An ab Initio Investigation of Lithium Ion Hydration

The structures, frequencies, and interaction energies of small lithium−water clusters, Li(H 2 O) 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(H 2 O) 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(H 2 O) 4 + ](H 2 O) n, n = 4,8). The second solvation sphere of four waters raises the frequency of the Li−O vibration by 18 cm -1 (7%).