Ab Initio Calculations of the Potential Surface for Rearrangement of 2,2,3,3-Tetrafluoromethylenecyclopropane to 1-(Difluoromethylene)-2,2-difluorocyclopropane

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