| Abstract
| - (4/4)CASSCF and CASPT2 calculations have been performed to understand the reason thataddition of a second pair of geminal fluorines to methylenecyclopropane lowers the barrier to rearrangementby 6.7 kcal/mol more than addition of the first pair. Our calculations duplicate this experimental finding byDolbier and co-workers. Our computational results confirm Dolbier's conjecture, that the non-additive loweringof Ea for the rearrangement of 2,2,3,3-tetrafluoromethylenecyclopropane (9) to 1-(difluoromethylene)-2,2-difluorocyclopropane (11) is due to destabilization of 9 by the presence of the vicinal CF2 groups in thisfluorocarbon. In the course of exploring the potential energy surface for the rearrangement of 9, we havelocated a bond-stretch isomer (20) that differs from 9 by inversion of both CF2 groups. The enthalpy of 20is computed to be 21.9 kcal/mol higher than that of 9, but direct interconversion of these two “bond-stretchinvertomers” requires passage over a TS whose enthalpy is calculated to be 11.7 kcal/mol higher than thatof 20.
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