Wang Yin-Feng, Li Zhi-Ru, Wang Fang-Fang, Sun Chia-Chung
State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun, 130023, PR China.
Phys Chem Chem Phys. 2009 Jan 21;11(3):455-62. doi: 10.1039/b812336e. Epub 2008 Nov 6.
To study the inter-ring sigma/(pi/pi) covalent interactions between non-radical pi-systems, five structures of cyclodimers of benzene (C6H6)2 with all the real frequencies, i.e. o-p'-dibenzene (), the pentacyclic dimer (), p-p'-dibenzene (), syn-o,o'-dibenzene (), and hexaprismane (), are obtained at the MP2/6-311G(d,p) level. Five inter-ring bonding mode types forming the inter-ring multicenter multielectron sigma/(pi/pi) covalent bonds are represented: , ring-edge type between a butterfly-shaped ring and a planar ring (4-center 4-electron bond); , edge-edge and ring-ring types between two identical butterfly-shaped rings (8-center 8-electron bond); , ring-ring type between two identical butterfly-shaped rings (4-center 4-electron bond); , edge-edge type between two identical planar rings (4-center 4-electron bond); and , face-face type between two identical planar rings (12-center 12-electron bond). The order of the large inter-ring interaction energies at the MP2/6-311+G(3d, 2p)+BF level is -99.15 (with two inter-ring C-C bonds)> -98.57 (with four C-C bonds)>-85.76 (with two C-C bonds)>-61.35 (with two C-C bonds)>-60.40 kcal mol(-1) (with six C-C bonds). However, this does not show an obvious relationship between the interaction energy and the number of the inter-ring C-C bonds. The reason is that the number of decisive influencing factors of the inter-ring interaction energy is not one but five: the number of the favorable inter-ring C-C single bonds, the number of the unfavorable four-membered rings themselves, the participating number of the four-membered rings in unfavorable interaction among those rings, the number of the favorable non-planar melted six-membered ring, and the weak inter-ring pi/pi interaction (between two pi bonds in different rings).
为了研究非自由基π体系之间的环间σ/(π/π)共价相互作用，在MP2/6 - 311G(d,p)水平上获得了苯（C6H6）2环二聚体的五种结构，即邻 - 对' - 二苯（）、五环二聚体（）、对 - 对' - 二苯（）、顺式邻，邻' - 二苯（）和六棱柱烷（），且所有频率均为实频。文中展示了形成环间多中心多电子σ/(π/π)共价键的五种环间键合模式类型：，蝶形环与平面环之间的环边缘型（4中心4电子键）；，两个相同蝶形环之间的边缘 - 边缘型和环 - 环型（8中心8电子键）；，两个相同蝶形环之间的环 - 环型（4中心4电子键）；，两个相同平面环之间的边缘 - 边缘型（4中心4电子键）；以及，两个相同平面环之间的面 - 面型（12中心12电子键）。在MP2/6 - 311 + G(3d, 2p)+BF水平下，环间相互作用能较大的顺序为 - 99.15（有两个环间C - C键）> - 98.57（有四个C - C键）> - 85.76（有两个C - C键）> - 61.35（有两个C - C键）> - 60.40 kcal mol(-1)（有六个C - C键）。然而，这并未显示出相互作用能与环间C - C键数量之间存在明显关系。原因是环间相互作用能的决定性影响因素并非一个，而是五个：有利的环间C - C单键数量、不利的四元环自身数量、四元环在这些环之间不利相互作用中的参与数量、有利的非平面熔融六元环数量以及环间弱π/π相互作用（不同环中两个π键之间）。
