Cooperation-Enhanced N-H···π Hydrogen Bonds: Liquid Pyrrole and Its Mixture with Benzene.

Weak intermolecular interactions are central to the chemical and biological sciences as they dictate the stability, growth, and geometry of larger assemblies. Among weak interactions, NH···π hydrogen bonds are abundant in structural biology, where amines interact with aromatic systems: liquid pyrrole is the ideal test solvent containing both motifs. We therefore combined total neutron scattering and simulation-based refinement to study pure pyrrole and its mixture with benzene. The NH···π interaction between pyrroles is remarkably directional, with NH approaching the center of the ring perpendicularly at 2.11 Å. While the NH···π bond lengths are similar in pyrrole-pyrrole and pyrrole-benzene, the occurrence of the latter is suppressed by a factor of 2. This difference originates from cooperative mechanisms arising from the ability of pyrrole to donate and simultaneously accept a hydrogen bond. Our results clearly show that this traditionally weak interaction can become as short and directional as classical hydrogen bonds.