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首页> 外文期刊>Crystal growth & design >Rationalizing the Lacking of Inversion Symmetry in a Noncentrosymmetric Polar Racemate: An Experimental and Theoretical Study
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Rationalizing the Lacking of Inversion Symmetry in a Noncentrosymmetric Polar Racemate: An Experimental and Theoretical Study

机译：合理化非中心对称极性消旋体中反对称性的缺失：一项实验和理论研究

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The total charge density of PYRAC, a polar (Pca2_1) organic racemate with Z′ = 2, was derived from high-resolution single-crystal X-ray diffraction data at T = 100(2) K and periodic DFT calculations. The PYRAC asymmetric unit consists of a hydrogen-bonded pair of conformationally different enantiomers, A and B_i, where the subscript “i” indicates a reversed absolute configuration. The lattice stability was compared with that of centrosymmetric possibly competing structures, with the aim of understanding why a noncentrosymmetric lattice framework is obtained from a racemic mixture. The likelihood of specific intermolecular recognition processes among different conformers of PYRAC in the very first stages of nucleation was investigated by DFT simulations in vacuo. Two competing, equivalent interconversion pseudorotatory paths between the most stable A and the least stable B conformers were found. It results that molecules spend most of their time (≈53%) in the A conformation, whereas the B one is far less populated (≈7%). Therefore, centrosymmetric AAi adducts are formed very frequently in the reaction liquor, whereas the BBi ones are rare. Nevertheless, AAi pairs produce crystal forms with cohesive energies and densities significantly less favorable than those estimated for the noncentrosymmetric heterochiral ABi ones. Therefore, preference for Z′ = 2 in conjunction with noncentrosymmetric point and space groups results from the thermodynamic control of the crystallization process. The capability of forming extended hydrogen bond chains throughout the lattice appears to be a prerequisite to bind together the fundamental AB_i repeating units.
机译：PYRAC是Z'= 2的极性（Pca2_1）极性有机消旋体，其总电荷密度来自T = 100（2）K的高分辨率单晶X射线衍射数据和定期DFT计算。 PYRAC不对称单元由一对氢键结合的构象不同的对映体A和B_i组成，其中下标“ i”表示反向的绝对构型。将晶格稳定性与中心对称的可能竞争结构的晶格稳定性进行了比较，目的是理解为什么从消旋混合物中获得非中心对称的晶格骨架。通过真空中的DFT模拟研究了成核的第一阶段中不同构象的PYRAC之间特定分子间识别过程的可能性。发现了在最稳定的A和最不稳定的B构象异构体之间存在两个竞争的，等效的相互转换伪旋转路径。结果表明，分子在A构象中花费了大部分时间（约53％），而B则没有那么多（约7％）。因此，在反应液中非常频繁地形成中心对称的AAi加合物，而BBi则很少。然而，AAi对产生的晶体具有内聚能，并且密度明显低于非中心对称的异手性ABi晶体所估计的晶体。因此，结合结晶中心的热力学控制，导致Z'= 2以及非中心对称点和空间基团的优先选择。在整个晶格中形成延伸的氢键链的能力似乎是将基本的AB_1重复单元结合在一起的先决条件。

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