Pei Zhipeng, Magann Nicholas L, Sowden Madison J, Murphy Rhys B, Gardiner Michael G, Sherburn Michael S, Coote Michelle L
Institute for Nanoscale Science and Technology, Flinders University, Bedford Park 5042 South Australia, Australia.
Research School of Chemistry, Australian National University, Canberra 2601, Australia.
J Am Chem Soc. 2023 Jul 26;145(29):16037-16044. doi: 10.1021/jacs.3c04363. Epub 2023 Jul 18.
The ground-state structure of the parent -quinonedimethide (-QDM) molecule is generally represented in its closed shell form, i.e., as a cyclic, nonaromatic, through-conjugated/cross-conjugated hybrid comprising four C═C bonds. Nonetheless, -QDM has been theorized to contain a contribution from its open-shell aromatic singlet diradical form. VBSCF calculations identify an open-shell contribution of 29% to the structure, while CASPT2(16,16)/def2-TZVP and ωB97XD/aug--pVTZ calculations predict that dimerization proceeds along an open-shell singlet diradical pathway with a low (77 kJ/mol) barrier toward dimerization, which occurs by way of C-C bond formation between the exocyclic methylene carbons. A similar low (98 kJ/mol) barrier exists toward the reaction between a -QDM molecule and the radical trap TEMPO. These predictions are verified experimentally through the isolation of -TEMPO-trapped -QDM, its C-C coupled dimer, and by demonstrating that a mixture of -QDM and TEMPO can initiate the radical polymerization of -butyl acrylate at ambient temperature. In contrast to -QDM, tetracyanoquinone (TCNQ) neither dimerizes nor reacts with TEMPO, despite having a similar diradical character to -QDM. This lack of reactivity is consistent with both a higher kinetic barrier and a thermodynamically unfavorable process, which is ascribed to destabilizing steric clashes and polar effects.
母体醌二甲烷（-QDM）分子的基态结构通常以其闭壳形式表示，即作为一种环状、非芳香性、通过共轭/交叉共轭的杂化物，包含四个C═C键。尽管如此，理论上-QDM包含其开壳芳香单重态双自由基形式的贡献。VBSCF计算确定该结构的开壳贡献为29%，而CASPT2(16,16)/def2-TZVP和ωB97XD/aug--pVTZ计算预测二聚化沿着开壳单重态双自由基途径进行，二聚化势垒较低（77 kJ/mol），这是通过环外亚甲基碳之间形成C-C键实现的。-QDM分子与自由基捕获剂TEMPO之间的反应也存在类似的低（98 kJ/mol）势垒。通过分离TEMPO捕获的-QDM、其C-C偶联二聚体，并证明-QDM和TEMPO的混合物可以在室温下引发丙烯酸丁酯的自由基聚合，这些预测得到了实验验证。与-QDM相反，四氰基醌（TCNQ）既不发生二聚化也不与TEMPO反应，尽管它与-QDM具有相似的双自由基特性。这种缺乏反应性与较高的动力学势垒和热力学不利过程一致，这归因于不稳定的空间冲突和极性效应。
