Study of the structure of molecular complexes. V. Heat of formation for the Li⁺, Na⁺, K⁺, F^-, and Cl ^- ion complexes with a single water molecule

In order to obtain the heat of formation ΔH, for the ion-water complexes previously studied in the Hartree-Fock approximation in the first three papers of this series, we have computed the normal frequencies of the complexes, the zero-point energy correction to ΔH, and the molecular extra correlation energy. The main contribution to ΔH is due to the Hartree-Fock binding; the least important contribution results from the correlation effects. The Hartree-Fock binding varies from about 35 kcal/mole (Li+-H2O) to about 12 kcal/mole (Cl--H 2O); the zero-point correction is beween 1 and 2 kcal/mole; and the molecular extra correlation correction is less than 1 kcal/mole. The computation of ΔH is analyzed in order to estimate upper and lower bounds. We conclude that the calculated AH values are accurate to about 2.0 kcal/mole. Experimental data support this conclusion. In the Appendix, the potentials for water-ion complexes have been presented in the form of a simple analytical expansion. The expansion has been obtained by fitting the Hartree-Fock computed energies for the water-ion complexes.