RMN en Milieu Orienté, ICMMO, UMR CNRS 8182, Université Paris Sud 11, Bât. 410, F-91405 Orsay cedex, France.
J Phys Chem B. 2011 Oct 20;115(41):11793-804. doi: 10.1021/jp205958k. Epub 2011 Sep 27.
The conformation and interconversion dynamics of two derivatives of the 18-membered hexathia metacyclophane 1 and 2 were studied by (1)H NMR spectroscopy in isotropic solvents and by (2)H NMR in chiral liquid crystalline (CLC) solutions, as well as by molecular structure computations. For the analysis of the dynamic effects, we made use of the concepts of "average symmetry" and "isodynamic groups", introduced by Altmann (Altmann, Proc. R. Soc.1967, 184, A298). Compound 1, which is unsubstituted in the inner aromatic site, has, according to the NMR and molecular force field calculations, a boat shaped ground conformation with C(2) symmetry. It is highly flexible and in the NMR spectrum exhibits two successive dynamic processes. There is a low temperature (170-210 K, E(a) = 10.5 kcal/mol) alternate "wing flipping", which corresponds to interchange between pairs of enantiomers and results, in the fast exchange limit, in an average prochiral molecule with C(2v) symmetry. This process is followed, at higher temperatures (290-320 K, E(a) = 28.5 kcal/mol), by an umbrella flipping type inversion with an average structure of D(2h) symmetry. This second process involves averaging of effective enantiotopic into homotopic sites and can only be studied in chiral solvents. The origin of the chiral discrimination and of their stepwise averaging is discussed. Compound 2, which is substituted with methoxy groups at the inner sites of the benzene rings, is much less flexible and exhibits dynamic effects in the NMR spectrum only at temperatures above 370 K. We were able to study the kinetic parameters of this process in isotropic solvents (E(a) = 21.4 kcal/mol). As for 1, the detailed mechanism of this process can in principle be established using dynamic NMR in CLC; however, experimental limitation precluded us from doing so. Possible alternatives and their effect on the 1D and 2D exchange spectra in CLC are discussed in a concluding section.
我们通过（1）H NMR 光谱在各向同性溶剂中以及（2）H NMR 在手性液晶（CLC）溶液中研究了 18 元六噻环戊烷 1 和 2 的两个衍生物的构象和互变动力学，同时还进行了分子结构计算。为了分析动态效应，我们利用了 Altmann（Altmann，Proc. R. Soc.1967，184，A298）提出的“平均对称”和“等动力学基团”的概念。在内部芳构化位置未取代的化合物 1 根据 NMR 和分子力场计算，具有船形的地面构象，具有 C（2）对称性。它具有高度的灵活性，在 NMR 光谱中显示出两个连续的动态过程。在低温（170-210 K，E(a) = 10.5 kcal/mol）下存在交替的“翅膀翻转”，这对应于对映异构体对之间的交换，在快速交换极限下，得到平均的具有 C（2v）对称性的前手性分子。在较高温度（290-320 K，E(a) = 28.5 kcal/mol）下，随后发生伞式翻转型反转，平均结构为 D（2h）对称性。第二个过程涉及有效对映异位到同异位位点的平均化，只能在手性溶剂中进行研究。讨论了手性识别的起源及其逐步平均化的起源。在内部苯环的芳构化位置上用甲氧基取代的化合物 2 则不那么灵活，仅在高于 370 K 的温度下在 NMR 光谱中显示出动态效应。我们能够在各向同性溶剂中研究该过程的动力学参数（E(a) = 21.4 kcal/mol）。与 1 一样，该过程的详细机制原则上可以通过 CLC 中的动态 NMR 来建立；然而，实验限制使我们无法做到这一点。在结论部分讨论了可能的替代方案及其对 CLC 中 1D 和 2D 交换光谱的影响。
