FINE TUNING OF THE MILLS-NIXON EFFECT - INFLUENCE OF SUBSTITUENTS IN SOME 1,2-DIHYDROBENZOCYCLOBUTENES AND BENZOCYCLOBUTENES

A particular form of the 3-21 G ab initio procedure is employed in estimating structural features of 1,2-dihydrobenzocyclobutenes and benzocyclobutenes. The Mills-Nixon (MN) type of bond shortening is identified in the parent hydrocarbon systems. Its extent can be amplified, diminished, completely cancelled or changed in sign depending on the electronegativity of the substituents at position 3. It is established that the underlying mechanism is that of rehybridization of the atoms of the benzene ring.