Huckel molecular orbital method bond order

The simplest molecular orbital method to use, and the one involving the most drastic approximations and assumptions, is the Huckel method. One str ength of the Huckel method is that it provides a semiquantitative theoretical treatment of ground-state energies, bond orders, electron densities, and free valences that appeals to the pictorial sense of molecular structure and reactive affinity that most chemists use in their everyday work. Although one rarely sees Huckel calculations in the resear ch literature anymore, they introduce the reader to many of the concepts and much of the nomenclature used in more rigorous molecular orbital calculations. 

In his first publications Huckel has already pointed out the possibility of the calculation of the transition frequencies from the calculated energy levels. An objection to this was, however, that in the so-called zeroth approximation (p. 279) the value of the parameters both in the valence bond and in the molecular orbital method had to be chosen quite different in order to calculate either the resonance energy or the transition frequencies in agreement with observation (Sklar, Forster)26. 

We now work through in detail an example of the application of the Huckel method to a specific molecule in its simplest form, the calculation will be characterised by just two empirical parameters, a and / . Butadiene is selected as the example and the molecular-orbital energies and LCAO combinatorial coefficients calculated for it here will be used in later discussions of other quantities derivable from them (such as charge, bond order and free valence discussed in Chapter Four). Butadiene has four carbon atoms, the cr-bond connectivity of which may conveniently be depicted schematically as in Fig. 2-6. 

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