Bally Thomas, Maltsev Alexander, Gerson Fabian, Frank Daniel, de Meijere A
Department of Chemistry, University of Fribourg, Switzerland.
J Am Chem Soc. 2005 Feb 16;127(6):1983-8. doi: 10.1021/ja046961l.
Upon ionization by gamma-irradiation in frozen CFCl(3), or by X-irradiation in an Ar matrix, 2,2,3,3-tetramethylmethylenecyclopropane (MCP-Me4) readily undergoes ring opening to yield the radical cation of 1,1,2,2-tetramethyltrimethylenemethane (TMM-Me4). The hyperfine-coupling constants for TMM-Me4(.+) are (mT) -1.99 (2H), +0.53 (6H), and +0.19 (6H), and the singly occupied orbital closely resembles one of the two degenerate nonbonding pi-MOs (NBMOs) of trimethylenemethane (TMM). Due to the expected effect of the methyl substituents, this "symmetric" NBMO, psi(2+) (b(1)), is energetically favored relative to its "antisymmetric" counterpart, psi(2-) (a(2)), so that the ground state assumes a structure with (2)B(1) symmetry in the C(2v) point group. Calculations show that the ring opening in the primary radical cation MCP-Me4(.+) to yield TMM-Me4(.+) is spontaneous, whereas in the parent system (MCP(.+) --> TMM(.+) a low barrier does exist. In contrast to the previously investigated case of the radical cation of tetramethyleneethane, the "electromer" of TMM-Me4(.+), in which the unpaired electron occupies psi(2-), cannot be attained photochemically.
在冷冻的三氯一氟甲烷中进行γ辐照电离，或在氩气基质中进行X辐照时，2,2,3,3-四甲基亚甲基环丙烷（MCP-Me4）很容易发生开环反应，生成1,1,2,2-四甲基三亚甲基甲烷（TMM-Me4）的自由基阳离子。TMM-Me4(+)的超精细偶合常数为（mT）-1.99（2H）、+0.53（6H）和+0.19（6H），且单占据轨道与三亚甲基甲烷（TMM）的两个简并非键π分子轨道（NBMOs）之一非常相似。由于甲基取代基的预期效应，这个“对称”的NBMO，ψ(2+)（b(1)），相对于其“反对称”对应物ψ(2-)（a(2)）在能量上更有利，因此基态在C(2v)点群中具有(2)B(1)对称性的结构。计算表明，初级自由基阳离子MCP-Me4(+)开环生成TMM-Me4(+)是自发的，而在母体体系中（MCP(+)→TMM(+)）确实存在一个低势垒。与之前研究的四亚甲基乙烷自由基阳离子的情况相反，TMM-Me4(+)的“电子异构体”，即未成对电子占据ψ(2-)的情况，不能通过光化学方法得到。
