Mills–Nixon effect in heteroanalogs of cyclopropabenzene

Structural properties of cyclopropabenzene and its heteroanalogs are studied at the SCF 6-31G level of theory. It is found that the Mills–Nixon (MN ) type of localization within the benzene ring is present in these systems. The origin of the bond fixation lies in rehybridization at the carbon junction atoms. π-electron bond orders usually follow changes within the sigma framework but can be misleading sometimes. It is shown that a judicious choice of heteroatoms can considerably enhance the MN effect. A refined and better definition of the MN effect is offered. Finally, present calculations and analysis of the results in terms of hybridization model and π-bond orders strongly indicate that experimental ⁴J(H? C? C? Me) proton–proton spin–spin couplings over four bonds can be used in identification of the MN effect only with extreme caution.