Ab Initio Study of the CH3F···H2O Complex

Abstract

The CH3F···H2O complex has been studied using both the supermolecule approach through fourth-order Møller−Plesset perturbation theory (MP4) and perturbation theory of intermolecular forces. Nine configurations have been examined, seven of which were found to be attractive. The global minimum occurs when a bent C−F···H−O hydrogen bond is formed with the C···O distance of 6.15 a0 and the water molecule in the same plane as the hydrogen bond. The binding energy for this geometry is equal to 5291 μEh (3.32 kcal/mol) at the MP4 level of theory. When bond functions are included in the basis set, this configuration is further stabilized to 5739 μEh (3.60 kcal/mol). The two configurations where a hydrogen atom of water is closest to the carbon atom of fluoromethane are repulsive at all distances examined due to electrostatic interactions. The increase of the magnitude of the binding energy when the basis set includes bond functions is primarily due to increased attractiveness of dispersion energy. The electrostatic interaction is the most significant energy component for all seven attractive configurations at their radial minima, particularly for configurations where the C−F bond points toward the H2O molecule. The exchange and dispersion energies are, respectively, the second and third most important contributions to the interaction energy for the seven attractive configurations at their radial minima. The MP2 interaction energy is found to approximate the MP4 interaction energy qualitatively, but underestimates the attraction of the seven attractive configurations at their optimal intermolecular separations by 8−82 μEh. A model potential for the CH3F···H2O system has been developed.