Department of Physical Chemistry, Faculty of Science, University of Málaga, Andalucía Tech., E-29071 Málaga, Spain.
Phys Chem Chem Phys. 2019 Jan 30;21(5):2389-2396. doi: 10.1039/c8cp06974c.
Photochemistry of 3- and 4-methoxyphenyl azide at 266 nm has been studied by means of the complete active space self-consistent field (CASSCF) and multi-configurational second-order perturbation (MS-CASPT2) methods. Minima and interstate crossing points have been optimized using the CASSCF method. The calculations predict that the key step of the photolysis of both azides is a non-radiative process. However, an important difference is found when we compare the reactivity of both isomers of azide, deactivation of 3-methoxyphenyl azide (1) can occur via two reaction channels (internal conversion or intersystem crossing), which lead to formation of the dimer of 2H-azepine derivative (2a) and 3,3'-dimethoxyazobenzene (2b). In contrast, deactivation of 4-methoxyphenyl azide (3) takes place via a singlet to triplet intersystem crossing, which leads to formation of 4,4'-dimethoxyazobenzene (4). After initial deactivation, both isomers follow a cascade of surface crossings until they reach the final nitrenes, respectively. The reference active space for the two azides is 14 electrons in 13 orbitals and comprises the six π-type orbitals of the aromatic ring plus four σ-(N-N2) and five π-type orbitals of the -N3 moiety.
用完全活性空间自洽场 (CASSCF) 和多组态二级微扰 (MS-CASPT2) 方法研究了 3-和 4-甲氧基苯叠氮在 266nm 处的光化学。使用 CASSCF 方法优化了最小值和态间交叉点。计算预测，两种叠氮化物光解的关键步骤是非辐射过程。然而，当我们比较两种异构体的反应性时，会发现一个重要的区别，即 3-甲氧基苯叠氮（1）的失活可以通过两个反应通道（内转换或系间窜越）发生，这导致 2H-氮杂环庚烷衍生物（2a）和 3,3'-二甲氧基偶氮苯（2b）的二聚体的形成。相比之下，4-甲氧基苯叠氮（3）的失活通过单重态到三重态系间窜越发生，导致 4,4'-二甲氧基偶氮苯（4）的形成。初始失活后，两种异构体都经历一系列表面交叉，直到分别达到最终的氮宾。两种叠氮物的参考活性空间为 13 个轨道中的 14 个电子，包括芳环的六个 π 型轨道加上 -N3 部分的四个 σ-(N-N2) 和五个 π 型轨道。
