NMR-STUDY OF AROMATIC-AROMATIC INTERACTIONS FOR BENZENE AND SOME OTHER FUNDAMENTAL AROMATIC SYSTEMS USING ALIGNMENT OF AROMATICS IN STRONG MAGNETIC-FIELD

The solvent dependence of proton-proton dipolar couplings in a strong magnetic field was used to characterize the aromatic-aromatic interactions of benzene, hexafluorobenzene (HFB), naphthalene, and some monosubstituted benzenes under various conditions. The very accurate analysis of the spectral parameters of benzene also allowed observation of isotope effects on coupling constants and solvent effects on the C-13 isotope shifts. The results indicate that benzene, naphthalene, and the monosubstituted benzenes favor structures in which the aromatic ring is parallel-stacked with HFB. According to the concentration-dependent experiments, the benzene-HFB complex behaves like a dimer. For benzene-dimer, benzene-naphthalene, and some benzene-monosubstituted benzene complexes there is evidence for T-shaped structures; for benzene in water no clear preference is seen. The complexes express only in very high concentrations, which means that the interaction has significance only in conditions where the entropy of the complex formation is small or is compensated by some other interactions.