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

摘要

Interactions between two aromatic rings with various substituents in a near-sandwich configuration have been quantitatively studied 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 towards 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.