Complexation of Li+, Na+, and K+ by water and ammonia

The energetics of complexation and the resulting polarization of solvent mo lecules were investigated for complexation of lithium, sodium, and potassiu m cations with water and with ammonia molecules. The mono-, di-, and tri-co mplexes were investigated using a double-zeta, polarized, diffuse, and bala nced basis at both the molecular and atomic levels, using subsystem quantum mechanics. The stabilization at the molecular level during complexation re sults from large gains in the attractive energies overcoming the increases in repulsion. The cation-solvent interactions are closed-shell and involve only small transfers of electronic charge to the cations. On the atomic lev el, the cations are stabilized by inter-atom attraction, whereas the heavy atoms of the solvent molecules are destabilized by a small withdrawal of ch arge to the cation. Additional solvation continues to stabilize the cations , but each additional solvent molecule is less stabilized because of compet ition among solvent molecules. The solvent molecules are dipole polarized b y a transfer of electronic charge from the bonded hydrogen atoms towards th e heavy atom, a polarization that decreases with each additional solvent mo lecule and with increasing cation size. (C) 2000 Elsevier Science S.A. All rights reserved.