INTERACTIONS OF METAL-IONS WITH WATER - AB-INITIO MOLECULAR-ORBITAL STUDIES OF STRUCTURE, BONDING ENTHALPIES, VIBRATIONAL FREQUENCIES AND CHARGE-DISTRIBUTIONS - 1 - MONOHYDRATES

The formation and properties of a wide range of metal ion monohydrates , Mn+-OH2, where n = 1 and 2, have been studied by ab initio molecular orbital calculations at the MP2(FULL)/6-311++G*//MP2(FULL)/6-311++G* and CCSD(T)(FULL)/6-311++G**//MP2(FULL)/6-311++G** computational lev els. The ions M are from groups 1A, 2A, 3A, and 4A in the second, thir d, and fourth periods of the periodic table and the first transition s eries. Structural parameters, vibrational frequencies, bonding enthalp ies, orbital occupancies and energies, and atomic charge distributions are reported. Trends in these properties are correlated with the prog ressive occupancy of the s, p, and d orbitals. Except for K+-OH2 and C a2+-OH2, the O-H bond lengths and HOH angles are greater in the hydrat es than in unbound water. The M-O bond lengths decrease proceeding fro m group 1A --> 4A but become larger in proceeding from the second --> fourth period. The bonding enthalpies, Delta H(298)degrees, are found to be inversely linearly dependent on the M-O bond length Mn+ accordin g to equations of the form Delta H(298)degrees = A + B(1/M-O) for n = 1 and n = 2. Within each monohydrate the distribution of atomic charge reveals a small but definite transfer of charge from water to the met al ion. Compared to unbound water there is, in a metal-ion-bound water complex, an increase in the electronic (negative) charge on the oxyge n atom, accompanied by a (significantly) larger decrease in the electr onic charge on the hydrogen atoms. The bonding of the water molecule, although electrostatic in origin, is thus more complex than a simple i nteraction between a point charge on the metal ion, and the water dipo le.