Local Aromaticity in Polycyclic Aromatic Hydrocarbons: Electron Delocalization versus Magnetic Indices

Abstract

The local aromaticity of benzenoid rings is examined by means of the Polansky index (P) and generalized population analysis (GPA). The results are found to agree very well with recently published circuit-condensed ring currents and magnetic-energetic aromaticity indices, but no correlation is found with nucleus independent chemical shifts (NICS). This is usually seen as a manifestation of the more general multidimensional nature of aromaticity. This paper examines the sources for the observed correlations, showing that some indices give conflicting results because they inherently reflect different phenomena.

Correlations between Topological Ring-Currents, π-Electron Partitions, and Six-Centre Delocalisation Indices in Benzenoid Hydrocarbons

“…Figures 4 and 6 on the one hand, and of Figures 4 and 7 on the other, inevitably invite the general conclusion that -despite its superficially intuitive appeal -the "ring-current" index is out of step with the other two indices considered here -π-electron partitions and Δ 6 -values -as an index of what might be regarded as "local aromaticity". This conclusion agrees with the idea that aromaticity is a multi-dimensional and scaledependent phenomenon [12][13][14][15][16][17][18][19] -but see Ref. 15 for the opposite viewpoint.…”

“…Of the characteristic properties associated with conjugated systems, diamagnetic susceptibilities arising from π-electron ringcurrents [1][2][3][4] have been considered as one of the main diagnostics of aromaticity [5][6][7] -albeit a controversial one. [8][9][10][11] However, because of the so-called multidimensional character of aromaticity, [12][13][14][15][16][17][18][19] a quantitative measure requires the presence of several attributes, and this complicates quantitative assessments of the concept. One should note, however, that this multidimensional character has been discussed mostly for conjugated heterocycles, [20][21][22] whilst the present paper is about benzenoid hydrocarbons.…”

EL: the new aromaticity measure based on one-electron density function

“…It was postulated in many reports [4][5][6][7][8][9][10][11][12][13][14] and summarized in Tetrahedron report 520 [3] that aromaticity is a collective phenomenon and hence a variety of criteria should be used in the discussion of this term in any particular subject of investigation [15]. For this reason several measures of aromaticity were introduced on the basis of unique physical and chemical properties of the species considered to be aromatic.…”

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

“…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.…”