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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >~2H NMR Investigations of the Hexadecane/Urea Inclusion Compound
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~2H NMR Investigations of the Hexadecane/Urea Inclusion Compound

机译：十六烷/尿素包合物的〜2H NMR研究

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摘要

The molecular behavior of selectively deuterated n-hexadecane in the urea inclusion compound is studied between 100 and 290 K by means of dynamic ~2H NMR spectroscopy employing line shape studies and spin-spin and spin-lattice relaxation experiments. Particular emphasis is given to the changes of the guest molecule behavior close to a solid-solid phase transition occurring at lower temperatures which is accompanied by a distortion of the urea lattice. It is demonstrated that spin-lattice relaxation experiments are of particular help for the evaluation of the chain dynamics in such systems. A comprehensive computer analysis of the available experimental data could provide a detailed picture of the hexadecane chains in the low-temperature phase and above the solid-solid phase transition. In the low-temperature phase the alkyl chains are found to undergo fast but restricted rotational motions (rate constant ca. 10~7 s~(-1)). In addition, intramolecular trans-gauche isomerizations and methyl group rotation contribute to the spin relaxation of the alkane chain ends. In the high-temperature phase the alkyl chains rotate rapidly (rate constant ca. 10~(10) s~(-1)) and almost unrestricted around the channel long axis, giving rise to a dynamic (rotational) disorder of the embedded alkyl chains. It therefore is concluded that the phase transition can be assigned to a (dynamic) order-disorder transition. The alkane chains furthermore are found to be perfectly aligned with respect to the urea channel long axis. At the same time, they exhibit an internal flexibility gradient toward the chain ends. The conformational order significantly is altered at the phase transition. Thus, the trans population in the low-temperature phase is given by p_t = 0.76 while in the high-temperature phase a value of p_t = 0.95 has been derived.
机译：利用线形研究以及自旋-自旋和自旋-晶格弛豫实验，通过动态〜2H NMR光谱研究了尿素包合物中选择性氘化的正十六烷的分子行为。特别强调客体分子行为的变化接近在较低温度下发生的固-固相变，这伴随着尿素晶格的变形。结果表明，自旋晶格弛豫实验对于评估此类系统中的链动力学尤其有用。对可用实验数据进行全面的计算机分析可以提供低温相中和固-固相变以上的十六烷链的详细图像。在低温阶段，发现烷基链经历快速但受限制的旋转运动（速率常数约为10〜7 s〜（-1））。另外，分子内反式变位异构化和甲基旋转有助于烷烃链端的自旋松弛。在高温阶段，烷基链快速旋转（速率常数大约为10〜（10）s〜（-1）），并且几乎不受限制地围绕通道长轴旋转，从而导致嵌入的烷基发生动态（旋转）紊乱链。因此得出结论，可以将相变分配给（动态）有序-无序转变。此外，发现烷烃链相对于尿素通道长轴完全对齐。同时，它们在链端处表现出内部柔韧性梯度。构象顺序在相变处显着改变。因此，低温阶段的反式人口为p_t = 0.76，而高温阶段的反式人口的p_t = 0.95。

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