What is syncrystallization? States of the pH indicator methyl red in crystals of phthalic acid
What is syncrystallization? States of the pH indicator methyl red in crystals of phthalic acid
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DOI：
10.1021/ja0601181
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发表时间：
2006-04-26
影响因子：
15
通讯作者：
Kahr, B
中科院分区：
文献类型：
--
作者：
Benedict, JB;Cohen, DE;Kahr, B
以邻苯二甲酸（PA）与甲基红（MR）的共晶反应为例，对共晶的概念进行了重新研究。晶体在相邻的生长扇区中交替呈现红色和黄色。MR及其共晶的X射线结构，揭示MR在中性，两性离子和质子化状态，以及测量混合晶体的线性双折射和线性二向色性，被用来研究PA着色的机制。这些实验进行了补充力场计算的最低能量稳定的表面表示的小平面和能量的MR晶体上和晶体中，以及MR的分子轨道计算。两个MR物种中检测到PA具有不同的能量，极化，和面对选择性。对这些以前被认为是中性和质子化的MRS的结构进行分析，需要对氢键进行细致入微的分析。黄色和红色物质之间的本质区别在于MR羧酸质子是分子间还是分子内氢键。从PA和MR的共晶的检查得出的混合晶体结构的推论是不一致的偏振光谱信号谨慎使用化学计量的化合物作为稀固溶体的模型时。在加热混合晶体时，线性二向色性减小，并且取向，MR的伸长池分离并在垂直于伸长方向的方向上穿过本体。这些警棍随后结晶，留下宏观取向的晶体的MR丰富的相内PA。没有发现MR和PA同时结晶的证据;然而，加热时的MR重取向以及富含MR的相的分离和重结晶是不同的过程，可以被同结晶的字面意义所包含。
The concept of syncrystallization was reinvestigated by focusing on phthalic acid (PA) grown with methyl red (MR). Crystals are alternately red and yellow in adjacent growth sectors. X-ray structures of MR and its cocrystals, revealing MR in the neutral, zwitterionic, and protonated states, as well as measurements of linear birefringence and linear dichroism of mixed crystals, were used to investigate mechanisms of PA coloring. These experiments were complemented by force field calculations of the lowest energy stable surfaces of expressed facets and energies of MR on and in crystals, as well as molecular orbital calculations of MR. Two MR species were detected in PA having distinct energies, polarizations, and face selectivities. Assignments of structures to these MRS, previously thought to be neutral and protonated, required a nuanced analysis of hydrogen bonds. The essential difference between yellow and red species is whether the MR carboxylic acid proton is inter- or intramolecularly hydrogen bound. Inferences about mixed crystal structure drawn from an examination of cocrystals of PA and MR are inconsistent with polarization spectroscopy signaling caution when using stoichiometric compounds as models of dilute solid solutions. Upon heating mixed crystals, linear dichroism diminishes and oriented, elongated pools of MR separate and pass through the bulk in directions perpendicular to the direction of elongation. These batonnets subsequently crystallize leaving macroscopic oriented crystals of a MR-rich phase within PA. No evidence was found for the simultaneous crystallization of MR and PA; however, the MR reorientation on heating as well as the separation and recrystallization of a MR-rich phase are distinct processes that could be embraced by the literal meaning of syncrystallization.