Exceptionally pyramidalized olefins: A theoretical study of the cyclopropenyl fused tricycles tricyclo[3.2.1.0(2,4)]oct-2(4)-ene, tricyclo[3.2.1.0(2,4)]octa-2(4),6-diene, tricyclo[3.2.2.0(2,4)]non-2(4)-ene, and tricyclo[3.2.2.0(2,4)]nona-2(4),6-diene

RHF, MP2, and TCSCF ab initio theory and B3LYP, B3PW91, and SVWN density fu nctional theory were used to study the series of cyclopropenyl-fused tricyc les 9-12. In each of 9-12, the cyclopropenyl double bond is exceptionally p yramidalized (butterfly angle psi similar to 41-50 degrees) with both endo and exo bent isomers. In the norbornyl systems (9 and 10), the endo bent is omers are more stable than the exo bent isomers, whereas in the bicyclo[2.2 .2]octadiene 12 the reverse is true with the exo bent isomer being the low energy form. The activation barriers for the endo/exo interconversions are calculated to be relatively low (Delta H-not equal similar to 6-13 kcal/mol ).