Resonance, among bond structures frequency

II, plus some additional terms hence, according to the fundamental ideas of quantum mechanics, if it were possible to carry out an experimental test of the electronic structure Oust would identify structure I or structure II, each structure would be found for the molecule to the extent determined by the waee function. The difficulty for benzene and for other molecules showing electronic resonance is to devise an experimental test that could be carried out quickly enough and that would distinguish among the structures under discussion. In benzene the frequency of Kekuld resonance is only a little less than the frequency of the bonding resonance of electron pairs, so that the time required for the experiment is closely limited. 

The four different kinds of carbon-carbon bonds are represented by 1-2, 1-9, 2-3, and 9-10. (Every other carbon-carbon bond is equivalent to one of these four. For example, 6-7 is equivalent to 2-3, 7-8 is equivalent to 1-2, etc.) The bond with the greatest double-bond character should be the shortest. Among the three resonance structures shown in Fig. 9-33, the frequency of double bonds for the various bond types is as follows 2 in 1-2 [in (a) and (c)], 1 in 1-9 [in (b)], 1 in 2-3 [in (b)], and 1 in 9-10 [in (c)]. Bond 1-2 is expected to have the greatest double-bond character and to be the shortest. This prediction is found to be true experimentally. The above four bonds are found to have lengths of 136.5, 142.5, 140.4, and 139.3 pm, respectively. 

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