Topic
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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TL;DR: In this paper, the authors measured the nuclear relaxation in water and showed that the increase of relaxation rate is observable in natural water (0.037% O17), and becomes very appreciable in water enriched in O17.
Abstract: Measurements of the nuclear relaxation in water are reported. The transverse relaxation rate (1/T2) of the proton resonance is pH dependent. The effect is shown to be due to a spin‐spin splitting of the proton resonance by O17 (spin 5/2), which is only partially averaged out by proton exchange. The increase of relaxation rate is observable in natural water (0.037% O17), and becomes very appreciable in water enriched in O17. Additional information can be obtained by measuring relaxation rates in the presence of an rf field H1, using a method due to Solomon. A study of the width of the O17 resonance as a function of pH is in quantitative agreement with the results of the proton resonance. The observations provide a direct determination of the rate constants of the exchange reactions: H2O+H3O+→ lim k1H3O++H2O and H2O+HO−→ lim k2HO−+H2O. It is found that k1=(10.6±4)×109 liter mole−1 sec−1 and k2=(3.8±1.5)×109 liter mole−1 sec−1. The spin‐spin interaction between H and O17 in water is determined as 92±15 cps. In the Appendices, theoretical equations for the exchange contribution to the relaxation rate are derived.
475 citations
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TL;DR: A two-cluster drifting state with zero magnetization forms spontaneously at very small temperatures; at larger temperatures an initial density modulation produces this state, which relaxes very slowly, which suggests the possibility of exciting magnetized states in a mean-field antiferromagnetic system.
Abstract: We study the dynamics of a fully coupled network of N classical rotators, which can also be viewed as a mean-field XY Heisenberg (HMF) model, in the attractive (ferromagnetic) and repulsive (antiferromagnetic) cases. The exact free energy and the spectral properties of a Vlasov-Poisson equation give hints on the values of dynamical observables and on time relaxation properties. At high energy (high temperature T) the system relaxes to Maxwellian equilibrium with vanishing magnetization, but the relaxation time to the equilibrium momentum distribution diverges with N as ${\mathit{NT}}^{2}$ in the ferromagnetic case and as ${\mathit{NT}}^{3/2}$ in the antiferromagnetic case. The N dependence of the relaxation time is suggested by an analogy of the HMF model with gravitational and charged sheets dynamics in one dimension and is verified in numerical simulations. Below the critical temperature the ferromagnetic HMF mode shows a collective phenomenon where the rotators form a drifting cluster; we argue that the drifting speed vanishes as ${\mathit{N}}^{\mathrm{\ensuremath{-}}1/2}$ but increases as one approaches the critical point (a manifestation of critical slowing down). For the antiferromagnetic HMF model a two-cluster drifting state with zero magnetization forms spontaneously at very small temperatures; at larger temperatures an initial density modulation produces this state, which relaxes very slowly. This suggests the possibility of exciting magnetized states in a mean-field antiferromagnetic system.
475 citations
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TL;DR: In this article, the specific absorption rate (SAR) values of aqueous suspensions of magnetite particles with different diameters varying from 7.5 to 416nm were investigated by measuring the time-dependent temperature curves in an external alternating magnetic field (80 kHz, 32.5 kA/m).
473 citations
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TL;DR: In this paper, a steady state is established where there is a dynamic balance between the effect of the pulses and spin relaxation, and the deviation of the intensity of the free induction signal from its thermal equilibrium value is an exponential function of the pulse interval with time constant equal to the spin-lattice relaxation time.
Abstract: When a nuclear spin system is subjected to a repetitive sequence of strong radiofrequency pulses, a steady state is established where there is a dynamic balance between the effect of the pulses and spin relaxation. Under certain readily satisfied pulse conditions, the deviation of the intensity of the free induction signal from its thermal equilibrium value is an exponential function of the pulse interval with time constant equal to the spin–lattice relaxation time. The determination is unaffected by spin–spin relaxation provided that the interval between pulses is long enough to permit all transverse components of magnetization to be eliminated, and provided precautions are taken to inhibit spin‐echo formation. Through Fourier transformation of the transient response, high resolution spectra with many component resonances may be studied, and the spin–lattice relaxation times of the individual lines determined. The technique lends itself particularly well to repeated accumulation of the transient signal for the purpose of improving sensitivity. It has been applied to the problem of determining the spin–lattice relaxation rates of the eight different carbon‐13 resonances in 3,5‐dimethylcyclohex‐2‐ene‐1‐one. The results span a range from 2.6 to 39 sec, and are in good agreement with those obtained by applying 180°–t–90° sequences to the same sample.
470 citations
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TL;DR: In this paper, a theoretical framework for the interpretation of NMR data from water nuclei (1H, 2H, and 17O) is presented, and the possibilities and limitations of the NMR technique in answering the fundamental questions about water structure and dynamics in heterogeneous systems are discussed.
Abstract: Nuclear magnetic resonance (NMR) data from water nuclei (1H, 2H, and 17O) can provide much information about the state of water in heterogeneous systems. In the present work, we present a theoretical framework for the interpretation of such data and discuss the implications of the theory. Due to the local anisotropy in heterogeneous systems, it is necessary to consider two components of water motion: a fast anisotropic reorientation superposed on a more extensive slow motion. On the basis of the experimentally verified assumption that these motions occur on different time scales, we develop a ’’two‐step’’ model of relaxation, showing that both motions may give important contributions to the relaxation. We derive a simple expression for the relevant correlation function, valid for isotropic systems. Anisotropic systems are also treated, making use of a new symmetry theorem for time correlation functions. The proof of this theorem is given in an Appendix. The magnitudes of the water 2H and 17O quadrupole coupling constants are estimated to 0.222 and 6.67 MHz, respectively. Results of ab initio quantum chemical calculations are presented, demonstrating the insensitivity of the water 17O field gradient to nearby ionic species. The possibilities and limitations of the NMR technique in answering the fundamental questions about water structure and dynamics in heterogeneous systems are discussed. We suggest a novel interpretation of the well‐known invariance of the ratio of 1H and 2H splittings. Furthermore, we argue that the available NMR data are consistent with a short‐ranged (≲2 molecular layers) perturbation of the water tumbling rate and anisotropy.
467 citations