Neglect of Differential Overlap Models

Hiickel models of molecular electronic structure enjoyed many years of popularity, particularly the r-electron variants. Authors sought to extract the last possible amount of information from these models, perhaps because nothing more refined was technically feasible at the time. Thus, for example, the inductive effect was studied. The inductive effect is a key concept in organic chemistry a group R should show a - -1 or a —I effect (according to the nature of the group R) when it is substituted into a benzene ring. 

Wheland and Pauling (1959) tried to explain the inductive effect in terms of ar-electron theory by varying the ax and ySxY parameters for nearest-neighbour atoms, then for next-nearest-neighbour atoms and so on. But, as many authors have also pointed out, it is always easy to introduce yet more parameters into a simple model, obtain agreement with an experimental finding and then claim that the model represents some kind of absolute truth. 

A great failing of the Hiickel models is their treatment of electron repulsion. Electron repulsion is not treated explicitly it is somehow averaged within the spirit of Hartree-Fock theory. 1 gave you a Hiickel jr-electron treatment of pyridine in Chapter 7. Orbital energies are shown in Table 8.1. 

The next step came in the 1950s, with more serious attempts to include formally the effect of electron repulsion between the valence electrons. First came the 7T-electron models associated with the name of Pople, and with Pariser and Parr. You might like to read the synopses of their first papers. 

A Semi-Empirical Theory of the Electronic Spectra and Electronic Structure of Complex Unsaturated Molecules R. Pariser and R. G. Parr 

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Pople, Beveridge and Dobosh introduced the intermediate neglect of differential overlap model (INDO) in 1967. INDO is CNDO/2 with a more realistic treatment of the one-centre two-electron integrals. In the spirit of such models, the non-zero integrals were calibrated against experiment rather than being calculated fi om first principles. The authors concluded that, although INDO was a little better than... 

Anderson, W. P, Cundari, T. R., Zemer, M. C. (1991). An intermediate neglect of differential overlap model for second-row transition metal species. Int. J. Quantum Chem. 

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