Multidimensionality of delocalization indices and nucleus independent chemical shifts in polycyclic aromatic hydrocarbons

Abstract

The aromaticity and local-aromaticity of a large set of polycyclic aromatic hydrocarbons (PAHs) is studied using multicenter delocalization indices from generalized population analysis and the popular nucleus independent chemical shift (NICS) index. A method for the fast computation of the NICS values is introduced, using the so-called pseudo-pi-method. A detailed examination is made of the multidimensional nature of aromaticity. The lack of a good correlation between the NICS and the multicenter delocalization indices is reported and the grounds discussed. It is shown through a thorough statistical analysis that the NICS values arise not only from local aromaticity of the benzenoid rings, but also from other circuits. It is shown that the NICS indices do not reveal the individual aromatic nature of a specific ring, contrary to the delocalization indices.

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

“…Through statistical analysis, the present authors have previously shown that for a large set of PAH, the NICS values can be derived from MCBI, refuting the need to invoke a local analogue of the multidimensionality of aromaticity. It was also shown that ring current maps derived from MCBI, agree very well with ab initio computed ring current maps [1,22,23] .…”

“…The MCBI for all these molecules were calculated using the Pseudo-π approach [1,[28][29][30] . The MCBI were computed using the in home developed Kekule program which uses the Gaussian-03 [31] formatted checkpoint file.…”

“…The MCBI were computed using the in home developed Kekule program which uses the Gaussian-03 [31] formatted checkpoint file. The NICSzz [10,32] were also calculated in the Pseudo-π approach [1] using Gaussian-03. For the study of correlations between the NICS and multicenter indices, the ARTE-QSAR program was used [33] .…”

“…Supposing that for each circuit the NICS contribution and the multicenter index correlate and that the contribution of circuits that do not encircle the point at which the NICS is calculated is negligible [27] , the total NICS at the centre of a given six-membered ring X can be expressed as a linear combination of the six-centre index (SCI), the Naphthalene-like ten-centre indices (TCI) and the Anthracene-and Phenanthrene-like fourteen-centre indices (FCI-a and FCI-b) for the circuits that include X [1] :…”

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1

AbstractIn a recent contribution we examined the effect of ten-and fourteencenter circuits on the Nucleus Independent Chemical Shifts (NICS) using Multicentre Bond Indices (MCBI) [1] . In this study the non-local contributions to the NICS are further investigated for a larger set of polycyclic aromatic hydrocarbons (PAH).

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