Order fluctuations of the director in nematic thermotropic liquid crystals studied by nuclear magnetic resonance dipolar relaxation

Zamar, R. C.; Anoardo, Esteban; Mensio, O.; Pusiol, D. J.; Becker, St.; Noack, F.

doi:10.1063/1.476655

Cited by 20 publications

(14 citation statements)

References 11 publications

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“…In this article we generalize the meaning of relaxometry as an NMR experimental technique not exclusively restricted to the study of Zeeman order relaxation. Other possibilities worth to mention are T ID relaxometry [28,29] and T,P relaxometry [4,30,31].…”

Section: Relaxationmentioning

confidence: 99%

Fast-field-cycling NMR: Applications and instrumentation

Anoardo¹,

Galli²,

Ferrante³

2001

Appl. Magn. Reson.

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192

136

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Magnetic field cycling in nuclear magnetic resonance (NMR) experiments has been used since the early days of NMR. Originally such time-dependent magnetic field experiments were motivated to study cross relaxation, spin system thermodynamics and indirect detection of quadrupolar resonance. The first apparatus used mechanical or pneumatic systems to shoot the sample between two magnets, the typical "flying time" being a few hundreds of milliseconds. As a natural evolution of the experimental technique and the need to extend its application to samples with higher relaxation rates, faster magnetic field switching devices were developed during the last years. Special electric networks combined with sophisticated air core magnets allowed one to switch magnetic fields between zero and fields of the order of 0.5 T in a few milliseconds. Today we refer to this new generation of instruments as "fast-field-cycling" devices. The technique has been successfully used during the last years to obtain information on the molecular dynamics and order in different materials, ranging from organic solids, metals, polymers, liquid crystals, porous media to biological systems. At present it is also turning to be an important tool for the design of contrast agents for magnetic resonance imaging. Fast field cycling was mainly oriented to T, relaxometry as a unique technique offering a dynamic window of several decades, ranging from few kilohertz to several megahertz. However, there exist less conventional applications of the technique that can also provide relevant information concerning molecular dynamics, structure and molecular order. In this article we will briefly deal with basic aspects of the technique, its evolution, present-day relevant applications and the last improvements concerning specialized instrumentation.

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Section: Relaxationmentioning

confidence: 99%

Fast-field-cycling NMR: Applications and instrumentation

Anoardo¹,

Galli²,

Ferrante³

2001

Appl. Magn. Reson.

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192

136

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“…Contrarily to Zeeman order where this mechanism dominates the relaxation in the range of low frequencies, typically from few KHz to hundreds of KHz, the ODF have a high relative weight in the dipolar order relaxation even within the MHz Larmor frequency range [24].…”

Section: Introductionmentioning

confidence: 99%

Quasiequilibrium states in thermotropic liquid crystals studied by multiple-quantum NMR

Buljubasich

Monti

Acosta

et al. 2009

The Journal of Chemical Physics

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Previous work showed that by means of the Jeener-Broekaert (JB) experiment, two quasiequilibrium states can be selectively prepared in the proton spin system of thermotropic nematic liquid crystals (LC) in a strong magnetic field. The similarity of the experimental results obtained in a variety of LC in a broad Larmor frequency range, with crystal hydrates, supports the assumption that also in LC the two spin reservoirs into which the Zeeman order is transferred, originate in the dipolar energy and that they are associated with a separation in energy scales: a constant of motion related to the stronger dipolar interactions (S), and a second one (W) corresponding to the secular part of the weaker dipolar interactions with regard to the Zeeman and the strong dipolar part. We study the nature of these quasiinvariants in nematic 5CB (4'-pentyl-4-biphenylcarbonitrile) and measure their relaxation times by encoding the multiple quantum coherences of the states following the JB pulse pair on two orthogonal bases, Z and X. The experiments were also performed in powder adamantane at 301 K which is used as a reference compound having only one dipolar quasiinvariant. We show that the evolution of the quantum states during the build up of the quasi-equilibrium state in 5CB prepared under the S condition is similar to the case of powder adamantane and that their quasi-equilibrium density operators have the same tensor structure.In contrast, the second constant of motion, whose explicit operator form is not known, involves a richer composition of multiple quantum coherences on the X basis of even order, in consistency with the truncation inherent in its definition. We exploited the exclusive presence coherences ±4, ±6, ±8, besides 0 and ±2 under the W condition to measure the spin-lattice relaxation time T W accurately, so avoiding experimental difficulties that usually impair dipolar order relaxation measurement such as Zeeman contamination at high fields, and also superposition of the different quasiinvariants. This procedure opens the possibility of measuring the spin-lattice relaxation of a quasiinvariant independent of the Zeeman and S reservoirs, so incorporating a new relaxation parameter useful for studying the complex molecular dynamics in mesophases. In fact, we report the first measurement of T W in a liquid crystal at high magnetic fields. The comparison of the obtained value with the one corresponding to a lower field (16 MHz) points out that the relaxation of the W-order strongly depends on the intensity of the external magnetic field, similarly to the case of the S reservoir, indicating that the relaxation of the W-quasiinvariant is also governed by the cooperative molecular motions.

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“…On the contrary, only two constants of motion were observed in gypsum when the magnetic field B [010] and only one at an orientation that makes the protons at the water molecules equivalent. The fact that compounds having very different numbers of interacting spins (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25) in LCs and infinite in gypsum) admit the preparation of more than one dipolar QI, indicates that their occurrence is not exclusively determined by the size of the dipolar network. Instead, both the occurrence of multiple QIs and the many-spin character they show, do depend strongly on the topology of the spin network, since it determines the quantum dynamics of the spin system.…”

Section: Discussionmentioning

confidence: 99%

“…QE states are relevant observables of the spin system, providing relaxation parameters useful to study molecular motion in LC mesophases through their dependence on temperature and magnetic field. 14,20,21 They have been proposed as initial states for the excitation of MQC in NMR experiments 8,22,23 and used in spin counting experiments. 10,19 Besides, these states have been proposed as alternative to the Zeeman order in magnetic resonance imaging.…”

Section: Introductionmentioning

confidence: 99%

Evidence for several dipolar quasi-invariants in liquid crystals

Bonin¹,

González²,

Segnorile³

et al. 2013

The Journal of Chemical Physics

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The quasi-equilibrium states of an observed quantum system involve as many constants of motion as the dimension of the operator basis which spans the blocks of all the degenerate eigenvalues of the Hamiltonian that drives the system dynamics, however, the possibility of observing such quasiinvariants in solid-like spin systems in Nuclear Magnetic Resonance (NMR) is not a strictly exact prediction. The aim of this work is to provide experimental evidence of several quasi-invariants, in the proton NMR of small spin clusters, like nematic liquid crystal molecules, in which the use of thermodynamic arguments is not justified. We explore the spin states prepared with the Jeener-Broekaert pulse sequence by analyzing the time-domain signals yielded by this sequence as a function of the preparation times, in a variety of dipolar networks, solids, and liquid crystals. We observe that the signals can be explained with two dipolar quasi-invariants only within a range of short preparation times, however at longer times liquid crystal signals show an echo-like behaviour whose description requires assuming more quasi-invariants. We study the multiple quantum coherence content of such signals on a basis orthogonal to the z-basis and see that such states involve a significant number of correlated spins. Therefore, we show that the NMR signals within the whole preparation timescale can only be reconstructed by assuming the occurrence of multiple quasi-invariants which we experimentally isolate.

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Order fluctuations of the director in nematic thermotropic liquid crystals studied by nuclear magnetic resonance dipolar relaxation

Cited by 20 publications

References 11 publications

Fast-field-cycling NMR: Applications and instrumentation

Fast-field-cycling NMR: Applications and instrumentation

Quasiequilibrium states in thermotropic liquid crystals studied by multiple-quantum NMR

Evidence for several dipolar quasi-invariants in liquid crystals

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