Theoretically-derived, energy-based criteria for aromaticity

Abstract

Previous energy-based criteria for aromaticity developed from one electron eigenvalues (using methods ranging from Huckel to ab initio calculations) have not correlated well with experimentally derived criteria such as χ or chemical reactivity data, and are particularly poor for heteroaromatic compounds. Previous work has shown that criteria derived from the lowest π molecular orbitals of heteroaromatic compounds correlate well with χ and chemical reactivity, prompting us to modify Kollmar's procedure for determining aromatic stabilization energies by localizing the lowest π orbital rather than the entire π bonding manifold. This modified procedure was implemented with both the natural (NLMO) and Boys orbital localization methods, and was applied to a set of ten five-membered ring heteroaromatics and to pyridine and its diazine derivatives. The new criterion ΔπL correlated well with χ (R2 = 0.86 (NLMO); R2 = 0.92 (Boys)) for five-membered rings, and with theoretical measures of aromaticity for both sets of heteroaromatics. The Boys procedure gave better overall correlations and was markedly superior for pyrazole and pyridazine.