Aromaticity as a Quantitative Concept. 4. Less familiar five- and six-membered monocyclic heterocycles

An additional set of 23 monoheterocycles was subjected to AM1 calculations, and the characteristics thus determined, together with available experimental data, treated by PC analysis. Utilizing characteristic loadings previously determined for other monoheterocycles, scores for the 23 monoheterocycles were deduced. The new scores correlate well with those previously found, and with the chemical nature of the heterocycles. The dominant influence on the t1 is the nature of the hetero atoms present, whereas the dominant influence on t2 is the number of hetero atoms present. It is concluded that pyridine‐like nitrogen atoms have relatively little effect on clasical aromaticity: five‐membered rings are less aromatic than six‐membered, the presence of an oxygen atom has a particularly aromaticity‐reducing effect, whereas the effect of sulfur is much less than oxygen and only a little more than nitrogen. The predictive power for the present compounds is limited by the relative paucity of and some problems with the input data but suceeds well for Ix′, ΔN and ΔHf.