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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 theory of the direct effect of molecular rotations on longitudinal spin relaxation in pure quadrupole resonance is described and applied to the hexamethylene tetramine.
Abstract: Slow motional processes for which there is no motional narrowing can be important in spin-lattice relaxation. These relaxation mechanisms cannot be treated by perturbation theory. It is shown that the relaxation times are directly proportional to the motional correlation times. It is suggested that they can be used very effectively to measure the rate of motional processes in solids. A theory of the direct effect of molecular rotations on longitudinal spin relaxation in pure quadrupole resonance is described. The theory is applied to the ${\mathrm{N}}^{14}$ resonance in hexamethylene tetramine. Measurement of the longitudinal relaxation times in this molecule were carried out by a steady-state pulse method, and the rotational correlation times were determined. The results are in agreement with previous nmr measurements.

138 citations

Book
01 Jan 1980
TL;DR: In this article, the authors studied the spin exchange process in a two-electron system and measured the rate of spin exchange using the ESR spectrum of Nitroxide Biradicals.
Abstract: 1. Introduction.- 2. Theory of Spin Exchange.- 2.1 Exchange Interaction Between Two Paramagnetic Particles.- Dependence of Interaction Energy on Spin.- Spin-Hamiltonian of Exchange Interaction.- The Nature of Exchange Interaction. The Two-Electron System.- Interaction Between Many-Electron Systems. Delocalization and Spin Polarization.- Anisotropy of Exchange Interaction.- Semi empirical Estimates of Exchange Integral.- 2.2 Qualitative Description of Spin Exchange Process.- Spin Exchange.- Spin Exchange in a Two-Electron System.- Spin Density Matrix.- Spin Motion Due to Exchange Interaction.- 2.3 Formal Kinetics of Spin Exchange in Magnetically Dilute Solutions.- Binary Collision Approximation.- Rate of Bimolecular Spin Exchange Process.- Kinetics of Spin Exchange for Particles with Spins S = 1/2.- Kinetic Equations for Free Radicals Showing Hyperfine Structure (HFS) in Their ESR Spectra.- 2.4 Spectroscopic Manifestations of Spin Exchange.- Spectral Diffusion Due to Spin Exchange.- Shape of ESR Spectra.- Slow Exchange.- Fast Exchange.- Other Methods of Measuring the Rate of Spin Exchange.- Effect of Saturation.- Electron-Electron Double Resonance (ELDOR).- Electron Spin Echo.- 2.5 Theory of Sudden Collisions.- 2.5.1 Model of Sudden Collisions.- Basic Assumptions.- Equations for the Estimation of xc.- 2.5.2 Kinetic Equations.- Steady-State Equation for the Operator of Collision Efficiency.- 2.5.3 Spin Exchange Between Radicals.- Model System.- Spin Exchange Between Radicals in the Presence of Hyperfine Interaction.- 2.5.4 Spin Exchange Between Radicals with Anisotropic Interaction.- Rotation of Partners at the Moment of Their Contact.- Orientation Relaxation of Particles in Intervals Between Reencounters.- 2.5.5 Spin Exchange of Radicals with Other Paramagnetic Particles.- Spin Exchange Between Radicals and Spins SB with Long Times of Paramagnetic Relaxation.- Shape of ESR Spectra.- The Influence of the Relaxation of Spin SB on the Rate of Exchange.- Anisotropy of Spin Exchange.- 2.5.6 Spin Exchange in Biradicals.- The Model.- Kinetic Equations.- The ESR Spectrum of Nitroxide Biradicals.- 2.6 The Model of Diffusive Passage.- The Model.- An Equation for the Operator of Collision Efficiency.- The Kinetic Equations.- The Spin Exchange Between Two Kinds of Particles with Spins SA = SB = 1/2.- Comparison of Results Obtained from Models of Sudden Collisions and Those of Diffusive Passage.- 3. Experimental Measurement of Spin Exchange Rate.- 3.1 Experimental Measurement of Spin Exchange Rate Constants from ESR Spectra.- 3.1.1 Exchange Between Identical Particles with S = 1/2.- Doublet Spectrum with a Zero Initial Width.- Complex Spectra.- Determination of Rate Constants from Exchange Broadening of Complex Spectra.- Determination of Rate Constants from Exchange Shift of the Lines of Complex Spectra.- Determination of Rate Constants from Exchange Narrowing of Complex Spectra.- Peculiarities of Exchange Narrowing in Concentrated Solutions.- Comparison of Spin Exchange Rate Constants Measured by Different Techniques.- 3.1.2 Exchange Between Different Particles.- 3.2 Other Methods of Measuring the Spin Exchange Rate Constants.- 3.2.1 Continuous Wave Saturation Method.- 3.2.2 Electron-Electron Double Resonance.- 3.2.3 Electron Spin Echo.- 3.3 Separation of Exchange and Dipole-Dipole Contributions to Concentration Broadening of ESR Lines.- Theoretical Estimation of Dipole Broadening.- Experimental Estimation of Dipole-Dipole Contribution at Low Viscosity.- Separation of the Cases of Exchange and Dipole-Dipole Broadening According to Viscosity and Temperature Dependences.- 3.4 Separation of the Cases of Weak and Strong Exchange.- 4. Spin Exchange in Chemistry and Biology.- 4.1 Study of Diffusion Collisions in Solutions.- Hydrodynamic Model of Diffusion Collisions.- Potentialities of Spin Exchange.- 4.1.1 Collisions in Single-Component Solvents.- Spin Exchange Between Radicals.- Spin Exchange Between Free Radicals and Paramagnetic Complexes.- 4.1.2 Collisions in Complex Systems.- Binary Solvents.- Liquids Near Critical Points.- Polymeric and Heterogeneous Systems.- Collisions of Macroradicals.- 4.1.3 Study of Intramolecular Collisions of Paramagnetic Fragments.- 4.2 Estimation of the Energy of Exchange Interaction During Collisions.- 4.2.1 Free Radicals.- 4.2.2 Spin Exchange with Metal Complexes.- 4.2.3 Discussion of Results.- 4.3 Collisions of Charged Particles.- 4.3.1 Experimental Results.- Spin Exchange in Solutions of Anion Radical (S03)2N02-.- Spin Exchange Involving Charged Nitroxide Radicals.- Spin Exchange Between Anion Radicals in Organic Solvents.- 4.3.2 Comparison of Experiments on Spin Exchange with the Diffusion Theory of Collisions of Charged Particles in Liquid.- 4.3.3 Study of Collisions Between Aquo Complexes of Transition Metals.- Role of Counterions in Collisions of Aquo Complexes of Vanadyl.- An Example of the Process with Weak Influence of Electrostatic Repulsion.- 4.4 Biological Applications of Spin Exchange.- Paramagnetics Used as Spin Probes.- 4.4.1 Measurement of Binary Collision Rate of Molecules in Biological Systems.- 4.4.2 Study of Location of Paramagnetic Centers in Biopolymers.- Quantitative Interpretation of the Values of Kg for Spin Exchange Processes Involving Paramagnetic Centers of Biopolymers.- 4.4.3 Development of Individual ESR Spectra of Spin Labels via Spin Exchange.- 4.4.4 Spin Exchange Titration.- Kinetics of Reactions Involving Oxygen Molecules.- Coordination of Paramagnetic Ions by Macromolecules.- 4.4.5 Other Applications.- 4.5 Conclusion.- List of Frequently Used Notations and Abbreviations.- References.

138 citations

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the effect of geometric confinement on the dynamics of polyisoprene (PI) nanocomposites and found that the normal mode becomes faster with increasing clay content.
Abstract: Nanocomposites of organically modified clay nanoparticles and a polyisoprene (PI) matrix were prepared by solution-mediated intercalation, and their dynamics were investigated over a broad range of frequency and temperature by dielectric relaxation spectroscopy (DRS) and dynamic mechanical spectroscopy (DMS). The principal goal was to address the effect of geometric confinement and elucidate how the dynamics vary as a function of the type and concentration of clay and the molecular weight of PI. Dielectric spectra of nanocomposites with low-molecular-weight PI reveal no effect of clay loading on the average relaxation time for segmental and normal mode relaxation, but dc conductivity and interfacial polarization are affected. In nanocomposites with high molecular weight PI (in the entangled regime), however, a clear effect of clay loading on the average relaxation time for the normal mode process is observed. Most interestingly, it is found that the normal mode becomes faster with increasing clay content,...

138 citations

Journal ArticleDOI
01 Feb 1961
TL;DR: In this article, a theoretical treatment of spin-lattice relaxation appropriate for rare earth salts leads to predictions which agree both with the experimentally observed temperature dependence and with the magnitude of the relaxation time.
Abstract: The spin-lattice relaxation time of Ce3+ in cerium magnesium nitrate has been measured as a function of temperature and of magnetic fields up to 1000 gauss. In the temperature range between 3° and 1.9°K, the relaxation time is found to vary exponentially with inverse temperature, changing by a factor of 350. A theoretical treatment of spin-lattice relaxation appropriate to rare earth salts leads to predictions which agree both with the experimentally observed temperature dependence and with the magnitude of the relaxation time. The dominant relaxation process is one in which a phonon of high energy (» kT) is absorbed by the spin system resulting in a transition from one of the ground state doublet levels to the nearest excited level. Then, in a separate step, another high energy phonon is emitted and a transition is effected to the other level of the ground state doublet. The net result of this `two-step direct process' can be interpreted as spin-lattice relaxation; i.e. there results a net change in population between the ground state doublet levels which brings the spin system towards thermal equilibrium with the lattice. Such a mechanism, which is quite different from those which have been envisaged up to now for spin-lattice relaxation, should be appropriate for many of the rare earth salts.

138 citations

Journal ArticleDOI
L. Fleury1, Andreas Zumbusch1, Michel Orrit1, R. Brown1, J. Bernard1 
TL;DR: In this paper, the influence of low-energy matrix excitations on the line widths and fluorescence correlation of single terrylene molecules in polyethylene at helium temperatures was studied.

138 citations


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