Effective pair potentials, hydrogen bonding

V is due to a large extentto strain relaxation. Other possible contributions to redox potentials are electronic effects [189], specific hydrogen bonding [435], ion-pairing... 

Recognizing the fact that the hydrogen bond occurs only along four directions pointing to the vertices of a regular tetrahedron, we can use the four unit vectors introduced in Section 6.2 (see Fig. 6.2) to construct a HB potential function. This we denote by C/hb(Xi, Xg), so that the full effective pair potential is a superposition of three terms... 

Dipole moments per molecule are substantially enhanced in the chain compared to the dipole moments of the isolated molecules for all three model systems. This enhancement arises as a consequence of mutual polarization of interacting molecules. Unless effective pair potentials are used where this change of molecular properties is incorporated additive models of intermolecular interaction fail in the case of hydrogen bonded systems. 

One of the most important developments in the theory of water is the invention of a realistic effective intermolecular pair potential. Although others had made similar suggestions earlier 59>, it is Stillinger and co-workers 60> who have shown, with the aid of molecular dynamics calculations 3>, how satisfactory such a potential can be. The availability of a water-water potential drastically reduces the scope for parameterization in model based theories, and permits investigation of such concepts as "broken hydrogen bond. We shall frequently have occasion to call upon its properties in the following discussion. 

A solvent, in addition to permitting the ionic charges to separate and the electrolyte solution to conduct an electrical current, also solvates the discrete ions, by ion-dipole or ion-induced dipole interactions and by more direct interactions, such as hydrogen bonding to anions or electron-pair donation to cations. Lewis acidity and basicity of the solvents affect the latter. The redox properties of the ions at an electrode depend on their being solvated, and the solvation effects electrode potentials or polarographic half-wave potentials. 

Recent refinements on the atom-atom potential method include the development of accurate anisotropic model intermolecular potentials from ab initio electron distributions of the molecules. The non-spherical features in these charge distributions reflect features of real molecules such as lone pair and 7t-electron density, and therefore are much more effective at representing key interactions such as hydrogen bonding. 

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