Remarks on the Importance of Resonance

Of the views expressed in the literature, we find two of them to be partly compatible with our findings. The model developed by Shaik, Hiberty and coworkers [55-61] states that the a-system prefers equal bond lengths, while then-system prefers alternating bond lengths. The resonance of the n-system is also a symmetrising influence. 

The viewpoint expressed by Glendening et al. [62] is that the resonance between the structures is the key factor for delocalisation. We find this as well. When there is no resonance in cyclobutadiene (and benzene), the molecule becomes asymmetric. Just resonance is not enough, however. Both benzene and square cyclobutadiene have large resonance energies. 

We can therefore now conclude that there are three contributions that determine the geometry of benzene and cyclobutadiene. The first contribution is that of the a-system which prefers equal bond lengths. The other two contributing factors are the 7l-bonds and the 7l-resonance. The 7i-bonds by themselves give rise to two states with a minimum at one of the geometries with alternating bond lengths. The resonance provides interaction between the two states and stabilises the symmetric structure. 

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