Clarifying Aromaticity and Delocalization with the Principle of π-Electron Pair Interaction (PEPI)

Abstract

Valence bond (VB) theory and Molecular Orbital (MO) theory are foundational approaches to understanding chemical bonding. While MO theory describes delocalized orbitals across the molecule and offers quantitative rigor, VB theory aligns closely with classical chemical concepts, using localized bonds and hybridization for intuitive understanding. However, VB theory’s treatment of delocalized systems, such as aromatic compounds, relies on resonance structures, which are less efficient and may cause misconceptions compared to the methodology of MO theory. To address this, the Principle of π-Electron Pair Interaction (PEPI) is introduced as a heuristic framework to extend the qualitative power of VB theory. A visual guide is provided by PEPI to aid in understanding when π-electrons may resist delocalization due to pairing constraints. The model is intended to complement MO theory and is presented not as a physical principle, but as an interpretive aid that offers clarity in systems such as butadiene, benzene, and selected pericyclic reactions. It is demonstrated how PEPI can illuminate concepts such as aromaticity, antiaromaticity, misinterpretations of resonance, and stereoelectronic trends in a conceptually accessible manner. PEPI proposes that electron spin should be taken into account when evaluating resonance structures, particularly in the context of aromaticity. By reframing PEPI as a pedagogical tool, alignment is achieved with quantum mechanical models while the intuitive appeal of VB theory is preserved.