VALENCE-BOND CONCEPTS APPLIED TO THE MOLECULAR MECHANICS DESCRIPTION OF MOLECULAR SHAPES .2. APPLICATIONS TO HYPERVALENT MOLECULES OF THE P-BLOCK

A fascinating aspect of inorganic chemistry is the occurrence of compl icated and varied molecular shapes. However, these same features lead to difficulties in developing molecular mechanics (MM) methods that ar e suitable for inorganic molecules. In this paper we demonstrate that simple valence bond concepts can guide the construction of a new MM fo rce field for hypervalent molecules of the p-block of the periodic tab le. The primary difficulty in applying valence bond concepts to the MM description of hypervalent molecular shapes is the occurence of intri nsically delocalized bonding arrangements, such as the three-center fo ur-electron bond of XeF2. The inclusion of resonating configurations i nto the Mh? method provides a mechanism for surmounting the difficulti es presented by hypervalent molecules. By making the contributions of the individual configurations to the total potential energy function d ependent on the molecular geometry, we find that both equilibrium geom etries and fluxional pathways of hypervalent molecules can be modeled with impressive accuracy. This model, which we call HyperValent Valenc e Bond (HV-VB), is readily extended to hypervalent molecules containin g mixed ligands. By using the valence bond model to derive the HV-VB m ethod, the results of our MM computations indirectly become discrimina ting tests of the basic concepts of the model, The ideas that Pauling first presented more than six decades ago exhibit remarkable robustnes s.