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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Single-Crystal Magic-Angle Spinning ~(17)O NMR and Theoretical Studies of the Antiferroelectric Phase Transition in Squaric Acid
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Single-Crystal Magic-Angle Spinning ~(17)O NMR and Theoretical Studies of the Antiferroelectric Phase Transition in Squaric Acid

机译：单晶魔术角自旋〜（17）O NMR和方酸中反铁电相变的理论研究

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摘要

We report resolution-enhanced magic-angle sample spinning (MAS) ~(17)O NMR studies of the paraelectric-antiferroelectric phase transition (at T_C = 373 K) of the model hydrogen-bonded compound squaric acid (H_2C_4O_4). Utilization of single crystals in the MAS measurements yielded a factor of 4 decrease in the ~(17)O NMR line widths, as compared to powder samples. All four oxygens were clearly shown to be chemically different at T < T_C. This was akin to ~(13)C spectra, but the ~(17)O peaks are much more dispersed. The peak assignment was supported by quantum theoretical calculations of the ~(17)O isotropic chemical shifts using a pentamer model of the crystal structure below T_C. On raising the temperature, the four peaks merged and became a narrow doublet above T_C, in contrast to a singlet for ~(13)C. Also, the mean position of the doublet was not the average of the four low-temperature peaks. This observation suggests that the phase transition involves a distortion of the whole H_2C_4O_4 framework, and not just the order-disorder rearrangement of the H's i.e., future models of the transition should include a displacive component, in addition to an order-disorder part. The observation of the doublet at T > T_C implies that the two O-H…O chains retain their difference in the paraelectric phase as well. This is consistent with the one-dimensional Ising chain model, according to which the SQA lattice should be visualized as a mesh of two distinct and orthogonal one-dimensional chains, in contrast to the more prevalent two-dimensional C_4O_4 square-lattice model.
机译：我们报告了模型氢键键合的方酸​​（H_2C_4O_4）的顺电-反电相变（在T_C = 373 K时）的分辨率增强的魔角样品旋转（MAS）〜（17）O NMR研究。与粉末样品相比，在MAS测量中利用单晶可使〜（17）O NMR线宽减少4倍。清楚地表明，所有四种氧在T T_C处的双峰的观察表明，两条O-H…O链在顺电相中也保持了它们的差异。这与一维Ising链模型相一致，根据该模型，SQA晶格应可视化为两个不同且正交的一维链的网格，与更为普遍的二维C_4O_4方格模型相反。

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