Quantification of Aromaticity Based on Interaction Coordinates: A New Proposal

Abstract

Attempts to establish degrees of aromaticity in molecules are legion. In the present study, we begin with a fictitious fragment arising from only those atoms contributing to the aromatic ring and having a force field projected from the original system. For example, in benzene, we adopt a fictitious C6 fragment with a force field projected from the full benzene force field. When one bond or angle is stretched and kept fixed, followed by a partial optimization for all other internal coordinates, structures change from their respective equilibria. These changes are the responses of all other internal coordinates for constraining the bond or angle by unit displacements and relaxing the forces on all other internal coordinates. The "interaction coordinate" derived from the redundant internal coordinate compliance constants measures how a bond (its electron density) responds for constrained optimization when another bond or angle is stretched by a specified unit (its electron density is perturbed by a finite amount). The sum of interaction coordinates (responses) of all bonded neighbors for all internal coordinates of the fictitious fragment is a measure of the strength of the σ and π electron interactions leading to aromatic stability. This sum, based on interaction coordinates, appears to be successful as an aromaticity index for a range of chemical systems. Since the concept involves analyzing a fragment rather than the whole molecule, this idea is more general and is likely to lead to new insights.

A DFT study on 1,4‐dihydro‐1,4‐azaborinine annulated linear polyacenes: Absorption spectra, singlet‐triplet energy gap, aromaticity, and HOMO–LUMO energy modulation

“…Aromaticity is a multidimensional phenomenon, involving structural, electronic, magnetic and energetic properties of the molecules . The HOMA index, developed by Krygowski et al is one of the geometry‐based method to compare the aromaticity of a variety of organic molecules .…”

Effects of Multiple OH/SH Substitution on the H‐Bonding/Stability versus Aromaticity of Benzene Rings: From Computational Insights

“…In recent times (in 2016), using the IC parameter, a new technique as an electronic measure of aromaticity was proposed by Pandey and coworkers ‘Aromaticity Index Based on Interaction Coordinates (AIBIC)’ [55] . Computational experiments have been performed on methyl and fluorine substituted benzenes, fluorine substituted borazines, a few five and six‐membered heterocyclic systems, some mono‐ and polycyclic aromatic systems, and some large equilibrium structures containing distorted benzene and naphthalene as their constituents.…”

“…As model systems, we have chosen some OH/SH substituted benzene related species by taking two cases: (1) with IHB(s) and (2) without IHB(s). Hence, we now present another recipe for the stability versus aromaticity by emphasizing the remarkable energetic [relative energy (RE), interaction energy (IE) acquired from one of the very useful topological parameters such as potential electron energy density, V(r) and electron density, ρ(r) from the QTAIM tool), and HOMO‐LUMO energy gap (HLEG) [43,58] , geometric (HOMA), [44,45] electronic (PDI, AIBIC, and HBSBIC), [38,46,55] and magnetic [47,59] consequences of such synergistic connection.…”

Fluoromaticity: The Molecular Orbital Contributions of Fluorine Substituents to the π-Systems of Aromatic Rings

“…Since the discovery of benzene by Faraday in 1825, there has been significant controversy over the definition and even the cause of aromaticity, − but the classically accepted criteria for the determination of aromaticity typically includes the following: Reduced bond lengths within the ring compared to analogous acyclic unsaturated molecules Increased stability compared to analogous acyclic unsaturated molecules Increased probability of undergoing substitution reactions and a decreased probability of undergoing addition reactions Induction of a diatropic ring current by the application of an external magnetic field …”