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Symmetry 2010, 2(1), 284-319; doi:10.3390/sym2010284

Diagnosis of the σ-, π- and (σ+π)-Aromaticity by the Shape of the NICSzz-Scan Curves and Symmetry-Based Selection Rules

1
Laboratory of Inorganic and General Chemistry, Department of Chemistry, University of Ioannina, 451 10 Ioannina, Greece
2
Laboratory of Applied Quantum Chemistry, Faculty of Chemistry, Aristotle University of Thessaloniki, 541 24 Thessaloniki, Greece
*
Author to whom correspondence should be addressed.
Received: 16 December 2009 / Revised: 21 January 2010 / Accepted: 24 February 2010 / Published: 15 March 2010
(This article belongs to the Special Issue Aromaticity and Molecular Symmetry)
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Abstract

The NICSzz-scan curves of aromatic organic, inorganic and “all-metal” molecules in conjunction with symmetry-based selection rules provide efficient diagnostic tools of the σ-, π- and/or double (σ + π)-aromaticity. The NICSzz-scan curves of σ-aromatic molecules are symmetric around the z-axis, having half-band widths approximately less than 3 Å with the induced diatropic ring current arising from Tx,y-allowed transitions involving exclusively σ-type molecular orbitals. Broad NICSzz-scan curves (half-band width approximately higher than 3 Å) characterize double (σ + π)-aromaticity, the chief contribution to the induced diatropic ring current arising from Tx,y-allowed transitions involving both σ- and π-type molecular orbitals. NICSzz-scan curves exhibiting two maxima at a certain distance above and below the molecular plane are typical for (σ + π)-aromatics where the π-diatropic ring current overwhelms the σ-type one. In the absence of any contribution from the σ-diatropic ring current, the NICSzz(0) value is close to zero and the molecule exhibits pure π-aromaticity.
Keywords: aromaticity; delocalization; σ-; π-; (σ+π)-aromaticity; NICSzz-scan; density functional theory (DFT) aromaticity; delocalization; σ-; π-; (σ+π)-aromaticity; NICSzz-scan; density functional theory (DFT)
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This is an open access article distributed under the Creative Commons Attribution License (CC BY 3.0).

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Tsipis, A.C.; Depastas, I.G.; Tsipis, C.A. Diagnosis of the σ-, π- and (σ+π)-Aromaticity by the Shape of the NICSzz-Scan Curves and Symmetry-Based Selection Rules. Symmetry 2010, 2, 284-319.

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