The predictive power of aromaticity: quantitative correlation between aromaticity and ionization potentials and HOMO-LUMO gaps in oligomers of benzene, pyrrole, furan, and thiophene.

Aromaticity is a central and ubiquitous concept in organic chemistry, and is used extensively to explain various phenomena. Yet, aromaticity cannot be observed or measured as a property in its own right and, to date, only qualitative and semi-quantitative relationships have been described between aromaticity and an observable property. We now demonstrate for the first time a robust quantitative relationship between the HOMO-LUMO gap and adiabatic ionization potential of a polycyclic aromatic hydrocarbon oligomer - both measurable physical quantities - and its aromaticity, as quantified by the Nucleus Independent Chemical Shift (NICS) index. The agreement found for a range of structurally and electronically diverse oligomeric systems of varying lengths is so well-behaved as to enable accurate prediction of the properties of longer members of the respective oligomer family. The established correlation allows for preliminary screening of compounds geared towards functional use.