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 confo rmation of the complex is locked in a single well-defined conformation by a n 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 +/- 0.90 kJ mol (-1)) in this system. Although the interactions are very weak, the pentaflu orophenyl interaction is attractive, whereas the anthracene interaction is repulsive this is consistent with the dominance of pi -electron electrostat ic interactions. The nitropyrrole subunits used to control the conformation of these complexes lead to problems of aggregation and multiple conformati onal equilibria. The implications for the thermodynamic analysis are examin ed in detail, and the double-mutant-cycle approach is found to be remarkabl y robust with respect to such effects, since systematic errors in individua l experiments are removed in a pair-wise fashion when the cycle is construc ted.