Electron Densities and Frontier Orbital Energies

The presence in azoles of both pyrrole-like and pyridine-like heteroatoms leads to a highly perturbed n-electron distribution. As a result, these molecules often display along with ir-excessive centers, atoms with a rather high rr-deficiency, sometimes even higher than in typical azines. We first consider non-fused azoles (Table 1). 

On first sight it seems surprising that the pyrrole heteroatom in a fused imidazole does not prevent the appearance of a large positive charge at the neighbouring ring carbon atom. The reason for this is that the pyrrole nitrogen displays its TT-donor effect almost exclusively in the direction of the fused nucleus. 

Purine is also notable for its high rr-deficiency, 7//-purine being more Tr-deficient than the 9H-tautomer. Possibly this very circumstance is responsible for the natural occurrence of the less TT-deficient and therefore chemically more stable derivatives of 9//-purine. 

Transition from a neutral azole to an azolium cation strongly increases the TT-deficiency. In azines, the HOMO is normally occupied by the nonbonded electron pair of a nitrogen atom. By contrast, in azoles the HOMO is of the rr-type. This is supported by the parallel behavior of calculated HOMO 

Structure of Five-membered Rings with Two or More Heteroatoms 

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