Relaxation Time Experiments

These methods are nondestructive, and are particularly useful for atoms that have no - or only very short-lived - radioisotopes. There is a requirement for an isotope with a suitable nuclear spin, sensitivity and abundance, of which F and Li are important examples. These methods divide essentially into two classes (i) relaxation time experiments, where the diffusive motion of spins results in the loss the coherence of the spins and (ii) field gradient experiments, where the nuclear spin is used as a label on the atom, in a similar manner to a tracer experiment. 

The exact relationship between the relaxation times and the diffusion coefficient 

although the measurements provide accurate activation energies for diffusion they are difficult to correlate precisely with diffusion coefficients obtained with other techniques. However, the measurements are very rapid, the relaxation times being the order of seconds or less, and if there are two diffusion mechanisms operative with coefficients differing by an order of magnitude or more then they will have relaxation times that can be separated. 

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Anklam et al. [91] have attempted to measure the extensional rheological properties of w/o emulsions and HIPEs, using a nozzle-type viscometer. However, the results showed a dependence on the nozzle size used, and long relaxation times. Experiments on other non-Newtonian fluids indicated that it was not possible to obtain reliable results with this kind of instrument. 

Nuclear magnetic resonance (NMR) has been used to study segmental motions in block copolymer solutions. The mobility of protons in polymer chains in dilute solutions has been probed using high-resolution H NMR. Association of chains into micelles leads to a reduction in mobility in the core, which leads to a broadening of the respective NMR lines that has been studied for a number of systems, as described by Tuzar and Kratochvil (1993). The sol-gel transition in concentrated solutions has been located via ]H transverse relaxation time experiments, as outlined in Chapter 4. 

NMR spectroscopy and solid-state and 13C NMR relaxation-time experiments. However, the sensitivity of solid-state 13C NMR is not as high as that of Raman and IR spectroscopy. For instance, solid-state 13C NMR of sulfur-vulcanised EPDM could only be performed when the ENB unsaturation of EPDM was fully isotopically enriched with 13C NMR [124]. 

Sturz and DoUe measured the temperature dependent dipolar spin-lattice relaxation rates and cross-correlation rates between the dipolar and the chemical-shift anisotropy relaxation mechanisms for different nuclei in toluene. They found that the reorientation about the axis in the molecular plane is approximately 2 to 3 times slower than the one perpendicular to the C-2 axis. Suchanski et al measured spin-lattice relaxation times Ti and NOE factors of chemically non-equivalent carbons in meta-fluoroanihne. The analysis showed that the correlation function describing molecular dynamics could be well described in terms of an asymmetric distribution of correlation times predicted by the Cole-Davidson model. In a comprehensive simulation study of neat formic acid Minary et al found good agreement with NMR relaxation time experiments in the liquid phase. Iwahashi et al measured self-diffusion coefficients and spin-lattice relaxation times to study the dynamical conformation of n-saturated and unsaturated fatty acids. 

NMR structure calculation alone cannot describe the dynamic structure of protein. If structure calculations (Section 6) and NMR-derived information (Sections 8 and 9) are reliable enough, a poor convergence (if it remains in the NMR ensemble) likely arises from the internal mobility of atoms. The molecular motion is usually confirmed by order parameters of spins derived from relaxation time experiments.107-109... 

Some interesting effects have been observed in a series of H and relaxation time experiments using different organic liquids within silicas of known pore size under saturation conditions. It was observed that the relaxation times of polar molecules, such as pyridine, aniline and nitrobenzene, obeyed "... 

In this chapter we describe some molecular dynamics (MD) simulation studies (75-25) of this rich experimental system. After describing the simulation methods, we consider the origin of long relaxation times. Experiments and simulation results are consistent with the onset of a glass transition as the spacing between the confining... 

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