Ab initio calculations of the potential surfaces for rearrangement of methylenecyclopropane and 2,2-difluoromethylenecyclopropane. Why do the geminalfluorines have little effect on lowering the activation energy?

(4/4)CASSCF and CASPT2 calculations with the 6-31G* basis set have been per formed in order to understand the experimental observation that the geminal fluorines in 2,2-difluoromethylenecyclopropane (2) have only a small effec t on lowering the activation energies for its degenerate and non-degenerate methylenecyclopropane rearrangements, relative to the activation energy fo r the rearrangement of the hydrocarbon (1). As expected from previous exper imental and computational studies, the geminal fluorines are calculated to destabilize the three-membered ring in 2 thermodynamically. The small amoun t of kinetic destabilization of 2 is shown to be due to ii nearly equal des tabilization of the transition structures for its rearrangements. The high energies of the transition structures are attributed to the strong preferen ce of a CF2 group for a pyramidal geometry. This preference is found to des tabilize the transition structures both for forming a sigma bond to the flu orinated carbon in the degenerate methylenecyclopropane rearrangement of 2 and for making a pi bond to this carbon in the non-degenerate rearrangement of 2 to (difluoromethylene)cyclopropane (4).