Experimental Binding Energies for the Metal Complexes [Mg(NH3)n]2+, [Ca(NH3)n]2+, and [Sr(NH3)n]2+ for n=4-20 Determined from Kinetic Energy Release Measurements

A supersonic source of clusters has been used to prepare neutral complexes of ammonia in association with a metal atom. From these complexes the following metal-containing dications have been generated: [Mg(NH3)(n)](2+), [Ca(NH3)(n)](2+), and [Sr(NH3)(n)](2+), and for n in the range 4-20, kinetic energy release measurements following the evaporation of a single molecule have been undertaken using a high resolution mass spectrometer. Using finite heat bath theory, these data have been transformed into binding energies for individual ammonia molecules attached to each of the three duster systems. In the larger complexes (n > 6) the results exhibit a consistent trend, whereby the experimental binding energy data for all three metal ions are very similar, suggesting that the magnitude of the charge rather than charge density influences the strength of the interaction. From a comparison with data recorded previously for (NH3)(n)H+ it is found that the 2+ charge on a metal ion has an effect on the binding energy of molecules in complexes containing up to 20 solvent molecules. Although subject to comparatively large experimental errors, the results recorded for Ca2+ and Sr2+ when n <= 6 show evidence for the formation of an inner solvation shell containing up to 6 molecules. However, Mg2+ exhibits relatively low binding energies when n = S and 6, which suggests that a second shell starts to form before there are 6 ammonia molecules bound to the metal ion. This conclusion is supported by DFT calculations and it is proposed that these complexes could take the form [Mg(NH3)(4)(NH3)](2+) when n = 5 and either [Mg(NH3)(4)(NH3)(2)](2+) or [Mg(NH3)(5)(NH3)](2+) when n = 6. In each case, additional molecules are hydrogen bonded to one or more molecules in the inner solvation shell.