Electrostatic potential as a harbinger of cation coordination: CF₃SO₃- Ion as a model example

Abstract

The topography of molecular electrostatic potential (MESP) has been proposed as a predictive tool for studying ion-pair formation, using the trifluromethanesulfonate (triflate) ion, CF₃SO₃^- (Tf^-), as a test example. A model based on electrostatic docking of the cation (Li⁺, Na⁺, NH₄⁺) has been developed for studying the different possible coordinating geometries for these ion pairs. The interaction energies of different ion-pair structures estimated by the electrostatic docking model have been found to reproduce about 85% of the corresponding ab initio interaction energies. Furthermore, the geometries predicted by the present model indeed provide a good initial guess for a subsequent ab initio Hartree-Fock calculation. Ab initio local minima are usually found close to the respective geometries derived from the electrostatic docking, the bidentate structure being the most favored one. The topography-based approach thus seems to offer insights into primitive patterns of ion-pair formation. Electron correlation and the expansion of the basis set do not bring about any significant change in the MESP topography of the anion. The interionic bonds in the NH₄⁺Tf^-, however, are relatively sensitive to the electron correlation.