Pictorial representation and validation of Clar's aromatic sextet theory using molecular electrostatic potentials

Abstract

Topographical features of the molecular electrostatic potential (MESP) of a series of polycyclic aromatic benzenoid hydrocarbons have been analyzed at B3LYP/6-31+G(d,p) and MP2/6-31+G(d,p) levels of theory to study the relationship between π-electron distribution and Clar's aromatic sextet theory. For all the molecules, MESP isosurface plots provided an unambiguous visual representation of Clar's aromatic sextet theory. For molecules represented using a single dominant Clar structure, a complete agreement of Clar's theory was observed. For benzenoid structures showing more than one Clar structure, MESP isosurface lobes reflected the merger of possible Clar structures, and even in such cases, the most dominant configuration was clearly obtained in the MESP topography. The MESP at the ring critical point (V_rcp) of the π-electron cloud of every ring was found to be a good local measure of aromaticity. A hydrogenation scheme was introduced to calculate the aromatic stabilization energy (E_aroma) of the peripheral rings. E_aroma and V_rcp showed a good linear correlation. Furthermore, a linear correlation of V_rcp with the widely used geometric index of aromaticity, HOMA, was obtained. Thus, for a visual representation as well as rationalization of Clar's theory, MESP serves as an important descriptor of the π-regions, and V_rcp values give further insight into this theory by quantifying the local aromaticity of each ring in the polycyclic aromatic system. The theoretical results presented herein fully support the ideas of Clar's empirical sextet theory—a theory mainly based on experimental findings of the local reactivity of condensed aromatic systems.