ions. The oxonium ions are formed in relatively high abundance in many cases and are observed even when proton-transfer reactions would be expected to occur competitively owing to the acidity of some of the acylium 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 oxonium ions. The reaction with 1,3-dioxolane is also useful in distinguishing the most stable C2H3O+ ion, the acetyl cation, from its two stable isomers, O-protonated ketene and the oxiranyl cation. The thioacetyl cation, the only sulfur analog investigated, also reacts with dioxolane to form the corresponding oxirane addition product, indicating that the C2H4O addition reaction occurs and that it may be useful for identification of the thioacylium class and for the gas-phase generation of sulfur analogs of oxonium ions.