To What Extent Can Aromaticity Be Defined Uniquely?

Abstract

Statistical analyses of quantitative definitions of aromaticity, ASE (aromatic stabilization energies), RE (resonance energies), Lambda (magnetic susceptibility exaltation), NICS, HOMA, I5, and A(J), evaluated for a set of 75 five-membered pi-electron systems: aza and phospha derivatives of furan, thiophene, pyrrole, and phosphole (aromatic systems), and a set of 30 ring-monosubstituted compounds (aromatic, nonaromatic, and antiaromatic systems) revealed statistically significant correlations among the various aromaticity criteria, provided the whole set of compounds is involved. Hence, broadly considered, the various manifestations of aromaticity are related and aromaticity can be regarded statistically as a one-dimensional phenomenon. In contrast, when comparisons are restricted to some regions or groups of compounds, e.g., aromatic compounds with ASE > 5 kcal/mol or polyhetero-five-membered rings, the quality of the correlations can deteriorate or even vanish. In practical applications, energetic, geometric, and magnetic desriptors of aromaticity do not speak with the same voice. Thus, in this sense, the phenomenon of aromaticity is regarded as being statistically multidimensional.

Quantifying aromaticity with electron delocalisation measures

“…50 For this reason, it is widely accepted that the concept of aromaticity should be analysed by employing a set of aromaticity descriptors. 24,51,52 The importance of electron delocalisation in aromatic species is universally recognised. It is thus reasonable to employ electron delocalisation as a tool to construct new aromaticity indicators.…”

Multidimensionality of delocalization indices and nucleus‐independent chemical shifts in polycyclic aromatic hydrocarbons II: Proof of further nonlocality

“…Although originally well defined in terms of similarity between benzenoid rings [2] , over time it has been used for many different classes of molecules, making it a fuzzy concept. The lack of an immediate observable has resulted in a multiplicity of indices to quantify aromaticity, based on energetic [3] , geometric [4] , quantum chemical [5] or magnetic considerations [6][7][8][9][10] . A concept closely related to aromaticity is that of electron delocalization, of which the Multi Centre Bond Index (MCBI), recently introduced by our group [11][12][13][14] , has been shown to be a successful measure [15] .…”

“…In the family of magnetic measures of aromaticity, one of the most popular indices is the so-called Nucleus Independent Chemical Shift (NICS) [6,7,16,17] which is the negative of the magnetic shielding computed at the centre of the ring for which the degree of aromaticity is to be assessed. The lack of correlation between some of these indices has put forward suggestions that aromaticity is multidimensional phenomenon [3,[18][19][20][21] .…”

Aromatic molecular zones and fragments

“…The aromaticity of phosphole and arsole (X = PH and X = AsH) is matter in dispute more than 35 years. 9,[57][58][59][60] In phosphole and arsole the chemical bonds C-X are computed as "single" (B < α). Therefore, these cycles are non-aromatic, in TOPAZ meaning ("discontinuous conjugation").…”

“…In naphthalene (A = 608), anthracene (A = 531), [9] helicene (A = 445), fullerene C 60 (A = 252), indole (A = 448), for instance, the TOPAZ algorithm identifies unique polycyclic aromatic zones that include all the bonds between the heavy atoms.…”