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.

The gas-phase acidities of furan, thiophene, pyridine, 1-methylpyrrole, and a number of their methyl derivatives have been determined in a flowing afterglow apparatus. The AGacid (kcal/mol) and site of deprotonation of the parent heterocycles are as follows: furan 380 ± 3 (2-position), thiophene 373 ± 3 (2-position), pyridine 384 ± 3 (3-and/or 4-position), 1-methylpyrrole 386 ± 3 (methyl group). For the methylated species studied the acidities are 2-methylfuran 377 ± 3, 2-methylthiophene 373 ± 3, 3-methylthiophene 373 ± 3, 3-methylpyridine 371 ± 3, 2-methylpyridine 370 ± 3, and 4-methylpyridine 368 ± 3. The relative acidities of the hydrogens in the other sites in these molecules have been estimated by hydrogen-deuterium exchange studies and by other chemical methods. Anions (M -1) from these molecules have been allowed to react with N20, 02, COS, and CS2 and the ionic products determined. The gas-phase acidities have been compared with those calculated by the semiempirical methods AMI and MNDO.

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