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Relaxation (NMR)

About: Relaxation (NMR) is a research topic. Over the lifetime, 29342 publications have been published within this topic receiving 689851 citations.


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Journal ArticleDOI
TL;DR: In this paper, a density-functional investigation of static long-period spin-density waves (SDWs) in chromium was performed using both projector-augmented wave and muffin-tin-orbital techniques and the results showed that the ground state of Cr is predicted to be of commensurate antiferromagnetic order, longitudinal SDWs lead to an increased total energy.
Abstract: We present ab initio density-functional investigations of static long-period spin-density waves (SDWs) in chromium. Calculations were performed in the local-spin-density approximation with and without the generalized-gradient approximation, using both projector-augmented wave and muffin-tin-orbital techniques. The results show that for all cases the ground state of Cr is predicted to be of commensurate antiferromagnetic order, longitudinal SDWs lead to an increased total energy. The excitation energies associated with SDW formation are within the range of thermal energies even well below the N\'eel temperature. Structural relaxation of the SDW reveals that the stress relaxation has no significant effect on the energetics of the SDW phase. A possible scenario for the origin of the observed SDW state is developed.

115 citations

Journal ArticleDOI
J. P. Korb1
TL;DR: In this article, low-field NMR relaxation of water in calibrated granular packings leads to striking different pore-size dependencies of the relaxation times T1 and T2 when changing the amount of surface paramagnetic impurities.
Abstract: Nuclear magnetic relaxation is useful for probing physical and chemical properties of liquids in porous media. Examples are given on high surface area porous materials including calibrated porous silica glasses, granular packings, plaster pastes, cement-based materials and natural porous materials, such as sandstone and carbonate rocks. Here, we outline our recent NMR relaxation work for these very different porous materials. For instance, low field NMR relaxation of water in calibrated granular packings leads to striking different pore-size dependencies of the relaxation times T1 and T2 when changing the amount of surface paramagnetic impurities. This allows separation of the diffusion and surface limited regimes of relaxation in these macroporous media. The magnetic field dependence of the nuclear spin-lattice relaxation rate 1/T1(!0) is also a rich source of dynamical information for characterizing the molecular dynamics of liquids in porous media. This allows a continuous characterization of the evolving microstructure of various cementitious materials. Our recent applications of two-dimensional (2D) T1-T2 and T2-z-store-T2 correlation experiments have evidenced the water exchange in connected micropores of cement pastes. The direct probing of water adsorption time on a solid surface gives access to an original characterization of the surface nano-wettability of porous plaster pastes. We show that such a parameter depends directly on the physical chemistry of the pore surfaces. Lastly, we outline our recent measurements of wettability in oil/brine/reservoir carbonate rocks.

115 citations

Journal ArticleDOI
TL;DR: Pentacoordinate complex with a tridentate antenna-like ligand L(3) forms a dimer held by short π-π stacking with head-to-head contacts that shows superparamagnetic behavior at low temperature that depends upon the applied magnetic field.
Abstract: Pentacoordinate complex [CoL3Cl2] with a tridentate antenna-like ligand L3 forms a dimer held by short π–π stacking with head-to-head contacts at 3.4 A. The direct-current (dc) magnetic susceptibility and magnetization data confirm weak ferromagnetic interaction and a large-magnetic anisotropy, D/hc = 150 cm–1 and E/hc = 11.6 cm–1. The system shows superparamagnetic behavior at low temperature that depends upon the applied magnetic field. At Bdc = 0.2 T, a low-frequency peak at the out-of-phase susceptibility is seen (ν ∼ 0.3 Hz), whereas the onset of the second peak appears at ν > 1500 Hz, indicating the existence of two slow relaxation processes.

115 citations

Journal ArticleDOI
TL;DR: In this paper, the echo envelope modulations for an S = 1, I = 1/2 spin system including zero field splittings in the high field limit were calculated for the photoexcited triplet state of pentacene.
Abstract: Electron spin echo observations of the photoexcited triplet state of 0.1 mol % pentacene‐h14 and ‐d14 in p‐terphenyl crystals at room temperature are presented. Theory is presented for calculation of the echo envelope modulations for an S = 1, I = 1/2 spin system including zero field splittings in the high field limit. Echo envelope modulations due to proton and deuteron hyperfine interactions in the pentacene molecule have been observed. The echo decay data are used to calculate triplet state decay parameters.

114 citations

Journal ArticleDOI
TL;DR: It is concluded that water in these systems is highly mobile, that water molecules affected directly by a macromolecule tumble anisotropically about all axes relative to the macromolescule with correlation times in the region of 10 -9 s at 260 K and that these molecules exchange with water molecules free of the influence of the macROMolecule with a lifetime in the dynamically oriented state of the order of 10-6 s at temperatures around 300 K.
Abstract: The basic principles of nuclear spin relaxation, dielectric relaxation and quasielastic neutron scattering and their use in studying the motions of water molecules are outlined. A summary is given of the time scales associated with the translational and rotational motions of water molecules and of intermolecular proton exchange in pure liquid water. A model is then proposed for the dynamics of water molecules in heterogeneous systems involving regions having differing compositions, water molecules within each region existing in environments both affected by interaction with the macromolecular components and free of their influence and including exchange of water molecules between different environments and regions. The lifetime of the interaction of water molecules with the macromolecular components is assumed long compared with the time for rotation of such bound molecules. Exchange of protons between water molecules and between water molecules and macromolecules is also considered. The ways in which such processes would be expected to affect the observed nuclear magnetic resonance, dielectric and neutron scattering behaviour are outlined. Particular emphasis is placed on nuclear spin relaxation phenomena and the existence and observation of residual dipolar and quadrupolar splittings in the n.m.r. spectra of 1 H and 2 H (D) nuclei in water molecules in such systems, these splittings arising from water molecules dynamically oriented at water/macromolecule interfaces. Details are then given of particular studies of water molecule dynamics in heterogeneous systems using n.m.r., dielectric and neutron scattering techniques. The systems discussed include moist protein powders, protein solutions, phospholipid/ water and soap/water mesophases, clay/water systems and biological polymers and tissues. It is concluded that water in these systems is highly mobile, that water molecules affected directly by a macromolecule tumble anisotropically about all axes relative to the macromolecule with correlation times in the region of 10 -9 s at 260 K and that these molecules exchange with water molecules free of the influence of the macromolecule with a lifetime in the dynamically oriented state of the order of 10 -6 s at temperatures around 300 K. The ability of nuclear magnetic relaxation studies to distinguish water in different regions of a tissue is discussed and examples are given of the study of the rate of water transport across membranes using these techniques.

114 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
202227
2021652
2020582
2019614
2018638
2017645