Decoding Chemical Resonance in π-Conjugated Systems

Abstract

Understanding the electronic structure of (anti)aromatic poly- and macrocyclic π-conjugated systems remains a central challenge in contemporary chemistry, given their importance in materials science, molecular electronics, photophysics, and catalysis. While magnetically induced ring currents and NMR-based descriptors have long served as key indicators of (anti)aromatic character, the extent to which such magnetic observables reflect the full ground-state π-bonding architecture becomes less transparent in large and topologically complex systems. Here we introduce a wavefunction-based analytical protocol─ the π-Bond Delocalization Function (BDF_π)─ which maps the spatial organization of localized and delocalized π-bonding directly from the ground-state one-electron density matrix. Rather than relying on predefined resonance models or orbital localization schemes, the method extracts bonding topology from the complete occupied π-space. This enables a direct comparison between the global π-electron distribution and the more selective subset of frontier-orbital excitations that dominate magnetic response phenomena. Applied to representative poly- and macrocyclic systems frequently discussed in the context of global (anti)aromaticity, BDF_π reveals that magnetic ring currents and bonding delocalization need not coincide quantitatively in extended conjugated frameworks. Magnetic descriptors predominantly probe perturbative response channels, whereas the underlying resonance structure reflects the collective organization of the entire occupied π-manifold. By explicitly relating these two perspectives, the present framework provides a coherent and chemically transparent account of how resonance topology and experimentally observable magnetic behavior are connected─ and where they may diverge─ in large conjugated architectures.