A theoretical study of N-H•••π H-bond interaction of pyrrole: from clusters to the liquid

The N-H center dot center dot center dot pi H-bond interactions of clusters and liquid formed by pyrrole molecules were investigated by Quantum Mechanics (QM) and classical Molecular Dynamics (MD), respectively. Based on the optimized geometry at the B97-D/aug-cc-pVTZ level of theory including dispersion correction, the nature and the origin of N-H center dot center dot center dot pi H-bond interactions were unveiled by atoms in molecules (AIM), natural bond orbital (NBO), and energy decomposition analysis (EDA). Among them, the AIM analysis gives evidence to the presence of N-H center dot center dot center dot pi H-bond interactions, the NBO examination reveals that pi ->sigma* donor-acceptor orbital interaction is of great importance. EDA study indicates that N-H center dot center dot center dot pi interactions are governed by the electrostatic and dispersion term. Meanwhile, MD simulation with OPLS-AA (optimized potentials for liquid simulations all-atom) was applied to study the pure liquid pyrrole at different temperature. The results confirm the existence of the N-H center dot center dot center dot pi H-bond in the pure liquid pyrrole, and further characterized the structures of this H-bond which is somewhat different to the clusters.