Field-cycling NMR Relaxometry: Instrumentation, Model Theories and Applications
Chapter 7: Relaxometry at Very Low Frequencies by Rotating-frame Techniques for Complementing the Frequency Domain Explored by Field Cycling
Published:11 Oct 2018
Special Collection: 2018 ebook collectionSeries: New Developments in NMR
Daniel Canet, Sabine Bouguet-Bonnet, 2018. "Relaxometry at Very Low Frequencies by Rotating-frame Techniques for Complementing the Frequency Domain Explored by Field Cycling", Field-cycling NMR Relaxometry: Instrumentation, Model Theories and Applications, Rainer Kimmich
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Below 5 kHz, it is difficult (but possible with advanced techniques) to apply field-cycling relaxometry, which provides the evolution of the longitudinal relaxation rate (R1) as a function of the Larmor frequency. Moreover, residual dipolar or quadrupolar couplings, if not negligible, would make the interpretation of such measurements dubious. Rotating frame relaxation measurements constitute a possible remedy since, by varying the amplitude of the spin-lock radiofrequency field, it is possible to explore the frequency dependence of the relevant relaxation rate (R1ρ) down to very low frequencies. However, there is a problem in connecting the R1 and R1ρ dispersion curves because of the differences, in terms of spectral densities, of the theoretical expressions of these two relaxation rates. A method based on the random fields relaxation mechanism, has been developed and shown, through several examples, to be fairly efficient. Along with R1 measurements performed at high frequency with conventional NMR spectrometers, it proved possible to unravel, through a complete dispersion curve, all molecular motions occurring in complex systems.