Ordering in nematic liquid crystals from NMR cross-polarization studies |
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Authors: | Email author" target="_blank">K?V?RamanathanEmail author Neeraj?Sinha |
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Institution: | (1) Sophisticated Instruments Facility, Indian Institute of Science, 560 012 Bangalore, India;(2) Department of Physics, Indian Institute of Science, 560 012 Bangalore, India |
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Abstract: | The measurement of dipolar couplings between nuclei is a convenient way of obtatining directly liquid crystalline ordering
through NMR since the coupling is dependent on the average orientation of the dipolar vector in the magnetic field which also
aligns the liquid crystal. However, measurement of the dipolar coupling between a pair of selected nuclei is beset with problems
that require special solutions. In this article the use of cross polarization for measuring dipolar couplings in liquid crystals
is illustrated. Transient oscillations observed during cross polarization provide the dipolar couplings between essentially
isolated nearest neighbour spins which can be extracted for several sites simultaneously by employing two-dimensional NMR
techniques. The use of the method for obtaining heteronuclear dipolar couplings and hence the order parameters of liquid crystals
is presented. Several modifications to the basic experiment are considered and their utility illustrated. A method for obtaining
proton-proton dipolar couplings, by utilizing cross polarization from the dipolar reservoir, is also presented. |
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Keywords: | NMR cross polarization liquid crystals dipolar couplings ordering order parameter |
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