Gas-phase reactions of hydrated alkaline earth metal ions, M2+(H2O)(n) (M = Mg, Ca, Sr, Ba and n=4-7), with benzene

Gas-phase reactions of hydrated divalent alkaline earth metal ions and benz ene were investigated by electrospray ionization Fourier-transform mass spe ctrometry. Rate constants for solvent-exchange reactions were determined as a function of hydration extent for Mg2+, Ca2+, Sr2+, and Ba2+ clusters con taining four to seven water molecules each. All of the strontium and barium clusters react quickly with benzene. Barium reacts slightly faster than th e corresponding strontium cluster with the same number of water molecules a ttached. For calcium, clusters with four and five water molecules react qui ckly, whereas those with six and seven water molecules do not. Magnesium wi th four water molecules reacts quickly, but not when five through seven wat er molecules are attached. The slow reactivity observed for some of these c lusters indicates that the cation-pi interaction between the metal ion and benzene is partially screened by the surrounding water molecules. The react ivity of magnesium with seven water molecules is intermediate that of the h exa- and pentahydrate and the tetrahydrate. This result is consistent with the seventh water molecule being in the outer shell and much more weakly bo und. The unusual trend in reactivity observed for magnesium may be due to t he presence of mixed shell structures observed previously. These results ar e the first to provide information about the relative importance of cation- pi interactions in divalent metal ions as a function of metal hydration ext ent. Such studies should also provide a model and some insight into the rel ative binding affinities of divalent metal ions to aromatic residues on pep tides and proteins. (C) 2001 American Society for Mass Spectrometry.