Aromatic-aromatic ring interaction revisited with model compounds of Wilcox

Aromatic-aromatic nonbonded interactions have been reexamined using model compounds of Wilcox and collaborators (J. Am. Chem. Sec. 1994, 116, 4497). It was found that at low temperatures down to 210 degrees K, the population of the folded conformers (A) is higher than that of the unfolded conformers (B), suggesting that edge-to-face aromatic-aromatic ring interactions are in effect. However, the free energy difference between the two types of conformers did not vary linearly with temperature, which is against what we expected from the thermodynamic relationship of Delta G=Delta H-T Delta S. This suggests that in the presence of solvent molecules a free energy cancellation effect operates between the two conformers. Although A has a free energy gain of only similar to 0.5 kcal/mol over B in organic solvents, as obtained by subtracting the solvent-induced unfolding effect, it could still be a significant energy with respect to conformational preference.