COOPERATIVE EFFECTS IN THE STRUCTURING OF FLUORIDE WATER CLUSTERS - AB-INITIO HYBRID QUANTUM MECHANICAL MOLECULAR MECHANICAL MODEL INCORPORATING POLARIZABLE FLUCTUATING CHARGE SOLVENT/

A new hybrid quantum mechanical/molecular mechanical model of solvatio n is developed and used to describe the structure and dynamics of smal l fluoride/water clusters, using an ab initio wave function to model t he ion and a fluctuating charge potential to model the waters. Appropr iate parameters for the water-water and fluoride-water interactions ar e derived, with the fluoride anion being described by density function al theory and a large Gaussian basis. The role of solvent polarization in determining the structure and energetics of F(H2O)(4)(-) clusters is investigated, predicting a slightly greater stability of the interi or compared to the surface structure, in agreement with ab initio stud ies. An extended Lagrangian treatment of the polarizable water, in whi ch the water atomic charges fluctuate dynamically, is used to study th e dynamics of F(H2O)(4)(-) cluster. A simulation using a fixed solvent charge distribution indicates principally interior, solvated states f or the cluster. However, a preponderance of trisolvated configurations is observed using the polarizable model at 300 K, which involves only three direct fluoride-water hydrogen bonds. Ab initio calculations co nfirm this trisolvated species as a thermally accessible state at room temperature, in addition to the tetrasolvated interior and surface st ructures. Extension of this polarizable water model to fluoride cluste rs with five and six waters gave less satisfactory agreement with expe rimental energies and with ab initio geometries. However, our results do suggest that a quantitative model of solvent polarization is fundam ental for an accurate understanding of the properties of anionic water clusters. (C) 1998 American Institute of Physics. [S0021-9606(98)5043 2-1].