Rearrangements on the C6H6 potential energy surface and the topomerization of benzene

The benzene potential energy hyperface was examined employing hybrid Hartne-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 [1,13-C-13(2)]- and [1,4-C-13(2)]benzene: (a) benzene ring contraction to benzvalene (Delta G double dagger = 93.5 kcal mol(-1)) followed by ring opening to benzene; (b) degenerate rearrangement of benzvalene via a (1)A " prefulvene TS (Delta G double dagger = 95.0 kcal mol(-1) relative to benzene) generating [1,4-C-13(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 (Delta G double dagger = 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.