Comparison of the energy barriers to conformational inversion of the seven-membered rings in compounds of the types (I) and (II) shows that the change in delocalisation energy associated with the planar seven-membered rings is apparently due to increased Ar–C![[double bond, length as m-dash]](https://www.rsc.org/images/entities/char_e001.gif)
C–Ar and Ar–X–Ar conjugation, and is not a special property of the cyclic conjugated system.
M. Nógrádi, W. D. Ollis and I. O. Sutherland, J. Chem. Soc. D, 1970, 158 DOI: 10.1039/C29700000158
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