Veals Jeffrey D, Poland Kimberley N, Earwood William P, Yeager Spencer M, Copeland Kari L, Davis Steven R
Department of Chemistry and Biochemistry, University of Mississippi , University, Mississippi, United States.
Division of Mathematics and Sciences, Allen University , Columbia, South Carolina, United States.
J Phys Chem A. 2017 Nov 22;121(46):8899-8911. doi: 10.1021/acs.jpca.7b08227. Epub 2017 Nov 10.
The isomerizations of 3-aza-3-ium-dihydrobenzvalene, 3,4-diaza-3-ium-dihydrobenzvalene, and 3,4-diaza-diium-dihydrobenzvalene to their respective cyclic-diene products were studied using electronic structure methods with a multiconfigurational wave function and several single reference methods. Transition states for both the allowed (conrotatory) and forbidden (disrotatory) pathways were located. The conrotatory pathways of each structure all proceed through a cyclic intermediate 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. The 3,4-diaza-3-ium-dihydrobenzvalene structure has C symmetry, and there are four separate allowed and forbidden pathways for this structure. The 3-aza-3-ium-dihydrobenzvalene structure is C symmetric, and there are two separate allowed and forbidden pathways for this structure. For 3,4-diaza-3,4-diium-dihydrobenzvalene, there was a single allowed and single forbidden pathway due to the C symmetry. The separation of the barrier heights for all three molecules was studied, and we found the difference in activation barriers for the lowest allowed and lowest forbidden pathways in 3,4-diaza-3-ium-dihydrobenzvalene, 3-aza-3-ium-dihydrobenzvalene, and 3,4-diaza-diium-dihydrobenzvalene to be 9.1, 7.4, and 3.7 kcal/mol, respectively. The allowed and forbidden barriers of 3,4-diaza-diium-dihydrobenzvalene were separated by 3.7 kcal/mol, which is considerably less than the 12-15 kcal/mol expected based on the orbital symmetry rules. The addition of the secondary ammonium group tends to shift the conrotatory and disrotatory barriers up in energy (∼12-14 kcal/mol (conrotatory) and 5-10 kcal/mol (disrotatory) per secondary NH group) relative to the barriers of dihydrobenzvalene, but there is negligible effect on E,Z to Z,Z isomerization barriers, which remain in the expected range of greater than 4 kcal/mol.
使用具有多组态波函数的电子结构方法和几种单参考方法，研究了3-氮杂-3-鎓-二氢苯并戊搭烯、3,4-二氮杂-3-鎓-二氢苯并戊搭烯和3,4-二氮杂-二鎓-二氢苯并戊搭烯异构化为各自的环状二烯产物的过程。确定了允许（顺旋）和禁阻（对旋）途径的过渡态。每种结构的顺旋途径均通过环中具有反式双键的环状中间体进行：由于顺式和反式π键之间的轨道重叠不佳，这种反式双键破坏了π电子的芳香稳定性。3,4-二氮杂-3-鎓-二氢苯并戊搭烯结构具有C对称性，该结构有四条独立的允许和禁阻途径。3-氮杂-3-鎓-二氢苯并戊搭烯结构是C对称的，该结构有两条独立的允许和禁阻途径。对于3,4-二氮杂-3,4-二鎓-二氢苯并戊搭烯，由于C对称性，有一条允许途径和一条禁阻途径。研究了所有三种分子的势垒高度差异，我们发现3,4-二氮杂-3-鎓-二氢苯并戊搭烯、3-氮杂-3-鎓-二氢苯并戊搭烯和3,4-二氮杂-二鎓-二氢苯并戊搭烯中最低允许途径和最低禁阻途径的活化势垒差分别为9.1、7.4和3.7 kcal/mol。3,4-二氮杂-二鎓-二氢苯并戊搭烯的允许和禁阻势垒相差3.7 kcal/mol，这远低于基于轨道对称规则预期的12 - 15 kcal/mol。相对于二氢苯并戊搭烯的势垒，仲铵基团的添加倾向于使顺旋和对旋势垒在能量上向上移动（每个仲NH基团约12 - 14 kcal/mol（顺旋）和5 - 10 kcal/mol（对旋）），但对E,Z到Z,Z异构化势垒的影响可忽略不计，其仍在预期的大于4 kcal/mol的范围内。
