Nuclear Magnetic Resonance of Liquid Crystals

Dong, Ronald Y.

doi:10.1007/978-1-4684-0208-7

Cited by 223 publications

(111 citation statements)

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“…The relaxation of the W-order is much more efficient than the S one through the whole nematic temperature range [8,9,25]. Measurement of additional relaxation parameters other than the usual Zeeman relaxation time can help to disentangle the relevant spectral densities of the complex molecular motions in LCs [26]. However, dipolar relaxation experiments generally present certain degree of difficulty due to possible contamination with Zeeman magnetization of the signal observed in the 'dipolar channel' [27].…”

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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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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“…This liquid crystal is in a nematic mesophase and presents a molecular director mostly aligned with the external magnetic field in the range 294-319.5 K. Rotations and diffusion of the single molecules eliminate the intermolecular dipolar interactions. Then, the 1 H spin system can be considered as a finite set with different intramolecular dipolar interactions [47]. In particular, both phenyl groups are separated by a HC=N group.…”

Section: Loschmidt Echoes In Different Spin Networkmentioning

confidence: 99%

Quantum dynamics of excitations and decoherence in many-spin systems detected with Loschmidt echoes: its relation to their spreading through the Hilbert space

Sánchez

Levstein

Buljubasich

et al. 2016

Phil. Trans. R. Soc. A.

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In this work, we overview time-reversal nuclear magnetic resonance (NMR) experiments in many-spin systems evolving under the dipolar Hamiltonian. The Loschmidt echo (LE) in NMR is the signal of excitations which, after evolving with a forward Hamiltonian, is recovered by means of a backward evolution. The presence of non-diagonal terms in the non-equilibrium density matrix of the many-body state is directly monitored experimentally by encoding the multiple quantum coherences. This enables a spin counting procedure, giving information on the spreading of an excitation through the Hilbert space and the formation of clusters of correlated spins. Two samples representing different spin systems with coupled networks were used in the experiments. Protons in polycrystalline ferrocene correspond to an ‘infinite’ network. By contrast, the liquid crystal N -(4-methoxybenzylidene)-4-butylaniline in the nematic mesophase represents a finite proton system with a hierarchical set of couplings. A close connection was established between the LE decay and the spin counting measurements, confirming the hypothesis that the complexity of the system is driven by the coherent dynamics.

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“…Residual dipolar couplings (RDCs) with potentially high structural information content can be measured in partially aligned samples for which so-called alignment media are needed [1][2][3]. Next to classical liquid crystalline phases, mechanically stretched polymer gels form a second class of alignment media [4] which are applicable to a wide range of solutes and solvents [5][6][7][8][9][10][11][12].…”

Section: Introductionmentioning

confidence: 99%

Precise Measurement of RDCs in Water and DMSO Based Gels Using a Silicone Rubber Tube for Tunable Stretching

Kummerlöwe¹,

Halbach²,

Laufer³

et al. 2008

TOSPECJ

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Abstract:Residual dipolar couplings (RDCs) are of great interest for the structure determination of biomacromolecules as well as of organic molecules like synthetic or natural products. Their accurate measurement requires a proper degree of alignment for the molecule under investigation. As has been shown recently, a stretching device based on a flexible silicone rubber tube provides an easy, rapid, and reversible variation of alignment strength. We show in this article that such an apparatus is not limited to gelatin, but can also be used with covalently cross-linked hydrogels and even gels with polar organic solvents like DMSO. Using sucrose and a cyclic hexapeptide, we were able to demonstrate that the approximately linear relation of alignment with the extension factor of the stretched gel allows the measurement of RDCs with high precision if corresponding spectra are acquired at various alignment strengths. The method seems to be widely applicable to the structure determination of molecules of arbitrary size using standard 5 mm NMR equipment.

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Nuclear Magnetic Resonance of Liquid Crystals

Cited by 223 publications

References 0 publications

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

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

Quantum dynamics of excitations and decoherence in many-spin systems detected with Loschmidt echoes: its relation to their spreading through the Hilbert space

Precise Measurement of RDCs in Water and DMSO Based Gels Using a Silicone Rubber Tube for Tunable Stretching

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