T. Tsurusawa et S. Iwata, Theoretical studies of structures and ionization threshold energies of water cluster complexes with a group 1 metal, M(H2O)(n) (M = Li and Na), J PHYS CH A, 103(31), 1999, pp. 6134-6141
Water cluster complexes with a group 1 metal atom, M(H2O), (M = Li and Na),
for n = 3-6 were studied with ab initio MO methods. The singly occupied mo
lecular orbitals (SOMOs) are classified into three types: surface, quasi-va
lence, and semi-internal. It is the isomers of structures with semiinternal
SOMO that are responsible for the observed convergence of the ionization t
hreshold energy. They are the ion-pair complexes, M+(H2O)(m).(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 fr
om the cation M+(H2O)(m). The model also explains why the experimental ioni
zation threshold energy converges at n = 4.