The nature of the bonding of Li⁺ to H₂O and NH₃; A3 initio studies

Ab initio wavefunctions have been calculated for the complex of Li⁺ with NH₃ and H₂O in order to better characterize the nature of the bonding. Hartree-Fock and generalized valence bond calculations were performed using a double zeta basis plus polarization functions. The binding energies obtained at the GVB level are D_e (Li⁺ - NH₃) = 40.4 kcal/mol and D_e (Li⁺ - H₂O) = 37.6 kcal/mol, in reasonable agreement with experimental values. Model calculations indicate that the Li⁺ - base bond is basically electrostatic. Small basis sets were found to lead to D_e as large as 75 kcal/mol for Li⁺ - NH₃, a significant overestimation. Repulsions due to the Li⁺ core are responsible for keeping the Li⁺ too far away for significant relaxation effects.