Relaxation (NMR)

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

Electronic dephasing, vibrational relaxation, and solvent friction in molecular nonlinear optical line shapes

Abstract: The role of solvation dynamics in molecular nonlinear optical line shapes is analyzed using a reduced description based on the time evolution of the density matrix in Liouville space. Langevin equations are used to treat the coupling of the solvent to the molecular electronic and nuclear degrees of freedom. Electronic dephasing is calculated using a solvation coordinate which satisfies a generalized Fokker–Planck equation, and vibrational relaxation is related to the solvent viscosity and friction. The combined effect of both processes is incorporated into appropriate multitime correlation functions which are evaluated using a Liouville‐space generating function. The present eigenstate‐free approach is particularly suitable for calculating spectral line shapes as well as rate processes (isomerization, electron transfer) of large polyatomic molecules in condensed phases.

Polymer Chain Dynamics and NMR

Abstract: The universal features of polymer dynamics are specifically represented by laws for (anomalous) segment diffusion and chain relaxation modes. Nuclear magnetic resonance (NMR)-based techniques provide direct access to these phenomena. This in particular refers to NMR relaxation and diffusion studies. Methods suitable for this purpose are described in detail. Three basic classes of polymer dynamics models, namely the Rouse model, the tube/reptation model, and the renormalized Rouse models are outlined and discussed with respect to predictions for NMR measurands. A wealth of experimental NMR data are reviewed and compared with predictions of the model theories. It is shown that characteristic features of all three types of models can be verified in great detail provided that the model premisses are suitably mimicked in the experiments. Rouse dynamics is shown to be relevant for polymer melts with molecular weights below the critical value and for solutions of diminished entanglement effect. Features specific for the renormalized Rouse model reveal themselves in the form of high- and low-mode-number limits of the spin–lattice relaxation dispersion. These results are considered to mirror the analytical structure of the Generalized Langevin Equation. Finally, anomalous-diffusion and relaxation laws characteristic for the tube/reptation model can be perfectly reproduced in experiment if the polymer chains are confined in a nanoporous, solid matrix whereas bulk melts are not in accord with these predictions. The dynamics of chains confined in artificial tubes can be treated analytically assuming a harmonic radial potential for the polymer/wall interaction. These results derived for a real tube closely render the characteristic features of the original Doi/Edwards model predicted for a fictitious tube.

Structures of water and primary alcohol studied by microwave dielectric analyses

Abstract: By the use of the time domain reflectometry method dielectric measurements were carried out first on methanol mixtures with ethanol and 1‐propanol, and second, water mixtures with methanol, ethanol and 1‐propanol in the frequency range 10 MHz–20 GHz. The first mixtures show a Debye relaxation and logarithm of the relaxation time is given by a linear function of the mole fraction of methanol. These mixtures have the same chainlike cluster of pure alcohol. The second mixtures show the same trend of the relaxation time in a region 0≤xw 0.83 and that the cluster must be cyclic, consisting of six molecules.

Glauber dynamics in a single-chain magnet: From theory to real systems

Abstract: The Glauber dynamics is studied in a single-chain magnet. As predicted, a single relaxation mode of the magnetization is found. Above 2.7 K, the thermally activated relaxation time is mainly governed by the effect of magnetic correlations and the energy barrier experienced by each magnetic unit. This result is in perfect agreement with independent thermodynamical measurements. Below 2.7 K, a crossover towards a relaxation regime is observed that is interpreted as the manifestation of finite-size effects. The temperature dependences of the relaxation time and of the magnetic susceptibility reveal the importance of the boundary conditions.

Depolarized Rayleigh scattering and 13C NMR studies of anisotropic molecular reorientation of aromatic compounds in solution

Abstract: Measurements of reorientational relaxation times of simple aromatic compounds have been made by both depolarized Rayleigh light scattering and 13C NMR spin‐lattice relaxation. Combination of the reorientation times determined by these techniques makes it possible to extract the reorientation times about the different molecular axes. The viscosity dependence of reorientation times about the individual molecular axes has been measured for benzene, mesitylene, toluene, and nitrobenzene. The viscosity dependence, which is highly anisotropic, is discussed in terms of a ``slip'' model of reorientational motion.