Revisiting the Hunter-Sanders Model for π-π Interactions

The "Hunter-Sanders model" (J. Am. Chem. Soc. 1990, 112, 5525) is foundational to many chemists' understanding of interactions between aromatic systems. Carter-Fenk and Herbert (Chem. Sci., 2020, 11, 6758) recently upended that understanding by showing that the driving force for aromatic systems to adopt parallel displaced geometries arises from steric, not Coulombic, repulsion of the π-electron clouds. Carter-Fenk and Herbert also claimed to show that the original Hunter-Sanders potential fails to predict the geometries of a range of parallel and T-shaped dimers. Closer inspection reveals that the data supporting this latter claim are flawed. Correctly implemented, the Hunter-Sanders potential provides qualitatively correct predictions for these systems and performs particularly well for the T-shaped benzene dimer. Moreover, it predicts the preferred displacement direction for some stacked heterocyclic dimers and accurately captures the impact of a diverse group of substituents on the benzene sandwich dimer. This is inclusive of the fact that all substituents enhance stacking interactions in this geometry. Ironically, for substituted benzene dimers, the Hunter-Sanders potential provides data in accord with our Local, Direct Interaction Model but in contrast with the so-called "Hunter-Sanders model." At the same time, the Hunter-Sanders potential struggles to capture heteroatom and substituent effects in parallel displaced geometries in which the heteroatom/substituent is located over the other ring, leading to qualitatively incorrect predictions of the preferred displacement direction of substituted benzene dimers. Overall, many aspects of the Hunter-Sanders potential are flawed; however, others appear qualitatively correct.