Competitive charge transfer reactions in small [Mg(H2O)N]2+ clusters

Production of stable hydrated magnesium complexes of the general form [Mg(H2O)(N)](2+) (where 2 less than or equal to N less than or equal to 24) has been possible using the pick-up technique. Observations of ion intensities as a function of N together with data from collision induced dissociation processes (for ions in the range 3 less than or equal to N less than or equal to 10), indicates the existence of a closed solvation shell for N=6 to which additional water molecules are strongly bound. Collision-induced charge transfer in ions of all sizes yields solvated magnesium hydroxide ions Mg+OH(H2O)(N-M-2) accompanied by the loss of a hydronium ion, H3O+, and M water molecules. For N=3, 4, and 5, the above process is seen to be in competition with charge transfer to unprotonated water, and clusters of the general form Mg(H2O)(N-M)(+) are detected, where M now represents the total number of water molecules lost. These two separate loss channels are interpreted as being due to the presence of different structural (or transient) forms of those cluster ions where N less than or equal to 6. One structure corresponds to a highly symmetrical arrangement of the water molecules bonded directly to the magnesium dication, and is responsible for the formation of Mg(H2O)(N-M)(+) ions by charge transfer. In the second type of structure, at least one water molecule moves to an outer solvation shell, but remains hydrogen bonded to a molecule in the first shell. In this latter configuration, it is suggested that the formation of a salt-bridge structure may lower the barrier to proton transfer and lead to the loss of a hydronium ion. (C) 2000 American Institute of Physics. [S0021-9606(00)00714-5].