REVERSED MILLS-NIXON EFFECT IN BENZOBORIRENE AND BENZOCYCLOPROPENYL CATIONS

The structural properties of benzoborirene (2a) and the benzocyclopropylene cation (3a) are examined at the HF 6-31G* level of theory. It is shown that a pronounced reversed Mills-Nixon (MN) type of deformation takes place in these molecular systems within the aromatic fragment. This feature is rationalized by a strongly increased delocalization of pi electrons over the fused three-membered ring and rehybridization at the carbon junction atoms. The results are compared with those for the parent benzocyclopropene (1a) compound where MN distortion is operative.