Additivity of Substituent Effects in Aromatic Stacking Interactions

The goal of this study was to experimentally test the additivity of the electrostatic substituent effects (SEs) for the aromatic stacking interaction. The additivity of the SEs was assessed using a small molecule model system that could adopt an offset face-to-face aromatic stacking geometry. The intramolecular interactions of these molecular torsional balances were quantitatively measured via the changes in a folded/unfolded conformational equilibrium. Five different types of substituents were examined (CH₃, OCH₃, Cl, CN, and NO₂) that ranged from electron-donating to electron-withdrawing. The strength of the intramolecular stacking interactions was measured for 21 substituted aromatic stacking balances and 21 control balances in chloroform solution. The observed stability trends were consistent with additive SEs. Specifically, additive SE models could predict SEs with an accuracy from ±0.01 to ±0.02 kcal/mol. The additive SEs were consistent with Wheeler and Houk’s direct SE model. However, the indirect or polarization SE model cannot be ruled out as it shows similar levels of additivity for two to three substituent systems, which were the number of substituents in our model system. SE additivity also has practical utility as the SEs can be accurately predicted. This should aid in the rational design and optimization of systems that utilize aromatic stacking interactions.