Abstract

Aromaticity is a multidimensional concept and not a directly observable. These facts have always stood in the way of developing an appropriate theoretical framework for scaling of aromaticity. In the present work, a quantitative account of aromaticity is developed on the basis of cyclic delocalization of π-electrons, which is the phenomenon leading to unique features of aromatic molecules. The stabilization in molecular energy, caused by delocalization of π-electrons is obtained as a second order perturbation energy for archetypal aromatic systems. The final expression parameterizes the aromatic stabilization energy in terms of atom to atom charge transfer integral, onsite repulsion energy and the population of spin orbitals at each site in the delocalized π-electrons. An appropriate computational platform is framed to compute each and individual parameter in the derived equation. The numerical values of aromatic stabilization energies obtained for various aromatic molecules are found to be in close agreement with available theoretical and experimental reports. Thus the reliable estimate of aromaticity through the proposed formalism renders it as a useful tool for the direct assessment of aromaticity, which has been a long standing problem in chemistry.

Highlights

  • INTRODUCTIONThe phenomenon of aromaticity can be regarded as multidimensional since it cannot be described unambiguously by any single property based index and all indices do not always give consistent result

  • Aromaticity, a topic of enduring interest in Chemistry, is one such concept which lacks a proper definition and the term is used by most of the chemists with an intuitive notion.[1]

  • In spite of the inherent diversity, the aromaticity indices are found to stem from a single phenomenon, which is the cyclic delocalisation of electrons

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Summary

INTRODUCTION

The phenomenon of aromaticity can be regarded as multidimensional since it cannot be described unambiguously by any single property based index and all indices do not always give consistent result. Introduction of isodesmic[28] and homodesmotic reactions,[29] has made it possible to calculate the ASE from the energies of reactants and products, obtained experimentally from calorimetric measurements.[30] There are two basic approaches to measure aromaticity in terms of thermochemical data.1(b) In the first, the heat of formation of a conjugated system is compared with that of a nonconjugated model and the difference is taken as the resonance energy In this method, the ASE of benzene is found to vary in the range of 22 - 64 kcal/mol depending upon the choice of suitable nonconjugated model. The present method avoids energy comparison between different fragments and gets rid of the empiricism inherent in the design of an appropriate reference state.[33]

THEORETICAL DETAILS
COMPUTATIONAL METHODOLOGY
The onsite repulsion energy U
The intersite hopping integral tij
The spin orbital population niσ
Benzene
Pyridine
Pyrrole
Thiophene
CONCLUSION
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