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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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TL;DR: In this paper, the equations for the relaxation time of a nuclear spin in a molecule undergoing diffusion reorientation have been extended to include fully anisotropic reorientations for a number of relaxation mechanisms.
Abstract: The equations for the relaxation time of a nuclear spin in a molecule undergoing diffusion reorientation have been extended to include fully anisotropic reorientation for a number of relaxation mechanisms. The full rotational diffusion tensor—both principal diffusion constants and orientation of the principal rotation axes—can be found by the measurement of the NMR relation times of an appropriate set of nuclei in the molecule.
311 citations
TL;DR: In this article, measurements of dissociation field effect relaxation in aqueous poly-α,L-glutamic acid were carried out and the observed relaxation was attributed to the helix-coil transition.
Abstract: The measurements of dissociation field effect relaxation in aqueous poly-α,L-glutamic acid were carried out. From the feature of pH dependency of relaxation time, the observed relaxation was attributed to the helix-coil transition. The helical growth rate constant was calculated to be (2.9±2)×107 sec−1 from Schwarz’s equation.
309 citations
TL;DR: In this article, a general expression for the time-correlation function of the polymer concentration around equilibrium and examine its relaxation in some typical cases is given, which can be used for gelling solutions and explains previous dynamic light scattering experiments at the sol-gel transition.
Abstract: Phenomenological hydrodynamic equations are proposed for entangled polymer blends as generalization of those for polymer solutions. They can describe coupling between macroscopic flow and relative diffusion. The key concept we use is the "tube velocity" introduced by Brochard in the problem of mutual diffusion in polymer blends. As applications, (I) we give a general expression for the time-correlation function of the polymer concentration around equilibrium and examine its relaxation in some typical cases. It can be strongly influenced by the viscoelastic effect when the two polymers have different lengths. Our expression can also be used for gelling solutions and explains previous dynamic light scattering experiments at the sol-gel transition. (ii) Detailed calculations are performed for the case of a single rheological relaxation time (the Maxwell model). The steady state structure factor is obtained to Iinear order in macroscopic flow. (iii) We predict that composition inhomogeneity is created in mixtures oflong and short polymers undergoing nonuniform flow. Its origin is that the longer chains support stress more than the shorter ones and the resultant imbalance of stress causes relative motion of the two polymers. These results are applicable both to solutions and blends.
308 citations
TL;DR: A family of trigonal pyramidal iron(II) complexes supported by tris(pyrrolyl-α-methyl)amine ligands of the general formula [M(solv)(n)][(tpa(R))Fe] is presented, enabling an initial probe of how the ligand field influences the static and dynamic magnetic behavior.
Abstract: We present a family of trigonal pyramidal iron(II) complexes supported by tris(pyrrolyl-α-methyl)amine ligands of the general formula [M(solv)(n)][(tpa(R))Fe] (M = Na, R = tert-butyl (1), phenyl (4); M = K, R = mesityl (2), 2,4,6-triisopropylphenyl (3), 2,6-difluorophenyl (5)) and their characterization by X-ray crystallography, Mossbauer spectroscopy, and high-field EPR spectroscopy. Expanding on the discovery of slow magnetic relaxation in the recently reported mesityl derivative 2, this homologous series of high-spin iron(II) complexes enables an initial probe of how the ligand field influences the static and dynamic magnetic behavior. Magnetization experiments reveal large, uniaxial zero-field splitting parameters of D = -48, -44, -30, -26, and -6.2 cm(-1) for 1-5, respectively, demonstrating that the strength of axial magnetic anisotropy scales with increasing ligand field strength at the iron(II) center. In the case of 2,6-difluorophenyl substituted 5, high-field EPR experiments provide an independent determination of the zero-field splitting parameter (D = -4.397(9) cm(-1)) that is in reasonable agreement with that obtained from fits to magnetization data. Ac magnetic susceptibility measurements indicate field-dependent, thermally activated spin reversal barriers in complexes 1, 2, and 4 of U(eff) = 65, 42, and 25 cm(-1), respectively, with the barrier of 1 constituting the highest relaxation barrier yet observed for a mononuclear transition metal complex. In addition, in the case of 1, the large range of temperatures in which slow relaxation is observed has enabled us to fit the entire Arrhenius curve simultaneously to three distinct relaxation processes. Finally, zero-field Mossbauer spectra collected for 1 and 4 also reveal the presence of slow magnetic relaxation, with two independent relaxation barriers in 4 corresponding to the barrier obtained from ac susceptibility data and to the 3D energy gap between the M(S) = ±2 and ±1 levels, respectively.
308 citations
TL;DR: In this article, the effect of the magnetic dipolar interaction of a nuclear spin with the thermal average of an electronic spin (Curie spin) is modulated by molecular motion but not by the electronic spin relaxation.
308 citations