Computational and experimental evidence of N–H…π and cooperative πN…π∗ interactions in pyrrole…benzene and pyrrole…ethylene heterodimers at low temperatures

Abstract

The interaction between pyrrole with benzene and ethylene was investigated using matrix isolation infrared spectroscopy and ab initio computations. Computations indicated the global minimum structure of pyrrole-benzene and pyrrole-ethylene heterodimers are stabilized by π…σ∗ (N–H…π) and a cooperative πN…π∗ interactions. The present work for the first time, reports that pyrrole … benzene heterodimer is stabilized by both N–H…π and πN…π∗ interactions, where the latter induces a partial stacking in contrast to the T-shaped structure predicted by previous experimental and theoretical reports. To provide experimental proof, the heterodimers were trapped in N2 and Ar matrixes, and probed through infrared spectroscopy. Natural bond orbital (NBO) and energy decomposition (ED) analyses revealed that the heterodimers were stabilized by charge-transfer, electrostatic and dispersion interactions. To further understand and delineate the existence of πN…π∗ interaction (which results from partial π-stacking), computations were performed on the heterodimers of pyrrole with several acyclic and cyclic π-donors.