Wu Yong, Xue Ying, Kim Chan Kyung
Department of Chemistry, Inha University, Incheon 402-751, Korea.
J Comput Chem. 2008 Jun;29(8):1250-8. doi: 10.1002/jcc.20887.
In this study, 14 norbornadiene (NBD) dimers and the thermal dimerization mechanism were studied using the hybrid density functional theory (B3LYP) and the second-order multiconfigurational perturbation theory (CASPT2). In the process of dimerization, the biradical stationary points were located using the unrestricted, broken-spin, symmetry approach. The pathways were divided into eight parts to aid the analysis of their mechanisms. Our results indicated that the process for the formation of the cage-like heptacyclo[6.6.0.0.260.3130.411059.01014] tetradecane (HCTD, D14) is highly exothermic (92.15 kcal/mol), indicating that D14 is the most stable NBD dimer. However, the formation of D14 is very difficult to achieve kinetically because of a higher barrier in the thermal dimerization. On the contrary, the formation of exo-cis-exo (D5) is kinetically favorable, but thermodynamically unfavorable at higher temperature. Therefore, the combination of both thermodynamic and kinetic factors indicated that the formation of exo-exo (D9), which resembles the product of the pseudo-Diels-Alder addition, is most likely in the NBD dimerization.
在本研究中，使用杂化密度泛函理论（B3LYP）和二阶多组态微扰理论（CASPT2）研究了14种降冰片二烯（NBD）二聚体及其热二聚化机理。在二聚化过程中，采用无限制、破缺自旋、对称方法定位双自由基驻点。将反应途径分为八个部分以辅助分析其机理。我们的结果表明，笼状七环[6.6.0.0.260.3130.411059.01014]十四烷（HCTD，D14）的形成过程是高度放热的（92.15千卡/摩尔），这表明D14是最稳定的NBD二聚体。然而，由于热二聚化中存在较高的势垒，D14的形成在动力学上很难实现。相反，外型-顺式-外型（D5）的形成在动力学上是有利的，但在较高温度下在热力学上是不利的。因此，热力学和动力学因素的综合表明，类似于伪狄尔斯-阿尔德加成产物的外型-外型（D9）的形成最有可能发生在NBD二聚化过程中。
