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

## TL;DR

This study explores how pyrrole molecules form directional hydrogen bonds with aromatic rings, revealing insights into weak intermolecular interactions in chemistry and biology.

## Contribution

The paper introduces a novel understanding of cooperative mechanisms in N–H···π hydrogen bonds through experimental and simulation-based analysis.

## Key findings

- Pyrrole–pyrrole NH···π bonds are highly directional with a bond length of 2.11 Å.
- Pyrrole–benzene interactions are less frequent due to cooperative effects.
- N–H···π bonds can exhibit characteristics similar to classical hydrogen bonds.

## Abstract

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.