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An ab initio form of classical valence-bond theory

โœ Scribed by Roy McWeeny

Publisher
John Wiley and Sons
Year
1999
Tongue
English
Weight
181 KB
Volume
74
Category
Article
ISSN
0020-7608

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โœฆ Synopsis

In this work the classical valence-bond VB theory of the 1930s is recast ลฝ . in a fully ab initio modern form. The basic premises are simply i that valency is associated with singly occupied orbitals on the constituent atoms of a molecule, more ลฝ . tightly bound electrons being assigned to a ''core,'' ii that the valence electrons may be ลฝ . well described using resonance mixtures of alternative spin-paired structures; and iii that the structures used are not confined to a single orbital configuration but may include ลฝ . those of ''ionic'' type in which some orbitals are doubly occupied and others are empty and indeed spin-coupled functions based on arbitrary selections of nonorthogonal orbitals. The resultant multiconfiguration VB theory can thus accommodate all the basic pictorial concepts introduced by Pauling, Wheland, and others in the early days of quantum chemistry. The approach is mathematically simple, here being derived from first principles, and permits not only the optimization of structure coefficients but also that of the forms of both core and valence orbitals. An illustrative example, in which the benzene and pyridine molecules are discussed, along with their positive and negative ions, confirms the generality and feasibility of the approach.

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