Domingo Luis R, Pérez-Prieto Julia
Instituto de Ciencia Molecular/Departamento de Química Orgánica, Universidad de Valencia, Polígono La Coma s/n, 46980 Paterna, Valencia, Spain.
Chemphyschem. 2006 Mar 13;7(3):614-8. doi: 10.1002/cphc.200500431.
The ground- (S0) and lowest triplet-state (T1) pathways associated with dimerization of cyclohexadiene to give [2+2] and [4+2] cycloadducts have been theoretically studied at the UBLYP and UB3LYP levels of theory with the 6-31G* basis set. The DFT energies were validated by CCSD(T) single-point energy calculations. These cycloaddition reactions follow stepwise mechanisms with formation of bis-allylic biradical (BB) intermediates. In the S0 ground state, the interaction between two cyclohexadiene molecules with formation of BB intermediate IN(S0) has a large activation enthalpy of 32.0 kcal mol(-1). On the other hand, C-C bond-formation in the lowest triplet state (T1) leading to BB intermediate IN(T1) has a low activation enthalpy of 5.0 kcal mol(-1), but the subsequent ring closure involves a very large activation enthalpy of 43.4 kcal mol(-1). Triplet-to-singlet intersystem crossing from IN(T1) to IN(S0) favors cyclization to give the corresponding [2+2] and [4+2] cycloadducts.
采用6-31G*基组，在UBLYP和UB3LYP理论水平上对与环己二烯二聚反应生成[2+2]和[4+2]环加成产物相关的基态（S0）和最低三重态（T1）反应途径进行了理论研究。通过CCSD(T)单点能量计算对密度泛函理论（DFT）能量进行了验证。这些环加成反应遵循逐步机理，生成双烯丙基双自由基（BB）中间体。在S0基态下，两个环己二烯分子相互作用形成BB中间体IN(S0)，其活化焓较大，为32.0 kcal mol⁻¹。另一方面，在最低三重态（T1）下形成BB中间体IN(T1)的C-C键形成活化焓较低，为5.0 kcal mol⁻¹，但随后的闭环反应活化焓非常大，为43.4 kcal mol⁻¹。从IN(T1)到IN(S0)的三重态到单重态的系间窜越有利于环化反应，生成相应的[2+2]和[4+2]环加成产物。
