FAST-ATOM-BOMBARDMENT OF THE CYCLIC ACETALS - EVIDENCE INDICATING THEPREDOMINANT INVOLVEMENT OF CONDENSED-PHASE PROCESSES IN IONIZATION

A series of cyclic acetals, the 2-phenyl-1,3-dioxolanes, and their deu terated analogues were studied by electron ionization (EI), chemical i onization (CI), and fast-atom bombardment (FAB) mass spectrometry to g ain insight into the primary ionization processes for these compounds in FAB/liquid secondary ion mass spectrometry. Comparison of EI and CI data with that of FAB led to the conclusion that the predominant [M - H]+ ion observed in FAB for the nondeuterated cyclic acetals cannot t o a large extent be rationalized in thermodynamic terms by known gas-p hase ion-molecule reactions. Instead, a condensed-phase model in which the, multicharged transition state for hydride abstraction is better solvated than the transition state for proton transfer appears to be a plausible explanation for the FAB data obtained for the nonlabeled cy clic acetals; however, this explanation is not entirely sufficient to rationalize the FAB data for the deuterated cyclic acetals. For these compounds, a dramatic time dependence of protonation versus hydride ab straction is observed that suggests that beam-induced reactive species are responsible for hydride abstraction in the condensed phase. This time dependence can be interpreted in terms of a buildup of highly rea ctive beam-induced species in the bulk of solution. Comparison of the results obtained for deuterated acetals with different surface activit ies support this hypothesis.