CARBENE REARRANGEMENTS UNSURPASSED - DETAILS OF THE C7H6 POTENTIAL-ENERGY SURFACE REVEALED

The rearrangement of phenylcarbene (1) to 1,2,4,6-cycloheptatetraene ( 3) has been studied theoretically, using SCF, CASSCF, CASPT2N, DFT (B3 LYP), CISD, CCSD), and CCSD(T) methods in conjunction with the 6-31G, 6-311+G, 6-311G(2d,p), cc-pVDZ, and DZd basis sets. Stationary point s were characterized by vibrational frequency analyses at CASSCF(8,8)/ 6-31G and B3LYP/6-31G*. Phenylcarbene (1) has a triplet ground state ((3)A'') with a singlet-triplet separation (Delta E(ST)) of 3-5 kcal m ol(-1). In agreement with experiment, chiral 8 is the lowest lying str ucture on this part of the C7H6 potential energy surface. Bicyclo[4,1. 0]hepta-2,4,6-triene (2) is an intermediate in the rearrangement of 1 into 3, but it is unlikely to be observable experimentally due to a ba rrier height of only 1-2 kcal mol(-1). The enantiomers of 3 interconve rt via the (1)A(2) state of cycloheptatrienylidene (4) with an activat ion energy of 20 kcal mol(-1). The ''aromatic'' (1)A(1) state, previou sly believed to be the lowest singlet state of 4, is roughly 10 kcal m ol(-1) higher in energy than the (1)A(2) state, and, in violation of H und's rule, (3)A(2) is also calculated to lie above (1)A(2) in energy. Thus, even if (3)A(2) were populated, it is likely to undergo rapid i ntersystem crossing to (1)A(2). We suggest B-3(1)-4 is the metastable triplet observed by EPR.