Sigma Contribution to the Geometry of Benzene

Most recent studies have concluded that the regular hexagonal geometry of benzene results from the o bonds rather than the n bonds. This is reasonable since the a C-C bonds are stronger than the n bonds by about 20 kcal/mol. One would expect the a bonds at each carbon would have the same hybridization if at all possible, and thus in the absence of any other overriding factor, benzene should adopt its observed geometry in order to minimize its a energy. 

The structural preference for the n-bonds is not as clear. Berry noted that the antisymmetric C-C stretching mode, which effectively interconverts Kekule type structures, had an unusually low frequency, and concluded that the n-system preferred a Kekule-like structure.29 Similar conclusions have been derived from 

It has been suggested that equalization of bond lengths is one characteristic of aromatic compounds. However, the data in Table 2 shows that this is not a requirement for stabilization, and also shows that the n-system does have a large effect on the structure. 

One of the more useful criteria for aromatic character is derived from nmr chemical shifts. It is known that the protons of benzene are found at a lower field than ordinary olefinic protons, and it has been attributed to the ring current in the Ti-system which will reinforce the applied field at the protons. This has found confirmation in the observation that protons placed over an aromatic ring will be shifted upfield.39 A particularly striking effect is found with the planar cyclooctadecanonaene in which the outer hydrogens have an unusually large downfield shift (8 9.28) and the inner hydrogens have a remarkably large upfield shift (8 -2.99).40 

Many aromatic and antiaromatic compounds were included in this study, and it provides the best justification for the use of NMR chemical shifts in studying 

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