Quantifying Aromaticity at the Molecular and Supramolecular Limits:Comparing Homonuclear,Heteronuclear,and H-Bonded Systems

摘要

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.B_3N_3H_6 and N_3P_3F_6 resemble benzene in being aromatic,albeit to a lesser extent,while B_2N_2H_4 and N_2P_2F_4 are found to be aromatic,opposite to that for cyclobutadiene.A sigma-pi separation analysis has been performed to critically examine the contributions from the pi electrons compared to that from the a 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 supramdlecular systems.