Investigation of structure and dynamics in a photochromic molecular crystal by NMR crystallography.
Investigation of structure and dynamics in a photochromic molecular crystal by NMR crystallography.
复制标题
通过核磁共振晶体学研究光致变色分子晶体的结构和动力学。
DOI：
10.1002/mrc.4805
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发表时间：
2019
期刊：
影响因子：
--
通讯作者：
Dale BL
中科院分区：
文献类型：
--
作者：
Dale BL
通过单晶X射线衍射、多核魔角自旋核磁共振和第一性原理密度泛函理论（DFT）计算，研究了一种光致变色的苯胺，N‐（3,5‐二‐t‐丁基水杨基吡啶）‐4‐氨基吡啶。根据计算的化学位移对固态核磁共振数据的解释证实，该结构主要由基态烯醇互变异构体中的分子组成，而热激活的顺式酮和光异构的反式酮存在低能级缺陷，其居群太低，无法通过实验检测。变温13c核磁共振数据揭示了固态动力学的证据，发现这与丁基的快速旋转运动和吡啶环的180°翻转有关，与X射线衍射获得的时间平均结构形成对比。对完整晶体结构和孤立分子计算的化学位移的比较也揭示了分子间氢键的证据，包括吡啶环和相邻的亚胺碳，这促进了翻转运动。DFT计算还表明，晶体结构中的分子构象非常接近于孤立分子的能量最小值，表明环动力学是由于吡啶环的相当大的空间自由度而产生的，这也允许分子采用有利于光致变色的构象。
A photochromic anil,N‐(3,5‐di‐t‐butylsalicylidene)‐4‐amino‐pyridine, has been studied by single‐crystal X‐ray diffraction, multinuclear magic‐angle spinning NMR, and first‐principles density functional theory (DFT) calculations. Interpretation of the solid‐state NMR data on the basis of calculated chemical shifts confirms the structure is primarily composed of molecules in the ground‐state enol tautomer, whereas thermally activatedcis‐keto and photoisomerisedtrans‐keto states exist as low‐level defects with populations that are too low to detect experimentally. Variable temperature13C NMR data reveal evidence for solid‐state dynamics, which is found to be associated with fast rotational motion oft‐butyl groups and 180° flips of the pyridine ring, contrasting the time‐averaged structure obtained by X‐ray diffraction. Comparison of calculated chemical shifts for the full crystal structure and an isolated molecule also reveals evidence for an intermolecular hydrogen bond involving the pyridine ring and an adjacent imine carbon, which facilitates the flipping motion. The DFT calculations also reveal that the molecular conformation in the crystal structure is very close to the energetic minimum for an isolated molecule, indicating that the ring dynamics arise as a result of considerable steric freedom of the pyridine ring and which also allows the molecule to adopt a favourable conformation for photochromism.