An ab initio study of the photochemical decomposition of 3,3-dimethyldiazirine
An ab initio study of the photochemical decomposition of 3,3-dimethyldiazirine
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DOI：
10.1021/jo000856m
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发表时间：
2000-11-17
影响因子：
3.6
通讯作者：
Soto, J
中科院分区：
文献类型：
--
作者：
Bernardi, F;Olivucci, M;Soto, J
采用高水平从头计算方法，在6- 31 G * 和cc-pvdz基组下，对3，3-二甲基二氮杂环丙烷（DMD）的光化学分解进行了计算研究.用完全活性空间自洽场（CAS-SCF）方法优化了最小值和过渡态的几何构型以及两表面相交处的圆锥交点，并用二阶多参考微扰（CAS/MP2）理论重新计算了它们的能量.从DMD的n-pi* 激发态开始的反应路径被预测为通过非绝热机制发生，产生卡宾和分子二氮（均处于单重基态）作为主要产物;计算的势垒高度（1.0 kcal mol（-1））与单重激发态活化能（0.0-1.5 kcal mol（-1））的实验估计值吻合得很好。二甲基卡宾的基态是唯一发生1，2-氢转移的物质，是丙烯的唯一来源。计算的势能势垒高度为二甲基卡宾丙烯异构化（2.6 kcal mol（-1））与观察到的活化能（2.56 kcal mol（-1））吻合得很好。没有发现DMD的第一单重激发态重排的证据;这样的过程将导致比观察到的更高的活化能。因此，已排除了在激发态下与N-2挤出同时发生的1，2-氢迁移。
Photochemical decomposition of 3,3-dimethyldiazirine (DMD) has been computationally investigated by using high-level ab initio calculations in conjunction with the 6-31G* and cc-pvdz basis sets. The geometries of minima and transition states, as well as conical intersection points in the seam of crossing of two surfaces, have been optimized with the complete active space self-consistent field (CAS-SCF) method, and their energies, recalculated with second-order multireference perturbation (CAS/MP2) theory. The reaction path starting at the excited n-pi* state of DMD is predicted to occur via a nonadiabatic mechanism, giving carbene and molecular dinitrogen (both in their singlet ground states) as the main products; the computed barrier height (1.0 kcal mol(-1)) agrees well with the experimental estimate of the activation energy in the singlet excited state (0.0-1.5 kcal mol(-1)). Ground state of dimethylcarbene is the only species where a 1,2-hydrogen shift takes place, being the only source of propene. The calculated potential energy barrier height for dimethylcarbene to propene isomerization (2.6 kcal mol(-1)) agrees well with the observed activation energy (2.56 kcal mol(-1)). No evidence for rearrangement in the first singlet excited state of DMD has been found; such a process would lead to a higher activation energy than the observed one. Consequently, 1,2-hydrogen migration concurrent with N-2 extrusion in the excited state has been ruled out.