Fast-atom bombardment of the cyclic acetals: Evidence indicating the predominant involvement of condensed-phase processes in ionization.
A series of cyclic acetals, the 2-phenyl-l,3 dioxolanes, and their deuterated analogues were studied by electron ionization (EI), chemical ionization (CI), and fast-atom bombardment (FAB) mass spectrometry to gain 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 to a large extent be rationalized in thermodynamic terms by known gas-phase 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 cyclic 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 abstraction 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 reactive beam-induced species in the bulk of solution. Comparison of the results obtained for deuterated acetals with different surface activities support this hypothesis. (J Am Sot Mass.
Regional Center for Mass Spectrometry, Department of Chemistry, University of Montreal, Station A, P. O. Box 6128, H3C 3J7, Montreal, Canada.
G J
GJ
Paul
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S
Bourg
M J
MJ
Bertrand