AN IR SPECTRAL MEASURE OF CLASSICAL AROMATICITY IN 5-MEMBERED AND 6-MEMBERED RING HETEROCYCLES - AN ABINITIO STUDY

The carbon-hydrogen IR frequencies and intensities obtained from 4-31G molecular orbital calculations for 15 five- and six-membered ring heterocycles are subjected to a principal component analysis. The first principal component (PC) is found to discriminate between five- and six-membered ring compounds. The first PC scores are found to be correlated with a recently proposed measure of classical aromaticity. In consequence, the first PC score is suggested as a measure of classical aromaticity, entirely based on quantities that can be observed. The relative contributions of the IR intensities to the first PC indicate increasing charge localization in the less aromatic compounds.