Formation, acidity and charge reduction of the hydrates of doubly charged ions M2+ (Be2+, Mg2+, Ca2+, Zn2+)

There are two methods for producing in the gas phase doubly charged metal i on hydrates, M(H2O)(n)(2+) (or other ion ligand L MLn2+ complexes). In the clustering method, one starts with the naked ion M2+, and in the presence o f a third (bath) gas and water vapor, the ion hydrates form by ion-molecule clustering reactions. The second method is based on electrospray with whic h a spray of aqueous solutions containing the dissolved salts M2+ + 2X(-), leads to gas phase M(H2O)(n)(2+) with a distribution around n approximate t o 8. For M, which has a high second ionization energy, IE(M2+), both method s can fail to produce a full range of hydrates with a given n, because of t he interference of a charge reduction reaction which involves intramolecula r proton transfer. This reaction becomes possible at n = 2; (M(H2O)(2)(2+)) * = MOH+ + H3O+, and competes with the simple ligand loss: (M(H2O)(2)(2+))* = M(H2O)(2+) + H2O. The thermally excited (M(H2O)(2)(2+))* results in the clustering method by the exothermicity of the forward clustering reaction a nd in the electrospray method by the thermal declustering required to produ ce lower n ions. Ab initio calculations are presented for the energies of t he above reactions and transition states for Mg2+ and Ca2+. These show that the transition state for the charge reduction reaction is much lower than that for the simple ligand loss at n = 2. However, as n increases, the two transition states move closer together and above a given n = r, simple liga nd loss becomes dominant. The capabilities and limitations of the two metho ds to produce hydrates of a given n is discussed. Experimental results illu strate competing charge reduction and simple H2O loss for Be(H2O)(n)(2+) un der thermal equilibrium conditions at n approximate to 9. Charge reduction reactions when occurring in the forward clustering direction can be viewed as proton transfer reactions to the incoming H2O molecule. These can be gen eralized by examining the proton affinities of the MOH(H2O)(n)(+) ions, whi ch are obtained by ab initio calculations. Proton transfer from M(OH)(2))(n )(2+) can be induced not only by H2O but also by other bases B. Experimenta l results for the deprotonation of Zn(OH2)(n)(2+), n = 8 or 9, by NH3 are p resented. The charge reduction reactions by which a deprotonated ligand att ached to M is formed, can have synthetic utility. Examples are given for th e production of methylthiolate complexes which may be useful for modeling i on complexes in which one of the ligands is the deprotonated amino acid res idue cysteine. (Int J Mass Spectrom 185/186/187 (1999) 685-699) (C) 1999 El sevier Science B.V.