The spin-coupled (SC) theory of molecular electronic structure is introduced as the modern development of classical valence bond (VB) theory. Various important aspects of the SC wave function are described. Attention is particularly focused on the construction and properties of different sets of N-electron spin functions in different spin bases, such as the Kotani, Rumer and Serber. Applications of the SC description to a range of different kinds of chemical problems are presented, beginning with simple examples: the H2 and CH4 molecules. This is followed by the description offered by the SC wave function of more complex situations such as the insertion reaction of H2 into CH2(lA1), the phenomenon of hypervalence as displayed by molecules such as diazomethane, CH2N2, SF6 and XeF2. The SC description of the ground and excited states of benzene is briefly surveyed. This is followed by the SC description of antiaromatic systems such as C4H4 and related molecules.
The paper collects the answers of the authors to the following questions: Is the lack of precision in the definition of many chemical concepts one of the reasons for the coexistence of many partition schemes? Does the adoption of a given partition scheme imply a set of more precise definitions of the underlying chemical concepts? How can one use the results of a partition scheme to improve the clarity of definitions of concepts? Are partition schemes subject to scientific Darwinism? If so, what is the influence of a community's sociological pressure in the “natural selection” process? To what extent does/can/should investigated systems influence the choice of a particular partition scheme? Do we need more focused chemical validation of Energy Decomposition Analysis (EDA) methodology and descriptors/terms in general? Is there any interest in developing common benchmarks and test sets for cross‐validation of methods? Is it possible to contemplate a unified partition scheme (let us call it the “standard model” of partitioning), that is proper for all applications in chemistry, in the foreseeable future or even in principle? In the end, science is about experiments and the real world. Can one, therefore, use any experiment or experimental data be used to favor one partition scheme over another? © 2019 Wiley Periodicals, Inc.
Spin-coupled theory is used to investigate the bonding in several hypercoordinate and "normal octet" compounds of main group elements. It is found that d basis functions play much the same qualitative role in hypercoordinate and normal molecules, acting as polarization functions. There are no obvious demarcations in the energy penalty per bond of excluding such functions. No evidence is found to support the traditional notions of spdm hybridization. The spin-coupled approach, also known as the full-GVB model, provides a very clear and simple picture of the bonding in all of the molecules studied. In SF6, for example, the sulfur atom contributes six equivalent, nonorthogonal s p l i k e hybrids, which delocalize onto the fluorine atoms. Each of these two-center orbitals overlaps with a distorted F(2p) function, with the perfect-pairing spin function dominating. The spin-coupled description of PFs is entirely analogous, with remarkably little differentiation between axial and equatorial bonds. A key consideration for all of the hypercoordinate species studied is the polarity of the various bonds. It is suggested that less emphasis than hitherto be placed on the "octet rule" and that the so-called democracy principle be asserted: any valence electron can participate in chemical bonding if provided with sufficient energetic incentive. This idea is pursued for phosphorus and sulphur halides, for XeF2, and for the CHS-, SiHS-, and SiF5-ions. It is argued that there are no significant qualitative differences between the hypercoordinate nature of first-row, second-row, and noble gas atoms in appropriate chemical environments. IntroductionAn extraordinary amount has been written in the past sixty years or so on the question of the nature of the bonding in hypercoordinate (or hypervalent) molecules such as SF6 and PFs.
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