Azines molecular orbital energies

The present article is an attempt to review those studies of pyridinelike heterocycles (mono-azines) and, to a lesser extent, their analogues and derivatives that have interpreted the behavior and estimated various physico-chemical properties of the compounds by the use of data calculated by the simplest version of the MO LCAO (molecular orbital, linear combination of atomic orbitals) method (both molecular orbital energies and expansion coefficients). In this review, attention is focused upon the use of the simple method because it has been applied to quite extensive sets of compounds and to the calculation of the most diverse properties. On the other hand, many fewer compounds and physico-chemical properties have been investigated by the more sophisticated methods. Such studies are referred to without being discussed in detail. In a couple of years, we believe, the extent of the applications of such methods will also be wide enough to warrant a detailed review. 

The chemistry of cyclazines has not previously been reviewed. It is our aim to survey the methods of synthesis of these compounds and to show that the chemistry of cyclazines has stimulated discussion on theories of condensed -systems. This latter point may be illustrated by an example Early molecular orbital (MO) studies on cycl[3,3,3]azine (2) predicted a resonance energy greater than that for the highly stable... 

Other fundamental characteristics of heteroaromatic systems are their electron-donor and electron-acceptor properties. The energies of the highest occupied (HOMO) and the lowest unoccupied (LUMO) molecular orbitals (the frontier orbitals) can serve as measures of such properties. Pyridine-like heteroatoms lower the energies of all the MOs and compounds containing heteroatoms of this type can be expected to show more -acceptor and less r-donor character. In accord with this expectation (Table 2), -acceptor properties of azines decrease in the sequence l,2,4>5-tetrazine > pyrazine > pyridazine > pyrimidine > pyridine. 

In 1995 Soscun Machado and Hinchliffe [373] used Hartree-Fock 6-31G and 6-31G calculations to estimate the pK s of a set of mono cyclic and bicyclic azines. For three classes (see representatives below) they found good correlations (r = 0.91-0.99) between the energies of the highest occupied molecular orbital (HOMO) energies enoMo the pK s. 

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