Bond Theory versus Molecular Orbital

I am sure that he, a modest and generous person, would register some protest to these observations. Indeed, he always emphasised a balanced view even when allegiances to valence-bond theory versus molecular-orbital theory were almost divisive. His own words were,... 

Valence bond theory versus molecular orbital theory... 

D. Valence Bond Theory versus Molecular Orbital Theory... 

Through its ups and downs, the concept of orbital hybridization is most significative for the perspectives opened out by Quantum Chemistry from about 1930. Let us only mention all the disputes in which it was involved during almost one century with regard to tricky questions of molecular structure (i.e., the supremacy of valence-bond pictures over molecular orbitals or vice-versa, the physical content of the resonance theory, the localization versus delocalization dilemma and so on...). Furthermore, its study gives us a good example of the specificity of the scientific explanation among chemists (i.e., the rejection of reductionism to Physics [1]). 

The generally accepted valence bond and molecular orbital (MO) approach to the bonding of metal isocyanides has been well described in Treichel s review (6), and has been used to rationalize (i) variations in IR stretching frequencies between bonded and nonbonded isocyanides, and (ii) the better Tt-acceptor qualities of aryl versus alkyl isocyanide groups (53,54). In valence bond theory the canonical forms involved in metal isocyanide bonds are... 

For hydrogen bonds weaker than OH 0=C or NH 0=C, theory predicts smaller differences in the X-H bond lengths and negligible changes in the nonhydrogen bonds. TWo models have been studied theoretically with the ab-initio molecular orbital method at the HF/3-21G level of approximation. These are formaldehyde oxime cyclic dimer versus the monomer [378] and formaldehyde hydrazone cyclic dimer versus the monomer [379]. Except for the O-H and N-H bonds, these differences are too small to be detected by single crystal X-ray or neutron diffraction methods, as shown in Fig. 5.2. 

Fig. 5.1. Experimental P-O bond length versus bond overlap population calculated using extended Huckel molecular-orbital theory. See Bartell et al. (1970) for further details (after Bartell et al., 1970 reproduced with the publisher s permission).

Indeed, chemists think of atoms as the building blocks of molecules (and their assemblies), whereas the physically rooted Schrodinger equation thinks of molecules in terms of electrons and nuclei. Another example of such dislocation is the computationally convenient molecular orbital theory versus the chemically more intuitive valence bond theory. In this chapter we will introduce QCT, starting with QTAIM [2, 17, 18]. This theory will serve as a tool to bridge the gap between the numerical emptiness of modem wave functions and the wealth of chemical concepts. In an ideal world, chemical insight can indeed be safely extracted from modem wave functions. If this extraction persistently fails for a chemical concept such as aromaticity, for example, then the concept should be modified or abandoned. 

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