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 functional theory were used to study the series of cyclopropenyl-fused tricycles 9-12. In each of 9-12, the cyclopropenyl double bond is exceptionally pyramidalized (butterfly angle psi approximately 41-50 degrees ) with both endo and exo bent isomers. In the norbornyl systems (9 and 10), the endo bent isomers 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 (DeltaH() approximately 6-13 kcal/mol).