Self-decoupling

C.-F. Hoelger, E. Rossler, B. Wehrle, F. Aguilar-Parrilla and H.-H. Limbach, Self-decoupling of 15N-14N dipole-quadrupole couplings in 15N CPMAS NMR spectra and molecular motions in crystalline hydrazine sulfate, p-(diethylamino)benzaldehyde, and its solid solution in polycarbonate.. Phys. Chem., 1995, 99,14271-14276. 

The efficient relaxation of quadrupolar nuclei in solution generally results in self-decoupling, such that quadrupolar effects are generally not observed in NMR spectra of spin-1/2 nuclei. Self-decoupling may also be a factor in NMR spectra of solid samples this often manifests itself as an asymmetric line shape. By decreasing the temperature, the Ti relaxation time of the quadrupolar nucleus may increase sufficiently to allow acquisition of NMR spectra where the effects of residual dipolar coupling are sufficiently resolved. ... 

The time dependence of the perturbation related to the transition from the decelerating to the accelerating era could be calculated [17,18]. The calculation shows that the self-decoupling process is effective enough to explain why a ter this transition the variation of the constants is as small as observed in the present-time laboratory experiments. However, the calculated time dependence is also consistent with the observations of the variation of the electromagnetic fine-structure constant at z> 1 [19-21]. 

Rapid magnetic exchange self-decouples chlorine, bromine, and iodine nuclei... 

What one requires is a self-consistent picture of the interface, including both metal and electrolyte, so that, for a given surface charge, one has distributions of all species of metal and electrolyte phases. Unified theories have proved too difficult but, happily, it seems that some decoupling of the two phases is possible, because the details of the metal-electrolyte interaction are not so important. Thus, one can calculate the structure of each part of the interface in the field of the other, so that the distributions of metal species are appropriate to the field of the electrolyte species, and vice versa. 

Thus the existence of such inhomogeneity seems to be necessary to understand the decoupling of the self-diffusion from the viscosity for solutes having the same size as that of the solvent. The existence of such inhomogeneity has also been suggested by Taijus and Kivelson [89] from their computer simulation studies. 

The study is performed at reduced temperature T = 0.75 and reduced density p = 0.844-0.92. This is precisely the system studied in computer simulations [102]. The variation of the self-diffusion coefficient with the solute size is shown in Fig. 8, where the size of the solute molecule has been varied from 1 to 1/20 times that of the solvent molecule. In the same figure the computer-simulated values [102] are also plotted for comparison with the calculated results. The calculated results are in good agreement with the computer simulations. Both the theoretical results and the computer simulation studies show an enhanced diffusion for size ratios TZ TZ = 01/02) between 1.5 and 15. This is due to the sharp decoupling of the solute dynamics from the solvent density mode. 

The decoupling scenario can be clearly envisaged from. Fig. 9, which shows the time dependence of (a) the self-intermediate scattering function... 

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