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首页> 外文期刊>Zeitschrift fur Naturforschung. A, A journal of physical sciences >Deuteron NMR of Methyl Groups in the Tunneling Regime. A Single Crystal Study of Aspirin-CD_3
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Deuteron NMR of Methyl Groups in the Tunneling Regime. A Single Crystal Study of Aspirin-CD_3

机译：隧道体制中甲基的氘核NMR。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 CD_3 groups which display the so-called ±β, ±(|α| ± β) and ±(2|α| ± β) 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. Muller-Warmuth. The molecular system we study is aspirin-CD_3. We recorded spectra for differently oriented single crystals and measured spin-lattice relaxation times T_1 in a wide temperature range. At 12.5 K we exploit the dependence of the ±(|α| ± β) and ±(2|α| ± β) lines on the orientation of the applied field B_0 for determining the equilibrium orientation of the CD_3 group in the crystal lattice. The spectra display features which allow, by comparison with simulated spectra, a measurement of the tunnel frequency v_t. Its low temperature limit is (2.7 ± 0.1) MHz. It allows to infer the height V_3 of the potential V(φ) in which the CD_3 group moves, provided that this potential is purely threefold. We get V_3 = (47.2 ± 0.5)meV. The transition from the tunneling to the classical, fast reorienting regime occurs in the 15 K approx < T approx < 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 ±β 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 T_1-data we derive the temperature dependence of the correlation time τ_c of the reorientational jumps. The plot of log τ_c 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-CD_3.
机译：我们报告了CD_3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了旋转隧穿的特征，即所谓的±β，±（|α|±β）和±（2 |α|±β）线，可以与W. Muller-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-CD_3。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T_1。在12.5 K时，我们利用±（|α|±β）和±（2 |α|±β）线对施加场B_0的方向的依赖性来确定晶格中CD_3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v_t。其低温极限为（2.7±0.1）MHz。只要该电位是三倍，它就可以推断出CD_3组所移动的电位V（φ）的高度V_3。我们得到V_3 =（47.2±0.5）meV。从隧穿到经典的快速重新定向状态的转变发生在大约15 K的温度

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