A physical basis for the VSEPR model of molecular geometry

Bader RFW, Gillespie RJ, MacDougall PJ (1988) A physical basis for the VSEPR model of molecular geometry. J Am Chem Soc 110(22) 7329-7336... 

AIM theory provides a physical basis for the theory of Lewis electron pairs and the VSEPR model of molecular geometry. Equipped with computers and computer-generated, three-dimensional electron density maps, scientists are able to view molecules and predict molecular phenomena without even having to get off their chairs ... 

The Laplacian of the electron density plays a dominant role throughout the theory.191 In addition, Bader has shown that the topology of the Laplacian recovers the Lewis model of the electron pair, a model that is not evident in the topology of the electron density itself. The Laplacian of the density thus provides a physical valence-shell electron pair repulsion (VSEPR) basis for the model of molecular geometry and for the prediction of the reaction sites and their relative alignment in acid-base reactions. This work is closely tied to earlier studies by Bader of the electron pair density, demonstrating that the spatial localization of electrons is a result of a corresponding localization of the Fermi correlation hole. 

It must be reemphasized that hybridization is only a conceptual and a mathematical model that allows us to calculate molecular parameters. Changing hybridization is simply modifying that original model to suit a current need, just as the concept of h)4)ridization represents only a change to the model of atomic energy levels. Variable hybridization should not be considered more fundamental than the VSEPR model, just more mathematical. The ability to make quantitative predictions of molecular geometry and physical properties makes the variable hybridization model quite useful for some problems. On the other hand, the VSEPR model is also valuable as an intuitive basis for qualitatively correct predictions. As is so often the case, we need not decide which of two complementary models to adopt for all situations we need only to determine which best serves our purposes in a particular case. 

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