Stereochemical activity of s orbitals
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The dependence of the bond angles of AH (n=2, 3, 4, 5, 6) molecules on various energy parameters are studied by extended Hückel theory (EHT). The results suggest that, contrary to the expectations of the valence shell electron pair repulsion (VSEPR) model, the only important Pauli repulsions in “normal” covalent molecules (four or fewer electron pairs) are those between bond pairs. The driving force for bending in a molecule such as H2O is due primarily to the relative np-ns energy separation of the central atom in agreement with our previous ab initio calculations. Thus, H2O bends not because there are lone pair-lone pair or lone pair-bond pair repulsions but because the 2s orbital is lower in energy and the molecule can maximize its occupation only by bending. The np-ns energy separation also controls the degree to which three-center, four-electron (3c-4e) bonds bend toward the two-center, two-electron (2c-2e) bonds in molecules such as ClF3. In these “hypervalent” molecules the Pauli exclusion principle also contributes to the bending. However, this effect does not arise from lone pair-bond pair repulsions but from bond-pair attractions, i.e., the desire to delocalize the “electron-rich” 3c-4e bond into the “electron poor” 2c-2e bond. © 1978, American Chemical Society. All rights reserved.
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