Application of Natural Bond Orbital Analysis and Natural Resonance Theory to Delocalization and Aromaticity in Five-Membered Heteroaromatic Compounds.

The various measures of delocalization in all of the five-membered nitrogen and oxygen heteroaromatic compounds (azoles and oxoles) were obtained from MO calculations at the HF/6-31G level and the application of natural bond orbital analysis and natural resonance theory. The hydrogen transfer and aromatic energies of these compounds were also calculated. These were compared to the relative ranking of aromaticity reported by Katritzky from a principal component analysis of other measures of aromaticity. It was concluded that the extent of the transfer of electron density from the p(z)() orbital of the heteroatom to the rest of the pi system is the best measure of delocalization and "aromaticity" of these compounds. This indicated that all of the oxoles are less delocalized than any of the azoles because the electronegativity of the oxygen atom prevents the interaction of its lone pair of electrons with the pi system. For this reason the range of delocalization in the oxoles is also much narrower. The degree of delocalization in the pi system is the result of the geometry imposed by the sigma structure, which in turn is defined by the identity and arrangement of the atoms of the ring.