Abstract

Aromatic π-π interactions are now known to play a key role in a wide range of important problems, including the stereochemistry of organic reactions, organic host-guest chemistry and crystal packing, protein folding and structure, DNA and RNA base stacking, protein-nucleic acid recognition, drug design, and asphaltene (heavy crude oil) aggregation and fouling. However, detailed knowledge of the structures adopted by simple model aromatic liquids is currently lacking and essential to our fundamental understanding of π-π interactions in condensed matter. We have high-resolution neutron diffraction in conjunction with hydrogen/deuterium isotopic labeling to determine the structure of simple aromatic liquids, and more complex solutions of fullerene, graphene and carbon nanotubes. From our studies of benzene and toluene we have discovered the nature of aromatic π-π interactions in the liquid state by constructing six-dimensional spatial and orientational pictures of these systems. We find that in each case the nearest neighbour coordination shell contains approximately 12 molecules. At small molecular separations ( 5 A) the neighbouring aromatic rings are predominantly perpendicular, with two H atoms per molecule directed toward the acceptor’s π orbitals. The so-called “anti-hydrogen-bond” configuration, proposed as the global minimum for the benzene dimer, occurs only as a saddle point in our data. In the case of graphitic carbon nanostructures such as fullerene and graphene, charging in metal-ammonia solutions allows us to dissolve high concentrations of these solute molecules. We have then used neutron diffraction to obtain a detailed picture of the solvation of these particles, and to elucidate for example the nature of hydrogen bonding to the π-orbitals.