Rearrangements on the C₆H₆Potential Energy Surface and the Topomerization of Benzene

The benzene potential energy hyperface was examined employing hybrid Hartree−Fock/density functional theory (B3LYP), second-order perturbation theory (MP2), and the coupled-cluster method with single, double, and perturbative triple excitations [CCSD(T)] in conjunction with DZP and TZ2P basis sets. All stationary points were characterized by harmonic vibrational frequency analyses; intrinsic reaction coordinates were calculated for all transition structures at B3LYP/DZP. Final energies were evaluated at the CCSD(T)/DZP//B3LYP/DZP level and corrected for T = 1373 K. There are three competing mechanisms for the high-temperature intramolecular topomerization of [1,2- 13 C 2 ]benzene to [1,3- 13 C 2 ]- and [1,4- 13 C 2 ]benzene: (a) benzene ring contraction to benzvalene (Δ G ⧧ = 93.5 kcal mol - 1 ) followed by ring opening to benzene; (b) degenerate rearrangement of benzvalene via a 1 A‘‘ prefulvene TS (Δ G ⧧ = 95.0 kcal mol - 1 relative to benzene) generating [1,4- 13 C 2 ]benzene as a primary reaction product of [1,2- 13 C 2 ]benzene; (c) [1,2]-H shift in benzene to yield 2,4-cyclohexadienylidene, followed by ring contraction to bicyclo[3.1.0]hexa-1,3-diene (Δ G ⧧ = 96.7 kcal mol - 1 ) and ring opening to fulvene. As these mechanisms are all within 3.2 kcal mol - 1, it is unlikely that benzene topomerizes at 1373 K exclusively via one mechanism.