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Valence Bond Concepts Applied to the Molecular Mechanics Description of Molecular Shapes. 2. Applications to Hypervalent Molecules of the P-Block

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journal contribution
posted on 26.06.1996, 00:00 by Thomas Cleveland, Clark R. Landis
A fascinating aspect of inorganic chemistry is the occurrence of complicated and varied molecular shapes. However, these same features lead to difficulties in developing molecular mechanics (MM) methods that are suitable for inorganic molecules. In this paper we demonstrate that simple valence bond concepts can guide the construction of a new MM force field for hypervalent molecules of the p-block of the periodic table. The primary difficulty in applying valence bond concepts to the MM description of hypervalent molecular shapes is the occurrence of intrinsically delocalized bonding arrangements, such as the three-center four-electron bond of XeF2. The inclusion of resonating configurations into the MM method provides a mechanism for surmounting the difficulties presented by hypervalent molecules. By making the contributions of the individual configurations to the total potential energy function dependent on the molecular geometry, we find that both equilibrium geometries and fluxional pathways of hypervalent molecules can be modeled with impressive accuracy. This model, which we call HyperValent Valence Bond (HV-VB), is readily extended to hypervalent molecules containing mixed ligands. By using the valence bond model to derive the HV-VB method, the results of our MM computations indirectly become discriminating tests of the basic concepts of the model. The ideas that Pauling first presented more than six decades ago exhibit remarkable robustness.