Department of Chemistry and Biochemistry, University of Mississippi, University, MS, USA.
Phys Chem Chem Phys. 2013 Aug 28;15(32):13593-600. doi: 10.1039/c3cp51283e.
The isomerizations of 3-aza-benzvalene to pyridine and 3,4-diaza-benzvalene to pyridazine have been studied using ab initio methods with a multiconfigurational wavefunction. Transition states for both the allowed disrotatory and forbidden conrotatory pathways were located. The forbidden pathways proceed through an intermediate consisting of pyridine or pyridazine with a trans double bond in the ring: this trans double bond destroys the aromatic stabilization of the π electrons due to poor orbital overlap between the cis and trans π bonds. Due to the Cs molecular point group, there are two separate allowed and forbidden pathways for 3-aza-benzvalene. The separation of the barrier heights was of particular interest: the difference in activation barriers for the lowest allowed and lowest forbidden pathways in 3-aza-benzvalene was only 1.3 kcal mol(-1), and the lowest forbidden pathway actually had a 1.5 kcal mol(-1) lower barrier than the highest allowed one. The 3-aza-benzvalene structure allows energy crossing of the allowed and forbidden barriers. For 3,4-diaza-benzvalene, there was only a single allowed and single forbidden pathway, due to the C2v point group, and they were separated by 8.4 kcal mol(-1), more in line with the orbital symmetry rules.
已使用含多组态波函数的从头算方法研究了 3-氮杂苯并环戊烯向吡啶和 3,4-二氮杂苯并环戊烯向哒嗪的异构化。定位了允许的反式旋转和禁止的顺式旋转两种途径的过渡态。禁止途径通过一个中间体进行，该中间体由吡啶或哒嗪组成，环中具有反式双键：由于顺式和反式π键之间的轨道重叠不良，这个反式双键破坏了π电子的芳香稳定化。由于 Cs 点群，3-氮杂苯并环戊烯有两个单独的允许和禁止途径。特别关注的是势垒高度的分离：3-氮杂苯并环戊烯中最低允许和最低禁止途径之间的活化势垒差异仅为 1.3 kcal/mol，最低禁止途径的势垒实际上比最高允许途径低 1.5 kcal/mol。3-氮杂苯并环戊烯的结构允许能量越过允许和禁止的势垒。对于 3,4-二氮杂苯并环戊烯，由于 C2v 点群，只有一个允许的和一个禁止的途径，它们之间的分离为 8.4 kcal/mol，更符合轨道对称规则。
