Enthalpy (ΔH) and Entropy (ΔS) for π-Stacking Interactions in Near-Sandwich Configurations: Relative Importance of Electrostatic, Dispersive, and Charge-Transfer Effects

Abstract

Interactions between two aromatic rings with various substituents in a near-sandwich configuration have been quantitatively studied by using the triptycene derived molecular models. This model system allows a stacking arrangement of two arenes to assume a near-perfect face-to-face configuration in its ground state conformation. Comparing to our previous study of the parallel displaced configuration, repulsive interactions are predominant for most arenes currently studied. However, if one arene is strongly electron deficient (Ar₂ = pentafluorobenzoate), attractive interactions were observed regardless of the character of the other arene (Ar₁). For stacking interactions between Me₂NC₆H₄ and C₆F₅CO groups, a ΔH of −1.84 ± 0.2 kcal/mol and a ΔS of −2.9 ± 0.8 cal/(mol·K) were determined. The general trend in the attractive stacking interaction toward a pentafluorobenzoate is Me₂NC₆H₄ > Me₃C₆H₂ > Me₂C₆H₃ > MeC₆H₄ > MeOC₆H₄ > C₆H₅ > O₂NC₆H₄. The observed trend is consistent with a donor−acceptor relationship and the acceptor is a C₆F₅CO group.