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  • See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/51463793

    Aromaticity in heterocyclic analogues of benzene: Comprehensive analysis of

    structural aspects, electron delocalization and magnetic characteristics

    ArticleinPhysical Chemistry Chemical Physics July 2011

    DOI: 10.1039/c1cp20905aSource: PubMed

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    Irina V Omelchenko

    National Academy of Sciences of Ukraine

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    Oleg V Shishkin

    National Academy of Sciences of Ukraine

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    Leonid Gorb

    Institute of Molecular Biology and Genetics, Ukraine, Kiev

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    Jackson State University

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  • 20536 Phys. Chem. Chem. Phys., 2011, 13, 2053620548 This journal is c the Owner Societies 2011

    Cite this: Phys. Chem. Chem. Phys., 2011, 13, 2053620548

    Aromaticity in heterocyclic analogues of benzene: comprehensive analysisof structural aspects, electron delocalization and magnetic characteristics

    Irina V. Omelchenko,a Oleg V. Shishkin,*ab Leonid Gorb,cd Jerzy Leszczynski,c

    Stijn Fiase and Patrick Bultinck*e

    Received 24th March 2011, Accepted 31st May 2011

    DOI: 10.1039/c1cp20905a

    The degree of aromaticity of six-membered monoheterocycles with IVVI group heteroatoms

    (C6H5X, where X = SiH, GeH, N, P, As, O+, S+, Se+) was analyzed using the results of

    ab initio calculations at the MP2/cc-pvtz level. Values of common aromaticity indices including

    those based on electronic delocalization properties, structuraldynamic features and magnetic

    properties all indicate high aromaticity of all considered heterocycles. A decrease in aromaticity is

    observed with increasing atomic number of the heteroatom, except in the case of the pyrylium

    cation. However, not all types of indices or even different indices within the same type correlate

    well among each other. Ring currents have been obtained at the HF/cc-pvdz level using the

    ipsocentric formulation. Ring current maps indicate that in the case of cationic heterocycles the

    ring current persists in all molecules under consideration. The different conclusions reached

    depending on the type of index used are a manifestation of the fact that when not dealing with

    hydrocarbons, aromaticity is ill-defined. One should always express explicitly which property of

    the molecules is considered to establish a degree of aromaticity.

    Introduction

    Aromaticity is one of the fundamental concepts in organic

    chemistry.1 Qualitative definitions of aromaticity were formulated

    long ago2 on the basis of the structure of benzenoid hydrocarbons

    and their unusual properties namely their symmetric structure

    with bond length equalization, thermodynamic stability, reactivity,

    special magnetic properties etc. These compounds composed of

    only carbon atoms do not exhibit significant intrinsic polarization

    of the s-skeleton, creating very favorable conditions for p-electrondelocalization.

    Assessing the degree of aromaticity of cyclic conjugated

    systems requires some quantitative descriptors. Many indices

    of aromaticity have been suggested for this purpose. All earlier

    indices are based on experimental data only (e.g., reactivity

    in the DielsAlder reaction, burning or hydrogenation).1

    However, their values depend significantly on the experimental

    conditions, methods of experimental measurement, available

    reagents, etc. Therefore, comparison of the values obtained

    can be a very difficult task. Computational chemistry methods

    allow overcoming the limitations of having to use experimental

    data. Most traditional aromaticity indices are based on some

    common physico-chemical properties of aromatic molecules,3

    and can be divided into several groups.Minkin et al.1 distinguish

    (i) structural indices (Bir

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