NMR Relaxometry and Molecular Dynamics: Experimental Techniques

The measurement of nuclear magnetic resonance (NMR) relaxation times T1, T2, and T1p is possible using appropriate experimental equipment and suitable experimental conditions. Depending on each particular relaxation time different experimental techniques have to be used. Here is presented a brief review of the experimental setups and techniques most often used.

Due to the fact that the NMR signal detected has a signal-to-noise ratio that decreases with B0^2 some experimental techniques can be applied in practice only for B0 > 0.1 T (e.g., 1H-NMR frequency larger than 4 MHz). Above this magnetic field many radio-frequency (RF) pulse sequences can be used to measure the relaxation times Ti, T2, and Tip.

Inversion Recovery

For T1 the inversion recovery pulse sequence is the one mostly used. This RF pulse sequence is composed of a n-pulse, followed by a relaxation time т and a п/2-pulse. The FID signal is collected after

NMR ofLiquid Crystal Dendrimers

Carlos R. Cruz, Joao L. Figueirinhas, and PedroJ. Sebastiao Copyright © 2017 Pan Stanford Publishing Pte. Ltd.

ISBN 978-981-4745-72-7 (Hardcover), 978-981-4745-73-4 (eBook) www.panstanford.com the second pulse. A complete spin-lattice relaxation is required before repeating the pulse sequence, which requires a time тP ^ 5Ti.

The amplitude of the RF field B1 and the duration of the RF pulse are adjusted so that the effect upon the magnetization of the spins' system can be regarded, in a classical picture, as the rotation of the magnetization vector in the laboratory frame in a way that the angles defined by the initial (before the RF pulse) orientations have determined angles (ex: n, n/2), as referred to in Chapter 5. The pulse duration is thus obtained from the relation в = у B1te.

In a classical picture, the n-pulse flips the magnetization from its initial equilibrium state, aligned with the external magnetic field, M(0) = M0ez || B0ez, to an antiparallel state M(0 + ) = — M0ez. The n/2 pulse transfers the magnetization to the plane xy ± ez and allows for the detection of the free induction decay signal. During time т the magnetization component Mz evolves according to the solutions of Bloch's equations (Eq. 4.22)

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Depending on the acquisition system it is possible to include phase cycles in the relative phases of the RF field so that the both real and imaginary parts of the FID response and both positive and negative signals can be obtained in order to minimize and/or compensate for DC offsets.

 
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