Hydration energies and structures of alkaline earth metal ions, M2+(H2O)(n), n=5-7, M = Mg, Ca, Sr, and Ba

The evaporation of water from hydrated alkaline earth metal ions, produced by electrospray ionization, was studied in a Fourier transform mass spectro meter. Zero-pressure-limit dissociation rate constants for loss of a single water molecule from the hydrated divalent metal ions, M2+(H2O)(n) (M = Mg, Ca, and Sr for n = 5-7, and M = Ba for n = 4-7), are measured as a functio n of temperature using blackbody infrared radiative dissociation. From thes e values, zero-pressure-limit Arrhenius parameters are obtained. By modelin g the dissociation kinetics using a master equation formalism, threshold di ssociation energies (E-0,) are determined. These reactions should have a ne gligible reverse activation barrier; therefore, E-0 values should be approx imately equal to the binding energy or hydration enthalpy at 0 K. For the h epta- and hexahydrated ions at low temperature, binding energies follow the trend expected on the basis of ionic radii: Mg > Ca > Sr > Ba. For the hex ahydrated ions at high temperature, binding energies follow the order Ca > Mg > Sr > Ba. The same order is observed for the pentahydrated ions. Collis ional dissociation experiments on the tetrahydrated species result in relat ive dissociation rates that directly correlate with the size of the metals. These results indicate the presence of two isomers for hexahydrated magnes ium ions: a low-temperature isomer in which the six water molecules are loc ated in the first solvation shell, and a high-temperature isomer with the m ost likely structure corresponding to four water molecules in the inner she ll and two water molecules in the second shell. These results also indicate that the pentahydrated magnesium ions have a structure with four water mol ecules in the first solvation shell and one in the outer shell. The dissoci ation kinetics for the hexa- and pentahydrated clusters of Ca2+, Sr2+ and B a2+ are consistent with structures in which all the water molecules ale loc ated in the first solvation shell.