A universal scale of aromaticity for π‐organic compounds

Abstract

Aromaticity is an essential concept in chemistry, invented to account for the stability, reactivity, molecular structure, and properties of many organic and inorganic compounds. In recent years, numerous methods to quantify aromaticity based on the energetic, magnetic, structural, and electronic properties of molecules have been proposed but none of them is universal. The inability of establishing a universal scale of aromaticity based on a single parameter is due to the multidimensional character of this phenomenon. Consequently, aromaticity analyses should be carried out by employing a set of aromaticity descriptors on the basis of different physical manifestations of aromaticity. Here, we report a universal scale of aromaticity for pi-organic compounds based on the Euclidean distance between neurons in a self-organizing map. The most widely used aromaticity indicators have been used as molecular descriptors, and so our approach provides the first scale of aromaticity which contains the energetic, magnetic, and structural aspects of this property. The method is applicable to a wide variety of unsaturated organic compounds and allows quantification of both aromaticity and antiaromaticity. Additionally, the position of a compound on the bidimensional map determinates immediately the following: (a) the group (aromatic, nonaromatic, or antiaromatic) to which the system belongs, (b) their degree of pi-electronic delocalization, and (c) the similarity in aromaticity/antiaromaticity between different compounds. This new scale of aromaticity is able to indicate the expected order of aromaticity of analogues of fulvene and heptafulvene, heteroaromatic species, substituted benzenes, and functionalized cyclopentadienyl compounds.

Sensitivity of high-order-harmonic generation to aromaticity

“…ASE and χ G [11] measures consider pyrrole to be more aromatic than thiophene, whereas according to NICS(1) [31] measure thiophene is more aromatic. Other versions of NICS [1,22] indicate a classification that agrees with ASE and χ G .Onthe other hand, the HOMA [11] index gives comparable values for both molecules. Figure 4(a) shows the ratio of the harmonic signal of pyrrole to furan for three different intensities at 1825 nm.…”

“…For this part, we focus our study on the five-membered ring molecules pyrrole (C 4 H 5 N), thiophene (C 4 H 4 S), and furan (C 4 H 4 O) that differ by a heteroatom. This is because (i) most measures of aromaticity are affected by the molecular size, making it difficult to compare molecules with different ring sizes [22], and (ii) the degree of aromaticity in these molecules was intensively investigated in the literature using different indicators. We show that the relative ratios of the plateau harmonics follow the same sequence proposed by most measures.…”

On the 6- and 7-substituted chromone system. A computational study

“…However, occasionally, different aromaticity indices provide a concordant pattern of aromaticity [17]. Interestingly, for series of compounds, it has been shown that it is possible to derive a unified aromaticity index [18]. It is important that the π-electron structure of a substituted aromatic system is usually quite resistant to the substituent effect [19,20].…”

Aromatic character of heptafulvene and its complexes with halogen atoms

“…There is increasing interest in studying the structural chemistry of these compounds, and numerous papers have recently been devoted to quantifying the aromaticities and other properties of these compounds [31][32][33][34][35][36]. The reliability of geometry predictions of heptafulvene obtained through various computational methods has also been assessed [37].…”