Local Quadrupole Ellipticity as Predictor of Anion-Affinity in Nanographenes.

Anion binding to nanographenes is governed by noncovalent interactions, particularly anion-π interactions in electron-deficient aromatic regions and CH---anion hydrogen bonding in electron-rich domains. These interactions are primarily driven by electrostatic effects, with the quadrupole moment of the aromatic system playing a central role in determining the strength and directionality of anion-π binding. The perpendicular component of the quadrupole moment (Qzz) correlates with binding energies for both anion-π and CH---anion interactions, though polycyclic systems present challenges due to competing interaction modes. In this study, we investigate the role of the local quadrupole moment in anion binding across 171 Cl--aromatic complexes, comparing various descriptors including aromaticity indices, Fukui functions, and electron density at ring critical points. We find that electrostatic descriptors, particularly the local quadrupole moment, provide a more consistent and robust explanation for binding energies than conventional descriptors. Specifically, two geometric descriptors derived from the local quadrupole moment-the scale factor ( S R max $$ {S}_R^{max} $$ ) and the ellipticity ( e c ' $$ {e}_c^{\prime } $$ )-show good correlation with binding strength, with S R max $$ {S}_R^{max} $$ reflecting π-acidity and ellipticity quantifying charge distribution anisotropy. These descriptors are validated across fluorinated naphthalenes and nanographenes, demonstrating their general applicability. Regression models based on S R max $$ {S}_R^{max} $$ and e c ' $$ {e}_c^{\prime } $$ effectively predict binding energies, with enhanced accuracy when combined with polarization-dependent penalty functions, especially for larger nanographene systems. While the predictive model is still somewhat constrained by polarization effects, its simplicity, robustness, and transferability across a wide range of systems offer distinct advantages over more complex, multilayered machine learning models. These results underscore the critical role of quadrupole moment anisotropy in anion-π interactions and offer a practical framework for predicting anion binding affinities and designing π-acidic receptors.