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. Pi-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 pi-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.