An investigation of the ionization processes occurring in fast atom bombardment (FAB) for perhydro-3a,6a,9a-triazaphenalene, a compound with high proton affinity/basicity and hydride donor properties

Abstract

The ionization processes which take place in fast atom bombardment (FAB) mass spectrometry for perhydro-3a,6a,9a-triazaphenalene, a compound with strong proton affinity/basicity and hydride donor ability, are investigated. Chemical ionization (CI) and electrospray ionization (ESI) were also performed to facilitate elucidation of the FAB ionization mechanisms. At the onset of fast atom bombardment, protonation is favored over hydride abstraction, but upon continued bombardment the intensity of [M − H] + increases relative to [M + H] + and hydride abstraction becomes the dominant process. In CI, protonation is strongly favored with the relative intensity of [M − H] + much lower than that obtained in FAB. Since the reactive ionic species in CI have greater gas phase hydride ion affinities than those proposed for FAB, hydride abstraction in FAB cannot be described predominantly in gas phase terms. A condensed phase ionization model for FAB, where protonation and hydride abstraction occur in solution, can instead explain the difference in relative molecular ion intensities obtained in FAB and CI. ESI data of perhydro-3a,6a,9a-triazaphenalene further enhances the condensed phase ionization model proposed for FAB, as the negligible presence of [M − H] + suggests that beam-induced processes are responsible for the formation of this species in FAB. This interpretation would explain the increase in relative [M − H] + intensity upon continued fast atom bombardment, through the accumulation of beam-induced reaction products in solution. The protonation of perhydro-3a,6a,9a-triazaphenalene in FAB is also proposed to occur predominantly through beam-induced processes, as equilibrium processes in solution should not account for the absolute ion intensities obtained in FAB. The presence of a strong [M + H] + peak in ESI of perhydro-3a,6a,9a-triazaphenalene with methanol as solvent, is thought to be a consequence of the presence of NH 4 + in solution and not to reflect the predominant formation of this species in FAB through equilibrium processes in solution. ESI work with ethylene glycol as solvent supports this interpretation.