Quantifying aromaticity at the molecular and supramolecular limits: comparing homonuclear, heteronuclear, and H-bonded systems

Abstract

The aromatic/antiaromatic characteristics of B-N and P-N analogues of benzene and cyclobutadiene have been studied using quantum chemical methods. We use established parameters such as nucleus-independent chemical shifts, charge density at the ring critical point, and stabilization energies to quantify the nature of interactions in these molecular systems. B₃N₃H₆ and N₃P₃F₆ resemble benzene in being aromatic, albeit to a lesser extent, while B₂N₂H₄ and N₂P₂F₄ are found to be aromatic, opposite to that for cyclobutadiene. A σ-π separation analysis has been performed to critically examine the contributions from the p electrons compared to that from the s backbone. The structural aspects in the weak interaction limits such as the H-bonded cyclic trimers of HX (X = F, Cl, and Br) have also been investigated. Even in such weak interaction limits, these cyclic systems are found to be substantially stable. These H-bonded systems exhibit nonlocal polarizations across the full-perimeter of the ring that lead to aromaticity. We propose the term "H-bonded aromaticity" for such closed-loop weakly delocalized systems. This new formalism of aromaticity has the potential to explain structures and properties in supramolecular systems.