The effect of hydration on the electronic structure and stability of the superalkali cation Li₃

The interaction of the superalkali cation Li3+ with water molecules, as well as the structures and stability of the resulting water complexes are theoretically studied at the MP2/6-311++G(d,p) level. A great number of geometrical configurations were obtained for the Li3+(H2O)n (n = 1-5) complexes and Li3+ is found to have a maximum coordination number of four. Natural population analysis shows that the charge distribution of Li3+ becomes seriously uneven upon interaction with five water molecules, so it loses ring conjugation and splits in the lowest-energy isomer of Li3+(H2O)5. Localized molecular orbital energy decomposition analysis indicates a dominant contribution of electrostatic interactions to the binding of water molecules to Li3+, which is similar to the case of lithium ion hydrates. However, as the number of water ligands reaches five, the contribution of the exchange-repulsion energy exhibits a sharp increase and even exceeds that of the electrostatic term.