Study of the Structure of Molecular Complexes. I. Energy Surface of a Water Molecule in the Field of a Lithium Positive Ion

Abstract

In the Introduction we have outlined the technique adopted in this paper (and in others of this series to follow) in order to elucidate the structure and energetics of water molecules around positive ions. The raw data to be analyzed are obtained from the Hartree-Fock model. We have started by considering the H2O molecule making use of the bond energy analysis formalism. Next we report Hartree-Fock data for the Li+–H2O energy surface keeping the H2O geometry rigid. These data are analyzed in the same way as for the H2O molecule. In addition, the energy of rotation of the H2O molecule for a fixed Li–O distance has been studied in detail. Analytical expressions aimed at reproducing the energy surface data in a very simple and condensed form are presented. Finally, the effect of complete optimization of the geometry and use of a large basis set for Li+–H2O energy calculations is discussed. The most stable configuration for the Li+–H2O complex is planar and is predicted to have the following geometry: The Li–O and O–H bond lengths are 1.84 and 0.96 Å, respectively, the Li–O–H and H–O–H angles are 127.0° and 106.1°, respectively. The binding energy of the H2O molecule to the Li+ ion is 1.53 eV in the Hartree-Fock approximation.