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 π electrons compared to that from the σ 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.