Chemical bonding theoretical idea

The aim of the series is to present the latest fundamental material for research chemists, lecturers and students across the breadth of the subject, reaching into the various applications of theoretical techniques and modelling. The series concentrates on teaching the fundamentals of chemical structure, symmetry, bonding, reactivity, reaction mechanism, solid-state chemistry and applications in molecular modelling. It will emphasize the transfer of theoretical ideas and results to practical situations so as to demonstrate the role of theory in the solution of chemical problems in the laboratory and in industry. 

Of all the concepts used in chemistry, that of the chemical bond is one of the most useful and, at the same time, one of the most difficult. It is useful because it helps us to understand the structures of compounds and their properties, and it is difficult because it is not easy to relate it to the physical theories, such as quantum mechanics, that underlie chemistry. This is not to say that people have not attempted to find a connection between the chemical bond and quantum mechanics. The Lewis (1923) electron pair model and the orbital overlap model (Coulson 1961) are, perhaps, among the better known attempts, but all are a posteriori rationalizations, trying to explain the properties of the empirical nineteenth-century chemical bond in terms of twentieth-century physical concepts. It is unlikely that, left to themselves, theoretical chemists in the twentieth century would have ever created the idea of a chemical bond had not the concept already been central to the language of structural chemistry. To this day the chemical bond remains largely an empirical concept. 

Of course there was a reason for the retention of resonance-theoretic explanations in text-books such explanations offered conceptual simplicity and connection with the classical chemical-bonding ideas. Moreover starting in the 1970s and more especially so in the 1980s there turned out to be dramatic... 

There are a number of frequently occurring ideas in VB theory which often are used in slightly different manners by different authors, so that it may be well to try to formulate such ideas in a clear and precise maimer, at least for use in this present chapter. These ideas include that of VB bases, of VB diagrams (especially those more readily relatable to classical chemical bonding ideas), of VB models, of VB-theoretic solutions (or wave-functions for various models), of resonance, of resonating VB wave-functions, etc. 

Theoretical chemistry has two problems that remain unsolved in terms of fundamental quantum theory the physics of chemical interaction and the theoretical basis of molecular structure. The two problems are related but commonly approached from different points of view. The molecular-structure problem has been analyzed particularly well and eloquent arguments have been advanced to show that the classical idea of molecular shape cannot be accommodated within the Hilbert-space formulation of quantum theory [161, 2, 162, 163]. As a corollary it follows that the idea of a chemical bond, with its intimate link to molecular structure, is likewise unidentified within the quantum context [164]. In essence, the problem concerns the classical features of a rigid three-dimensional arrangement of atomic nuclei in a molecule. There is no obvious way to reconcile such a classical shape with the probability densities expected to emerge from the solution of a molecular Hamiltonian problem. The complete molecular eigenstate is spherically symmetrical [165] and resists reduction to lower symmetry, even in the presence of a radiation field. 

After finishing his first paper on the nature of the chemical bond, in 1931, Linus Pauling stopped basing his ideas on mathematical proofs. Chemists, he understood, were not trained to appreciate the difficult mathematics of quantum physics. To communicate with them, he developed his own theoretical style, made up in equal parts of a broad application of Erwin Schrodinger s wave mechanics, structural data from X-ray crystallography, other laboratory results from across the field of chemistry, and Pauling s own insights. 

For most theoretical chemists, debates about the theory of chemical bonding center around the relative merits of the several named theories and their variants. But there is another viewpoint, from which the strongest theory of chemical bonding is not a named numerical theory like any of those discussed so far but a robust graphical calculus that involves drawing and manipulating lines between pairs of atoms in the structural formulas of molecules. The initial idea is that such a line means a pair of valence electrons shared between the atoms at the ends of the hond. This basic idea then undergoes a number of extensions. 

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