Time-Domain Multiple-Quantum NMR

Weitekamp, D. P.

doi:10.1016/b978-0-12-025511-5.50009-8

Cited by 194 publications

(73 citation statements)

References 40 publications

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“…Walls and Y. Lin [5] presented a method for constructing a set of orthogonal constants of motion in terms of products of spin operators, starting from the Zeeman and secular dipolar energy. They found that the projection of the spin state after the JB sequence onto operators of multispin character tends to zero when the system (regular linear chains of N spins 1 2 ) has enough size: N>8 ("thermodynamic limit") [5,33].…”

Section: Discussionmentioning

confidence: 99%

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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: Discussionmentioning

confidence: 99%

“…An indication of the occurrence of two quasiinvariants, besides the Zeeman energy in LC in the nematic phase, is the variation of the dipolar signal shape with the preparation time t 12 in the JB pulse sequence [8,33]. Fig.…”

Section: Preparation Of Ordered Statesmentioning

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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“…[7] includes the distant-dipolar field B d (r) (Eq. [2]) after a correction for any residual magnetization (16), which for z-modulated magnetization gives…”

Section: B Expected Behavior Of Crazed Signalsmentioning

confidence: 99%

“…Observations of multiple-quantum (MQ) coherences in both liquid-and solid-state magnetic resonance contribute significantly to the analytical power of the spectroscopy by providing highly selective spectral editing filters and amplifying effects of frequency differences in chemical exchange and translational diffusion experiments (1)(2)(3)(4)(5)(6)(7). The simplest conventional experimental approaches for MQ excitation and detection involve three RF pulses (90 x --90 x -t 1 -90 x -t 2 ).…”

Section: Introductionmentioning

confidence: 99%

Experimental Characterization of Intermolecular Multiple-Quantum Coherence Pumping Efficiency in Solution NMR

Zhang

Lizitsa

Bryant

et al. 2001

Journal of Magnetic Resonance

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The behavior of intermolecular multiple-quantum coherences in a variety of simple liquids with different chemical and magnetic properties is investigated experimentally and modeled by numerical simulations based on modified Bloch equations. The effects of spin concentration, temperature, intramolecular conformational flexibility, chemical exchange, and spin-spin coupling on the formation of high-order coherences are examined. It is shown that any process that makes the Larmor frequency time-dependent may interfere with the formation of these coherences. Good agreement is achieved between experiments and simulation, using independently known values of the magnetization density, the rate constants for translational diffusion, spin-spin and spin-lattice relaxation, and radiation damping.

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“…In high field NMR, there are several methods to simplify a complicated spectrum [30], one of them is multiple quantum transition NMR spectroscopy [25,26,27,28,30]. In this method, a multiple quantum operator (pumping operator in optics) which is a linear combination of a set of different rank tensors is created by a particular pulse sequence [25]. The n-quantum coherence in the density matrix is then prepared by applying the multiple quantum operator to the spin system for length of times, 7, where the density matrix starts to evolve under the spin Hamiltonian.…”

Section: Order Selective Observation Of Zero Field Signalsmentioning

confidence: 99%

Scalar operators in solid-state NMR

Sun

1991

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Time-Domain Multiple-Quantum NMR

Cited by 194 publications

References 40 publications

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

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

Experimental Characterization of Intermolecular Multiple-Quantum Coherence Pumping Efficiency in Solution NMR

Scalar operators in solid-state NMR

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