Competitive gas-phase solvation of alkali metal ions by water and methanol

On the basis of mass spectrometric measurements, the Gibbs energies Delta G(x,y;x-1,y+1) for successive ligand exchange equilibria MW(x)A(y)(+) + A reversible arrow MW(x-1)A(y+1)(+) + W (W = H2O, A = CH3OH) were determined for M = Li (n = x + y = 2, 3, 4), Na (n = 2, 3, 4), K (n = 1, 2, 3), Rb (n = 1, 2), and Cs (n = 1, 2). The exchange equilibria were established in a "high"-pressure ion source at 10 Torr of bath gas (N-2) containing water and methanol in the millitorr range and using MA(n)(+) ions produced by electrospray. The corresponding entropies, Delta S-x,S-y;x-1,S-y+1, were obtained from estimates based on theoretical calculations, and these values together with Delta G(x,y;x-1,y+1) led to the enthalpies, Delta H-x,H-y;x-1,H-y+1. Ab initio computations of Delta H-x,H-y;x-1,H-y+1 for Li+ and Na+ systems were found to be in excellent agreement with the experimental values. Replacement of a water molecule with methanol is exothermic for small ion-solvent clusters, but the preferential takeup decreases with the total number of ligands and from Li+ to Cs+. This is ascribed to the increase in distance between the metal ion and the ligands, which increases the importance of the larger permanent dipole moment of water, relative to the larger polarizability of methanol.