Department of Chemistry, Lancaster University, Lancaster, UK.
Materials Science Institute, Lancaster University, Lancaster, UK.
Magn Reson Chem. 2019 May;57(5):230-242. doi: 10.1002/mrc.4805. Epub 2018 Dec 16.
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 activated cis-keto and photoisomerised trans-keto states exist as low-level defects with populations that are too low to detect experimentally. Variable temperature C NMR data reveal evidence for solid-state dynamics, which is found to be associated with fast rotational motion of t-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.
一种光致变色的邻位萘酚，N-(3,5-二叔丁基水杨醛基)-4-氨基吡啶，已经通过单晶 X 射线衍射、多核魔角旋转 NMR 和第一性原理密度泛函理论（DFT）计算进行了研究。基于计算化学位移对固态 NMR 数据的解释证实，该结构主要由基态烯醇互变异构体的分子组成，而热激活的顺式-酮和光异构化的反式-酮态则以低水平的缺陷存在，其浓度太低以至于无法通过实验检测到。变温 C NMR 数据揭示了固态动力学的证据，这与叔丁基基团的快速旋转运动和吡啶环的 180°翻转有关，这与 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的计算化学位移的比较也揭示了涉及吡啶环和相邻亚胺碳的分子间氢键的证据，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孤立分子的能量最小值，这表明环动力学是由于吡啶环具有相当大的空间自由度所致，这也允许分子采取有利于光致变色的构象。
