COMPUTATIONAL STUDIES OF CYCLOBUTADIENE AND BENZOCYCLOBUTENE FUSED TOP-QUINONE AND O-QUINONE

Cyclobutadiene and benzocyclobutenes fused to o- and p-quinone have be en studied by computational methods. Geometries were optimized at the B3LYP/6-31G level, and absolute NMR shielding values were calculated using the GIAO method with the HF/6-31G basis set. NICS values of the compounds 8b,c and 9b,c indicate strong antiaromatic character for cy clobutadiene units. However, 8a and 9a show negative NICS values where the quinodal system reduces the antiaromaticity significantly by forc ing these systems to possess a dimethylene-like structure. The calcula ted C-13 NMR chemical shifts of 6-9 and parent systems are in very goo d agreement with literature values.