Stability and reactivity of hydrated magnesium cations

Unimolecular fragmentation and bimolecular reactions with HCl of water clusters which nominally contain Mg⁺ cations were studied in an FT-ICR spectrometer. A cluster fragmentation and successive evaporation of single water molecules occurring on a millisecond timescale and driven by ambient black body radiation is triggering interesting intracluster reactions. Below a certain critical size (~17 water molecules) MgOH⁺ forms, and a hydrogen atom is ejected. Similarly bimolecular reactions of Mg_aq⁺ clusters with HCl result in a release of H atom and formation of MgCl_aq⁺. Both findings can be rationalized by assuming that the solvated Mg⁺ cations actually detach an additional electron forming a Mg_aq²⁺ and e_aq^- within clusters with more than 17 water molecules. Mg⁺ formed by recombination when not enough solvent is available to stabilize the separate charged species then reacts with a water molecule resulting in H-atom formation. Detailed studies of the ion reactions and fragmentation provide additional insights into the structure and stability of solvated magnesium cations.