A systematic study on interplay between intermolecular hydrogen bonding and aromaticity

Abstract

In this study, the interplay between aromaticity and intermolecular hydrogen bonding was investigated in some heterocyclic compounds using quantum mechanical calculations. The NH … O intermolecular hydrogen bonds between formamide and NH and CO functional groups of the selected compounds enhance the delocalisation of the six-membered ring electron cloud, resulting in an increase in the aromaticity of the ring. The contribution of various factors to the binding energy, including the stabilisation energy of the interplay between aromaticity and H-bonding, was estimated using two model systems. The relative binding energies and the estimated stabilisation energies of the aforementioned interplay can be explained by the weights of resonance structures calculated using the natural resonance theory (NRT) analysis with the natural bond orbitals (NBO) method. These weights reflect the tendency of ring nitrogen lone pairs to contribute to the π electron cloud of the ring. The results of the topological analysis of electron charge densities using the atoms in molecules (AIM) method were used to estimate the aromaticity of the ring. Aromaticity indices, particularly nucleus-independent chemical shifts (NICS) calculated one angstrom above the ring plane, are in good linear correlation with the estimated stabilisation energies and electron density at the H-bond critical points. The sum of the total nuclear spin–spin coupling 1J of adjacent atoms of the ring (Σ1J), and the J values of H-bond donor and H-bond acceptor atoms, 1HJNO, which are in good agreement with the energy data and aromaticity indices, can be used as measures of the interplay between aromaticity and H-bonding. The interplay between H-bonding and the aromaticity of the ring is clearly evident from the changes in the pattern of the molecular electrostatic potential (MEP) contour maps at 1.7 Å above the plane of the molecule upon H-bond complexation.