Slow Dynamics of Banana-Shaped Molecules: A Theoretical Approach to Analyze 2H-NMR T2 Relaxation Times

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• 作者：Valentina Domenici ; Diego Frezzato ; Carlo Alberto Veracini
• 刊名：Journal of Physical Chemistry B
• 出版年：2006
• 出版时间：December 14, 2006
• 年：2006
• 卷：110
• 期：49
• 页码：24884 - 24896
• 全文大小：223K
• 年卷期：v.110,no.49(December 14, 2006)
• ISSN：1520-5207

文摘

In the present work, we analyze pulsed deuterium NMR experiments performed on the isotropic and nematicphases of the banana-shaped liquid-crystalline mesogen 4-chloro-1,3-phenylene bis{4-4'-(11-undecenyloxy)benzoyloxy} benzoate (ClPbis11BB) selectively deuterated on the central ring. Starting from a previous evidenceof unusual slow dynamics in the isotropic phase (Domenici V. et al., J. Phys. Chem. B 2005, 109, 769), aquantitative and model-supported analysis of the deuterium NMR data is performed here by accounting forslow-motional modulation of the magnetic anisotropies through the full solution of the stochastic Liouvilleequation. Focusing on the quadrupolar echo experiments performed in the nematic phase, the analysis of thetransverse relaxation rate has been carried out by considering single-molecule motions and fluctuations ofthe local director. The main conclusions are: (a) director fluctuations are not relevant on driving the signalrelaxation; (b) molecular reorientations about transverse axes control the dynamic regime of the signal relaxationand impose a full slow-motional treatment; (c) the small amplitude tumbling of the molecule within the wellsof orientational potential occurs with characteristic times up to the microsecond. The outcome of our analysishas to be taken as indicative of very slow dynamics concerning out-of-plane motions of the molecules. Besidesthe specific application, this paper also offers the methodological tools to treat the pulsed deuterium NMRexperiment in the slow-motional regime of reorientational motions and provides a detailed comparison withthe usually employed fast-motional approximation.