An ab initio investigation of lithium ion hydration

The structures, frequencies, and interaction energies of small lithium-water clusters, Li(H2O)(n)(+), n = 1-4, were calculated by ab initio Hartree-Fock theory with small- and medium-sized basis sets (STO-3G, 3-21G, 6-31G*, 6-31G**, 6-31+G*, 6-31+G*(5d), 6-311G*). The interaction energies were corrected for basis set superposition error (BSSE) by Mayer's CHA/CE formalism. The CHA/CE/6-31+G*//HF/6-31+G* level gives an excellent description of the binding energy. The geometry and symmetric stretch frequency for Li(H2O)(4)(+) of 248 cm(-1) (255 cm(-1) expt) are well described at the HF/6-31G* level. The choice of partition of the supermolecule was demonstrated to be of minor importance (+/-4 kJ/mol). The binding energies were rationalized on crowding around the ion and a weakening Li-O interaction. The first ab initio calculation (STO-3G, 3-21G, 6-31G*, 6-31+G*) of a full second-solvation sphere of a metal cation is presented ([Li-(H2O)(4)(+)](H2O)(n), n = 4,8). The second solvation sphere of four waters raises the frequency of the Li-O vibration by 18 cm(-1) (7%).