Ab initio study of the CH3F center dot center dot center dot H2O complex

The CH3F...H2O complex has been studied using both the supermolecule approa ch through fourth-Order Moller-plesset perturbation theory (MP4) and pertur bation theory of intermolecular forces. Nine configurations have been exami ned, seven of which were found to be attractive. The global mini-mum occurs when a bent C-F...H-O hydrogen bond is formed with the C...O distance of 6 .15 a(0) and the water molecule in the same plane as the hydrogen bond. The binding energy for this geometry is equal to 5291 muE(h) (3.32 kcal/mol) a t the. MP4 level of theory. When bond functions are included in the basis s et, this configuration is further stabilized to 5739 muE(h) (3.60 kcal/mol) . The two configurations where a hydrogen atom of water is closest to the c arbon atom of fluoromethane are repulsive at all distances examined due to electrostatic interactions. The increase of the magnitude of the binding en ergy when the basis set includes bond functions is primarily due to increas ed attractiveness of dispersion energy. The electrostatic interaction is th e 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 inte raction energy for the seven attractive configurations at their radial mini ma. The MP2 interaction energy is found to approximate the MP4 interaction energy qualitatively, but underestimates the attraction of the seven attrac tive configurations at their optimal intermolecular separations by 8-82,muE (h). A model potential for the CH3F...H2O system has been developed.