Structures and fragmentations of electrosprayed Zn(II) complexes of carboxylic acids in the gas phase: Isomerisation versus desolvation during the last desolvation step

Zn²⁺-carboxylate ions formed in methanol/water solutions are transferred in the gas phase by electrospray. At low cone voltage, species observed in the source spectra correspond to solvated Zn²⁺-carboxylates: [RCOOZn, (CH₃OH)_n]⁺ (R=H, CH₃; n=1-3) ions. Under low energy collisions, all ions with n>1 lose exclusively methanol, mimicking some of the last steps of ion desolvation. However, ions with only one molecule of solvent behave differently: [HCOOZn, CH₃OH]⁺ eliminates carbon dioxide and [CH₃COOZn, CH₃OH]⁺ either loses the last molecule of methanol or fragments to give the acylium ion [CH₃CO]⁺. Labelling experiments as well as accurate molecular orbital calculations are used to explain this different behaviour of [RCOOZn, CH₃OH]⁺ ions which fragment (totally or partially) instead of losing the last molecule of solvent. It appears that the loss of the last molecule of solvent from [HCOOZn, CH₃OH]⁺ requires more energy than does its isomerisation into [CO₂, HZn, CH₃OH]⁺, precursor for the loss of CO₂. For [CH₃COOZn, CH₃OH]⁺, isomerisation processes and direct loss of methanol require very similar energies. In both cases, part of the gaseous ions formed after complete desolvation are chemically different from their precursors in solution.