Comparative molecular orbital study of the lower annulenes.

The assignment of resonance energies in conjugated molecules is a problem of longstanding interest. The idea that the pi-electrons in certain bonds should be (de)stabilized as a function of ring size, chain length or topological connectivity is a relatively straightforward concept. The qualitative aspects of this viewpoint (in particular the (4n + 2) rule), have been appreciated for some time. Nevertheless, it has only been during the last twenty years that truly quantitative measures of the resonance energy have been advanced. This development was initiated by the work of Dewar, in which he redefined the reference energy for calculations of the energy of aromatic stabilization. His approach has been successfully implemented by a number of other authors using a variety of pi-electron procedures.