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

Abstract

The energetics of complexation and the resulting polarization of solvent molecules were investigated for complexation of lithium, sodium, and potassium cations with water and with ammonia molecules. The mono-, di-, and tri-complexes were investigated using a double-zeta, polarized, diffuse, and balanced basis at both the molecular and atomic levels, using subsystem quantum mechanics. The stabilization at the molecular level during complexation results 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 level, the cations are stabilized by inter-atom attraction, whereas the heavy atoms of the solvent molecules are destabilized by a small withdrawal of charge to the cation. Additional solvation continues to stabilize the cations, but each additional solvent molecule is less stabilized because of competition among solvent molecules. The solvent molecules are dipole polarized by a transfer of electronic charge from the bonded hydrogen atoms towards the heavy atom, a polarization that decreases with each additional solvent molecule and with increasing cation size.