A supramolecular system for quantifying aromatic stacking interactions.

A supramolecular complex for investigating the thermodynamic properties of intermolecular aromatic stacking interactions has been developed. The conformation of the complex is locked in a single well-defined conformation by an array of H-bonding interactions that force two aromatic rings on one end of the complex into a stacked geometry. Chemical double-mutant cycles have been used to measure an anthracene-aniline interaction (+0.6 +/- 0.8 kJ mol(-1)) and a pentafluorophenyl-aniline interaction (-0.4 +/- 0.9 kJ mol(-1)) in this system. Although the interactions are very weak, the pentafluorophenyl interaction is attractive, whereas the anthracene interaction is repulsive: this is consistent with the dominance of pi-electron electrostatic interactions. The nitropyrrole subunits used to control the conformation of these complexes lead to problems of aggregation and multiple conformational equilibria. The implications for the thermodynamic analysis are examined in detail, and the double-mutant-cycle approach is found to be remarkably robust with respect to such effects, since systematic errors in individual experiments are removed in a pair-wise fashion when the cycle is constructed.