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Deuteron NMR of methyl groups in the tunneling regime. A single crystal study of aspirin-CD3

机译：隧道区域中甲基的氘核直流。阿司匹林-CD 3 的单晶研究

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We report the first single crystal deuteron NMR spectra of CD3 groups which display the socalled ±ß, ±(|α| ± ß) and ±(2|α| ± ß) lines characteristic of rotational tunneling in a sufficiently clear manner to allow a quantitative comparison with the respective theory developed in 1988 by the group of W. Müller-Warmuth. The molecular system we study is aspirin-CD3. We recorded spectra for differently oriented single crystals and measured spin-lattice relaxation times T1 in a wide temperature range. At 12.5 K we exploit the dependence of the ±(|α| ± ß) and ±(2|α| ± ß) lines on the orientation of the applied field B0 for determining the equilibrium orientation of the CD3 group in the crystal lattice. The spectra display features which allow, by comparison with simulated spectra, a measurement of the tunnel frequency vt. Its low temperature limit is (2.7 ± 0.1) MHz. It allows to infer the height V3 of the potential V(φ) in which the CD3 group moves, provided that this potential is purely threefold. We get V3 = (47.2 ± 0.5) meV. The transition from the tunneling to the classical, fast reorienting regime occurs in the 15 K ≲ T ≲ 35 K temperature range. In this range we observe a broadening, merging and eventually narrowing of the ± |α| and ±2|α| lines in very much the way predicted by Heuer. His theory, however, must be extended by taking into account all librational levels. The behaviour of the ± ß lines in the transition temperature range signalizes a reduction of the observable tunnel frequency with increasing temperature. This reduction allows an independent measurement of the potential height and represents a test of the assumption of a purely threefold potential. From the T1 -data we derive the temperature dependence of the correlation time Ƭc of the reorientational jumps. The plot of log Ƭc vs. 1 /T follows a straight line for more than five decades. From its slope we get yet another independent number for the potential height. It agrees well with the other ones, which confirms the assumption of the essentially threefold potential V(φ) in aspirin-CD3.
机译：我们报告了CD3基团的第一个单晶氘核NMR谱图，这些谱图以足够清晰的方式显示出旋转隧穿特性的所谓的±ß，±（|α|±ß）和±（2 |α|±ß）线，以允许与W.Müller-Warmuth小组在1988年提出的理论进行定量比较。我们研究的分子系统是阿司匹林CD3。我们记录了不同取向的单晶的光谱，并在很宽的温度范围内测量了自旋晶格弛豫时间T1。在12.5 K时，我们利用±（|α|±ß）和±（2 |α|±ß）线对所施加场B0的方向的依赖性来确定晶格中CD3基团的平衡方向。频谱显示功能允许通过与模拟频谱比较来测量隧道频率vt。其低温极限为（2.7±0.1）MHz。只要该电势是三倍，它就可以推断出CD3组在其中移动的电势V（φ）的高度V3。我们得到V3 =（47.2±0.5）meV。从隧穿过渡到经典的快速重定向过程发生在15 K T T 35 K温度范围内。在此范围内，我们观察到±|α|的扩大，合并和最终变窄。和±2 |α|线条与Heuer的预言非常相似。但是，他的理论必须通过考虑所有的自由层次来扩展。 ±ß线在转变温度范围内的行为表明，随温度升高，可观察到的隧道频率降低。这种减小允许对电位高度进行独立测量，并且代表对纯三重电位假设的测试。从T1数据中，我们得出了方向性跃迁相关时间Ƭc的温度依赖性。 logƬc与1 / T的关系曲线沿直线延伸了超过五十年。从其斜率，我们获得了潜在高度的另一个独立数字。它与其他化合物很好地吻合，这证实了阿司匹林-CD3的基本三倍电势V（φ）的假设。

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