Theoretical Studies of Structures and Ionization Threshold Energies of Water Cluster Complexes with a Group 1 Metal, M(H2O)n (M = Li and Na)

Water cluster complexes with a group 1 metal atom, M(H2O)n (M = Li and Na), for n = 3-6 were studied with ab initio MO methods. The singly occupied molecular orbitals (SOMOs) are classified into three types: surface, quasi-valence, and semi-internal. It is the isomers of structures with semiinternal SOMO that are responsible for the observed convergence of the ionization threshold energy. They are the ion-pair complexes, M+(H2O)2·(H2O) l·(H2O)n-m-l-, and their vertical ionization energies (VIEs) are determined by the local structure of (H2O)n-m-l- and the electrostatic potential from the cation M+(H2O)m. The model also explains why the experimental ionization threshold energy converges at n = 4.