[reaction: see text] Triptycene-derived compounds have been prepared to serve as conformational equilibrium reporters for direct measurements of arene-arene interactions in the parallel-displaced orientation. A series of such compounds bearing arenes with different substituents were synthesized, and the ratios of the syn and anti conformers were determined by variable-temperature NMR spectroscopy. The syn conformer allows attached arenes to interact with each other while the anti conformer does not. The free energies derived from the syn/anti ratios in chloroform range from slightly positive (0.2 kcal/mol) to considerably negative (-0.98 kcal mol) values. The interactions between the arenes bearing electron-donating groups (EDG) are either negligible or slightly repulsive, while the interactions between arenes bearing electron-withdrawing groups (EWG) are attractive. Intermediate free energy values are obtained for those compounds bearing arenes with one EDG and one EWG.

[structure: see text] Attractive interactions between substituted arenes in the parallel displaced configuration have been quantitatively studied using triptycene-derived molecular conformational reporters. Charge-transfer bands are observed for models where the interactions are between strong donor and acceptors. Substituent effects on the strength of the aromatic interaction follow opposite trends for strongly electron-deficient arenes and mildly perturbed arenes. The free energy of interactions for models with strong electron donors and acceptors does not follow a linear correlation in the Hammett plot. Electrostatic models alone do not account for the nonlinearity of the free energy-substituents plot.

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 (Ar 2 = pentafluorobenzoate), attractive interactions were observed regardless of the character of the other arene (Ar 1 ). For stacking interactions between Me 2 NC 6 H 4 and C 6 F 5 CO 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 Me 2 NC

[reaction: see text] Triptycenes with C1-MeO/RCOO (R = H, Me, Et, i-Pr, CF3) and C9-XC6H4CH2 (X = Me, H, F, CN, CF3) have been prepared to determine lone pair-arene interactions in the off-center configuration. The ratios of the syn and anti conformers were determined by low-temperature NMR spectroscopy. The syn conformer allows the attached arene and the MeO/ester to interact with each other while the anti conformer does not. The free energies of interaction have been derived from the syn/anti ratios. Compound 7 in the ester series with X = H and R = CF3 is the only compound that shows a slightly repulsive interaction (0.08 kcal/mol). Compound 2e in the MeO series with X = CF3 exhibits an attractive interaction (-0.47 +/- 0.05 kcal mol). All other compounds show smaller attractive interactions.