Mass Spectrometry and Gas-Phase Chemistry of Bismuth-Oxido Clusters

The mass spectrometric characterisation of bismuth-oxido clusters is described together with tandem mass spectrometric experiments aimed at investigating their gas-phase chemistry. Clusters with different ligand shells (nitrate, salicylates, and methacrylate) and different sizes ({Bi₆ O₈ }, {Bi₂₂ O₂₆ }, and {Bi₃₈ O₄₅ }) were selected for this study. The following aspects were addressed by (tandem) electrospray ionisation Fourier-transform ion-cyclotron-resonance (ESI-FTICR) mass spectrometry: 1) Electrospray ionisation was used successfully for the generation of intact cluster ions, but led to rather complicated distributions of charge states and solvent adducts. The cluster core remained intact. 2) Mass spectrometry was used to follow the cluster growth from {Bi₆ O₈ } to {Bi₂₂ O₂₇ } and finally {Bi₃₈ O₄₅ } clusters. These experiments revealed that careful tuning of the conditions is required to avoid precipitates forming that hamper ESI ionisation. The stages of growth strongly depend on the experimental conditions. 3) Fragmentation experiments in the gas phase performed with the {Bi₂₂ O₂₇ } clusters obtained in the growth experiments resulted in the preferential formation of {Bi₆ O₈ } fragments and point to the particular stability of this cluster and its role in cluster growth. 4) Solution-phase ligand-exchange reactions on {Bi₂₂ O₂₆ } salicylate clusters were qualitatively monitored by mass spectrometry, but were limited by the complicated ion series formed that led to overlapping isotope patterns. 5) A number of fragmentation reactions occurred within the ligand shell when the clusters surrounded by an organic ligand shell were subjected to infrared multiphoton dissociation experiments. 6) For the nitrate clusters, fragmentation within the core was observed yielding {Bi₆ O₈ } fragments together with the corresponding clusters of reduced size.