Applications of H\"uckel-Su-Schrieffer-Heeger method. I. Carbon-carbon
  bond lengths in policyclic benzenoid hydrocarbons

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Abstract

The equilibrium carbon-carbon bond lengths in \(\pi\)-electron hydrocarbons are very sensitive to the electronic ground-state characteristic. In the recent two papers by Stolarczyk and Krygowski (to appear soon) a simple quantum approach, the Augmented H\"uckel Molecular Orbital (AugHMO) model, is proposed for the qualitative, as well as quantitative, study of this phenomenon. The simplest realization of the AugHMO model is the H\"uckel-Su-Schrieffer-Heeger (HSSH) method, in which the resonance integral \(\beta\) of the HMO model is a linear function the bond length. In the present paper the HSSH method is applied in a study of carbon-carbon bond lengths in a set of 34 selected policyclic aromatic hydrocarbons (PAHs). This is exactly the set of molecules analyzed by Rieger and M\"ullen (\textit{J. Phys. Org. Chem.} \textbf{2010}, \textit{23}, 315) in the context of their electronic-excitation spectra. These PAHs have been obtained by chemical synthesis, but in most cases no diffraction data (by X-rays or neutrons) of sufficient quality is available to provide us with their geometry. On the other hand, these PAHs are rather big (up to 96 carbon atoms), and \textit{ab initio} methods of quantum chemistry are too expensive for reliable geometry optimization. That makes the HSSH method a very attractive alternative. Our HSSH calculations uncover a modular architecture of certain classes of PAHs. For the studied molecules (and their fragments -- modules) we calculate the values of the aromaticity index HOMA.

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Chemistry at the interior atoms of polycyclic aromatic hydrocarbons

Lawrence T Scott (2015)

Reactions that form new σ-bonds to interior carbon atoms of polycyclic aromatic hydrocarbons were unknown before the fullerene era. For more than 150 years, chemical reactions that make new covalent bonds to polycyclic aromatic hydrocarbons (PAHs) have been confined almost exclusively to substitution and addition reactions on the perimeters of the compounds (“edge chemistry”). The “interior” atoms of PAHs, those belonging to three rings, almost never engage in new σ-bond-forming reactions. A compound with no edges, C 60 , was the first polycyclic carbon π-system observed to exhibit such reactivity. More recently, smaller subunits of C 60 , which we call geodesic polyarenes, have also been found to exhibit “fullerene-type chemistry” at their interior carbon atoms. These reactions are all reviewed together here for the first time. The review ends with speculation that σ-bond-forming reactions may also be observed someday even in certain planar, benzenoid PAHs, although no examples have yet been reported.