THEORETICAL-STUDIES ON THE STRUCTURE OF M- ION-PAIRS M = LI+, NH4+ - THE ROLE OF ELECTROSTATICS AND ELECTRON CORRELATION(BF4)

A systematic investigation of the M+BF4- (M = Li or NH4) ion-pair conf ormers has been carried out using an electrostatic docking model based on the molecular electrostatic potential topography of the free anion . This method provides a guideline for the subsequent ab initio molecu lar orbital calculations at the Hartree-Fock (HF) and second-order Mol ler-Plesset perturbation theory (MP2) levels. It has been demonstrated that the model presented here yields more than 75% of the HF interact ion energy when Li+ is the cation involved and more than 90% for the c ase of NH4+. Inclusion of MP2 correlation in the HF-optimized geometri es leads to stationary point geometries with different numbers of imag inary frequencies and in some places where the energies of two adjacen t conformers are very close, the energy rank order is altered. The HF lowest-energy minima for the Li+BF4- and NH4+BF4- show a bidentate and tridentate coordinating cation, respectively, whereas at the MP2 leve l, this ordering is reversed.