Electron-hydrogen bonds and OH harmonic frequency shifts in water cluster complexes with a group 1 metal atom, M(H2O)n (M=Li and Na)

Abstract

The harmonic vibrational frequencies for M(H2O)n (M=Li and Na) are calculated with ab initio molecular orbital (MO) methods. Three types of isomers, surface, quasivalence and semi-internal, exhibit the characteristic frequency shifts ΔνOH. The calculated spectral patterns are related to the geometric conformations around the localized electron {e} in the cluster. The downward shifts of OH frequencies are strongly correlated with the lengthening of OH bond distances, as is well known in the hydrogen bonds. The magnitude of the downward shift is as large as that for the hydrogen bond. The correlation of the shifts with the distance between the center of the electron {e} and the hydrogen atom reveals two types of interaction schemes. The characteristics of the structural unit OH{e}HO in M(H2O)n and its OH modes are common with those found in water anion clusters. The interaction between the localized electron and OH bonds can be called an electron-hydrogen bond.