Quantifying Vertical Resonance Energy in Aromatic Systems with Natural Bond Orbitals

Abstract Natural bond orbitals (NBOs) provide the familiar Lewis type (2c–2e − ) localized description of a molecule. Interactions between nearly filled (2e − π or σ) orbitals and empty (π* or σ*) anti‐bonding orbitals represent delocalization in the system and creates a framework to study stereoelectronic interactions. Here we show that deleting the interactions (NBODel) between π and π* orbitals in aromatic systems and acquiring the energy with the NBO program provides a highly intuitive and quantitative picture of π‐aromaticity that correlates with the well‐established nucleus‐independent chemical shift (NICS) method. This natural bond orbital resonance energy (NBO‐RE) measures the vertical resonance energy (VRE) of aromatic systems without the use of an external reference structure. The NBO‐RE method is applicable to the study of local aromaticity in polycyclic aromatic hydrocarbons (PAHs) and other non‐planar systems.