Electrostatic Topographical Viewpoint of π-Conjugation and Aromaticity of Hydrocarbons.

A molecular electrostatic potential (MESP) topographical study has been conducted for a variety of conjugated hydrocarbons at B3LYP/6-311+G(d,p) level of theory to understand their π-conjugation features and aromaticity. The value of MESP minimum (V_m) is related to the localized/delocalized distribution of π-electron density. The V_m are located interior to the rings in polycyclic benzenoid hydrocarbons (PBHs) whereas they lie outside the boundary of the rings in antiaromatic and in fused systems consisting of aromatic and antiaromatic moieties. The V_m points lie on top and bottom of the π-regions in linear polyenes and annulenes, while a degenerate distribution of CPs around the midpoint region of triple bonds is observed in alkynes. The eigenvalues λ₁, λ₂ and λ₃ of the Hessian matrix at V_m (MESP minima) are used to define the aromatic character of the cyclic structures. The eigenvalues follow the trend λ₁ > λ₂ > λ₃ ≅ 0 in PBHs, λ₁ > λ₂ > λ₃ ≅ 0 in linear polyenes and λ₁ > λ₂ > λ₃ ≠ 0 in antiaromatic systems. The difference in the aromatic character of PBHs from that of benzene is related with the deviations ∆λ₁, ∆λ₂ and ∆λ₃. The total deviation ∑_(i=1)³(∆λ_i ) is found to be ≤ 0.011 a.u. for all aromatic systems and lies between 0.011 to 0.035 a.u. for all non-aromatic systems. For antiaromatic systems its value is found to be above 0.035 a.u. Further, ∑_(i=1)³(∆λ_i ) gives a direct interpretation of Clar's aromatic sextet structures for PBHs. In summary, MESP topography mapping is a powerful technique to quantify the localized and delocalized π-electron distribution in a variety of unsaturated hydrocarbon systems.