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The basic tenet of the valence shell electron pair repulsion (VSEPR) model, that the Pauli exclusion principle determines the molecular geometry, has been examined by direct ab initio calculations on H2O. The calculations include one-electron models, which have Pauli forces but no electron-electron repulsions, and Hartree models, which have electron-electron repulsion but no Pauli forces. The latter calculations strongly suggest that as the Pauli restriction is removed the H2O angle decreases. This behavior, which is opposite that expected by VSEPR, indicates that the dominant Pauli repulsions in H2O are between the bond pairs. The calculations show that the optimum hybridization is relatively independent of the H-O-H angle. Thus, the hybrids appear to have some intrinsic existence and are not formed solely to provide better O-H bonding and are not always directed toward the hydrogens. The intrinsic hybridization is shown to be controlled by the number of electrons in the valence shell. This result provides a connection between traditional localized valence concepts and Walsh's rule. The driving force for the formation of these hybrids is, of course, the system's desire to keep the lower energy 2s orbital as fully occupied as possible, i.e., as a lone pair. Thus, the stereochemical activity of the lone pairs is, in reality, the stereochemical activity of the 2s orbital. A simple model based on these concepts is useful in predicting the geometry of a variety of AHn systems, particularly why the angles decrease so dramatically when a first-row central atom is replaced with one from the second row, i.e., from H2O toH2S. 1978, American Chemical Society. All rights reserved.
Journal of the American Chemical Society
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