Study of the structure of molecular complexes. III. Energy surface of a water molecule in the field of a fluorine or chlorine anion

The interaction between F− or the Cl− anion and water has been computed in the Hartree-Fock approximation with a large basis set of Gaussian functions: The obtained energies and wavefunctions are therefore close to the Hartree-Fock limit. We find that the geometrical configuration of maximal stability corresponds to a hydrogen-bonded configuration between the water and the anion; however, the hydrogen bond is somewhat bent with a deviation from linearity of about 4.5° for F−–H2O and about 14.6° for Cl−–H2O. At the most stable configuration the oxygen-anion distance is 4.75 a.u. for F− and 6.26 a.u. for Cl−. At the equilibrium configuration the Hartree-Fock binding energy is 23.54 and 11.86 kcal/mole for the F−–H2O and the Cl−–H2O complex, respectively. The energy surfaces are analyzed in terms of an energy partitioning consisting of the energy of the water molecule in the field of the anion, the anion in the field of the molecule of water, and of the interaction energy between the water molecule and the anion.