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Frequency Dependent NMR Spin Relaxation Studies of Deuterons and Protons in Nematic Liquid Crystals

 

作者: K.H. Schweikert,   F. Noack,  

 

期刊: Molecular Crystals and Liquid Crystals Science and Technology. Section A. Molecular Crystals and Liquid Crystals  (Taylor Available online 1992)
卷期: Volume 212, issue 1  

页码: 33-44

 

ISSN:1058-725X

 

年代: 1992

 

DOI:10.1080/10587259208037246

 

出版商: Taylor & Francis Group

 

关键词: spin relaxation;liquid crystal;frequency dependence;field-cycling;order fluctuations;Pincus-Blinc-Model

 

数据来源: Taylor

 

摘要:

As first discussed by Pincus and Blinc, order fluctuations of the director in a nematic liquid crystal should lead to a characteristic square-root dependence of the nuclear magnetic relaxation times T1of the considered atoms on the Larmor frequencyv, i.e. T1-v1/2. The significance of this process is violently disputed in the literature, essentially because standard NMR spectrometers do not allow a broad frequency variation sufficient to separate different dispersion laws reliably. Using a newly developed field-cycling NMR spectrometer with a fast switchable 1.2 T detection field and fast Fourier transform data processing capabilities, it became possible to extend previous proton relaxation studies to selective frequency dependent deuteron T1measurements in the range between 500 Hz and 7 MHz for various deuterated nematogens (PAA-d6, PAA-d8, PAA-d14, MBBA-d6). Compared with previous proton results in the same frequency range, the relaxation dispersion of the deuterons at identical sites is relatively small in all studied samples, and unlike the proton T1the deuteron T1does not show a fully developed square-root regime. However, the process is clearly detectable at medium and low Larmor frequencies between about 5 kHz and 500 kHz, where the Pincus-Blinc model dominates the proton spin relaxation. Thus the first deuteron field-cycling measurements support the previous proton results that nematic order fluctuations are not visible by T1at standard high Larmor frequencies in the megahertz range.

 

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