Benzene ring deformations calculations

Campanelli, A. R. Arcadi, A. Domenicano, A. Ramondo, F. Hargittai, I. Molecular structure and benzene ring deformation of three ethynylbenzenes from gas-phase electron diffraction and quantum chanical calculations, J. Phys. Chem. A 2006,110, 2045-2052. 

In the FIR spertrum of PS (Fig. 4, curve 3) [5, 29], similar calculations allowed the absorption band at 410 cm (the calculated value is 421 cm ) to be assigned to skeletal vibrations coupled with the 9b mode of the benzene ring. The low-intensity ateorption band at 325 cm (the calculated value is 358 cm ) is mainly accounted for by a C-C-C deformation of the backbone chain. A more intensive complicated band at 216 cm (the calculated value is 243 cm ) is assigned to the 9b mode of benzene rings overlapped by drain deformation mode. Absorption bands at 245 and 80 cm remained unrecognized in the scope of the approach considered. They will be di ussed below. 

SC calculations have subsequently also been carried out on many aromatic systems, such as heterocyclic five- and six-membered rings, on naphthalene and on azulene. For naphthalene and azulene, with 10 n electrons, the orbitals obtained are very similar to those of benzene, with the exception of the two orbitals localized at each of the C atoms which bridge the two rings. These orbitals display a three-way deformation, towards each of the three adjacent carbon atoms. In addition equation (12) shows that for a 10-electron system there are 42 possible spin functions which should be taken into account. But since the SC orbitals are fully optimized, it turns out that the only spin functions which play any significant role in these molecules are those corresponding to the Kekul structures (in the case of naphthalene, structures (3), (4), and (5) in Section 3) and that the contribution of the other 39 structures may be neglected. 

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