A new hybrid quantum mechanical/molecular mechanical model of solvation is developed and used to describe the structure and dynamics of small fluoride/water clusters, using an ab initio wave function to model the ion and a fluctuating charge potential to model the waters. Appropriate parameters for the water–water and fluoride–water interactions are derived, with the fluoride anion being described by density functional 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 interior compared to the surface structure, in agreement with ab initio studies. An extended Lagrangian treatment of the polarizable water, in which the water atomic charges fluctuate dynamically, is used to study the dynamics of F(H2O)4− cluster. A simulation using a fixed solvent charge distribution indicates principally interior, solvated states for 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 confirm this trisolvated species as a thermally accessible state at room temperature, in addition to the tetrasolvated interior and surface structures. Extension of this polarizable water model to fluoride clusters with five and six waters gave less satisfactory agreement with experimental energies and with ab initio geometries. However, our results do suggest that a quantitative model of solvent polarization is fundamental for an accurate understanding of the properties of anionic water clusters.

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The full topology of L(r), which is defined as minus the Laplacian of the electron density, ∇2ρ, has recently been explored for the water molecule (Coord. Chem. Rev. 2000, 197, 169). In this work, we have investigated the changing topology during the “umbrella” inversion in ammonia. The maxima in L(r) are points of local charge concentration, which can be associated with the electron pairs of VSEPR theory. We examine changes in three valence shell charge concentration (VSCC) and three depletion (VSCD) graphs as a function of the angle between the C 3 axis and a hydrogen. Through the use of planar graphs, the transition mechanisms can be easily rationalized. The previously noted double maxima in L(r), corresponding to the lone-pair of nitrogen, found at the transition state for inversion is shown to persist for geometries distorted considerably from planar. The transitions between structures in the valence shell charge concentration and charge depletion graphs do not occur simultaneously.

Within the framework of quantum chemical topology (QCT) the function L(r), which equals the negative of the Laplacian of the electron density, has been proposed before as a physical basis for the valence shell electron pair repulsion (VSEPR) model. The availability of a new algorithm to integrate property densities over the basins of L(r) enabled a re-evaluation of this physical basis. We optimised a set of nine molecules at B3LYP/6-311+G(2d,p) level and partitioned the corresponding L(r) function for each molecule into basins. For the first time we visualise these basins in L(r), by directly showing their boundaries. We identify the basins in L(r) with the domains of the VSEPR model. Observations drawn from the populations and volumes of L-basins are contrasted with the three subsidiary VSEPR postulates. We find unexpectedly small populations, nearer to one than to two, for non-hydrogen cores and bonding domains, and populations much larger than two for non-bonding domains. We conclude that non-bonding or lone pairs have larger domains than bonding pairs in the same valence shell, in accordance with VSEPR. We also confirm that double and triple bond domains are larger than single-bond domains. However we cannot substantiate the effect of the electronegativity of central atom or ligand on the volume of bonding domains. In summary, the full topology of L(r) supports two out of three subsidiary VSEPR postulates.

A new algorithm for location of the critical points in general scalar fields is described. The new method has been developed as part of an on-going process to exploit the topologic analysis of general 3D scalar fields. Part of this process involves the use of topologic information to seed the critical point search algorithm. The continuing move away from topologic studies of just the electron density requires more general algorithms and the ability to easily "plug in" new functions, for example, the Laplacian of the electron density ( triangle down (2)rho), the Electron Localisation Function (ELF), the Localised Orbital Locator (LOL), the Lennard-Jones function (LJF), as well as any new functions that may be proposed in the future. Another important aspect of the current algorithm is the retention of all possible intermediate information, for example, the paths describing the connectivity of critical points, as well as an ability to restart searches, something that becomes increasingly important when analysing larger systems. This new algorithm represents a core part of a new local version of the MORPHY code. We distinguish nine universal types of gradient paths.