Department of Chemistry and the Center for Advanced, Scientific Computing and Modeling, University of North Texas, 1155 Union Circle, #305070, Denton, Texas 76203-5070, USA.
J Phys Chem A. 2010 Mar 18;114(10):3683-90. doi: 10.1021/jp911705y.
(U)B3LYP calculations with the 6-31+G(d) and 6-311+G(2df) basis sets have been carried out on cyclooctatetraenes 6 and 7, in which the COT ring is tetrakis-annelated with alpha-dithio- or alpha-diselenocarbonyl groups. Transferring two electrons from the high-lying b(1g) and e(u) sigma MOs in 6 and 7 into the unoccupied, nonbonding, COT pi orbital is computed to be energetically favorable. The lowest D(4h) electronic state is calculated to be (3)E(u), which formally contains 10 pi electrons in the eight-membered ring and has two unpaired electrons in sigma MOs. The (3)E(u) state undergoes a first-order Jahn-Teller distortion to form 6d and 7d, in which the pair of one-electron holes in the sigma MOs is stabilized by the formation of two, two-center, three-electron bonds between pairs of chalcogen atoms that are diagonally across the eight-membered ring from each other. The corresponding open-shell singlets are computed to be about 1 kcal/mol lower in energy than the Jahn-Teller distorted triplets. Molecules 6i and 7i, in which the C-C bond in one four-membered ring is cleaved, are computed to be lower in energy than 6d and 7d. However, a substantial barrier is predicted to separate each of the two pairs of isomers so that 6d and 7d should, at least in principle, be isolable.
（U）已使用 6-31+G(d) 和 6-311+G(2df) 基组对环辛四烯 6 和 7 进行了 B3LYP 计算，其中 COT 环被 α-二硫代或 α-二硒代羰基基团四元环合。将 6 和 7 中高能 b(1g)和 e(u)σMO 中的两个电子转移到未占据的非键合 COT π轨道被计算为能量有利的。最低 D(4h)电子态计算为(3)E(u)，其在八元环中正式包含 10 个π电子，并且在σMO 中有两个未配对电子。(3)E(u)态经历一级 Jahn-Teller 畸变以形成 6d 和 7d，其中σMO 中的一对单电子空穴通过在彼此对角的八元环上的两对硫属原子之间形成两个二中心三电子键来稳定。相应的开壳 singlet 被计算为比 Jahn-Teller 扭曲三重态低约 1 kcal/mol。分子 6i 和 7i，其中一个四元环中的 C-C 键被切断，被计算为比 6d 和 7d 能量更低。然而，预测存在一个相当大的势垒来分离每对异构体，因此 6d 和 7d 应该至少在理论上是可分离的。
