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

Area of Science:

• Physical Chemistry
• Structural Biology
• Supramolecular Chemistry

Background:

• Weak intermolecular interactions govern molecular assembly and function.
• NH···π hydrogen bonds, involving amines and aromatic systems, are prevalent in biological structures.
• Liquid pyrrole serves as an ideal model system due to its amine and aromatic functionalities.

Purpose of the Study:

• To investigate the nature and strength of NH···π hydrogen bonds in liquid pyrrole.
• To compare pyrrole-pyrrole NH···π interactions with pyrrole-benzene interactions.
• To elucidate the cooperative effects influencing these weak interactions.

Main Methods:

• Total neutron scattering experiments were performed on pure pyrrole and a pyrrole-benzene mixture.
• Simulation-based refinement was employed to analyze the structural data.
• Quantitative analysis of intermolecular distances and geometries was conducted.

Main Results:

• The NH···π interaction in pyrrole is highly directional, with NH approaching the aromatic ring center perpendicularly at 2.11 Å.
• Pyrrole-pyrrole NH···π bond lengths are similar to pyrrole-benzene interactions.
• The frequency of pyrrole-benzene NH···π interactions is reduced by a factor of two compared to pyrrole-pyrrole interactions.

Conclusions:

• The NH···π interaction in pyrrole exhibits significant directionality and strength, akin to classical hydrogen bonds.
• Cooperative effects, stemming from pyrrole's dual hydrogen bond donor/acceptor capability, enhance its self-interaction.
• These findings challenge the perception of NH···π bonds as exclusively weak interactions and highlight their importance in molecular recognition.