Citethis:Phys. Chem. Chem. Phys.,2011,13 ,2053620548 PAPER .chemistry.1 Qualitative definitions

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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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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