Ab initio MC-SCF study of thermal and photochemical [2 + 2] cycloadditions
Ab initio MC-SCF study of thermal and photochemical [2 + 2] cycloadditions
复制标题
热和光化学 [2 2] 环加成的从头算 MC-SCF 研究
DOI：
10.1039/ft9949001617
复制
发表时间：
1994
期刊：
影响因子：
--
通讯作者：
M. Robb
中科院分区：
文献类型：
--
作者：
F. Bernardi;A. Bottoni;M. Olivucci;A. Venturini;M. Robb
对一些典型的极性和非极性 [2 + 2] 环加成的研究结果在理论水平上提出，其中双自由基和两性离子机制可以以平衡的方式处理（CAS-SCF）。反应物的极性通过包含供体/受体取代基（二氰乙烯与羟基乙烯的环加成）或通过用杂原子（硅乙烯和甲醛）取代烯属CC双键的碳原子来建模。在每种情况下记录了两种不同类型的反应机制：（i）两步双自由基机制和（ii）协调机制。描述两步机制的表面拓扑结构几乎与取代基或杂原子引入的极性无关。相反，描述协同机制的表面拓扑结构对取代基和杂原子敏感。在非极性系统（例如乙烯二聚化）中，不存在协同的同步或异步supra-supra途径；然而，在极化 π 系统中，我们已经能够找到真正的超-超过渡态。由于这种协调的过渡态处于高能量，只有当考虑溶剂效应时，它最终才会变得重要。还提出了通过最低激发单线态 (S1) 和三线态 (T1) 发生的光化学 [2 + 2] 二聚化的研究。计算表明，S1 势能表面的最低能量区域以奇点（即尖点）为中心，该奇点对应于 S1 和 S0 表面之间的圆锥形交叉点。这种拓扑特征的存在似乎与从简单的 2,3-二取代烯烃高度立体定向地形成环丁烷有关。从非极性反应物到极性反应物时势表面拓扑的变化以及激发态衰变过程的性质都可以使用相同的简单 VB 模型合理化。
The results of a study of few prototypical polar and non-polar [2 + 2] cycloadditions are presented at a level of theory where biradical and zwitterionic mechanisms can be treated in a balanced way (CAS-SCF). The polarity of the reactants is modelled either through the inclusion of donor/acceptor substituents (cycloaddition of dicyanoethene to hydroxyethene) or through the replacement of a carbon atom of the ethylenic CC double bond with a heteroatom (silaethene and formaldehyde). Two different types of reaction mechanism are documented in each case: (i) a two-step biradical mechanism and (ii) a concerted mechanism. The surface topology describing the two-step mechanism turns out to be almost independent of the polarity introduced by the substituents or by heteroatoms. In contrast, the topology of the surface describing the concerted mechanism is sensitive to substituents and heteroatoms. In non-polar systems (e.g. ethene dimerization) a concerted synchronous or asynchronous supra–supra pathway does not exist; however, in polarized π-systems we have been able to locate a true supra–supra transition state. Since this concerted transition state is at high energy, it will eventually become important only when solvent effects are considered.A study of the photochemical [2 + 2] dimerization occuring via the lowest excited singlet (S1) and triplet states (T1) is also presented. The computations demonstrate that the lowest-energy region of the S1 potential-energy surface is centered on a singularity (i.e. a cusp) corresponding to a conical intersection between the S1 and S0 surfaces. The presence of this topological feature seems to be related to the highly stereospecific formation of cyclobutane from simple 2,3-disubstituted alkenes.Both the change in potential surface topology on moving from non-polar to polar reactants and the nature of the excited-state decay process are rationalized using the same simple VB model.