The Angular Overlap Model and Vibronic Coupling in Treating s-p and d-s Mixing – a DFT Study
The steric and energetic lone pair effect induced by s-p mixing can be parameterised within the angular overlap model (AOM), as is shown for a series of halide molecules and complexes mainly of the fifth main group (oxidation state +III) with the coordination numbers 3 to 6. The derived effective AOM parameters espeff, defined for the average bond lengths of the distorted structures, decrease the softer the considered entity is, and seem to be transferable within groups of chemically related species. We base the analysis on results of DFT calculations, using specifically Kohn-Sham MO diagrams in comparison to corresponding many-electron energies. There is definite experimental evidence for s-p and for d-s mixing as well, as is demonstrated by the DFT analysis of spectral data for some model complexes. The equivalence of the vibronic coupling approach on the MO level with the AOM is emphasized; the advantage of the AOM is, however, that angular and radial (bond length) contributions can be cleanly separated.
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