Nuclear magnetic resonance spectroscopy relaxation mechanisms

Secondary relaxations are usually measured either by mechanical methods such as dynamic mechanical spectroscopy or (somewhat less often) by electrical methods such as dielectric relaxation spectroscopy [159], The existence of Tp is generally ascribed to the onset of a significant amount of some kind of motion of the polymer chains and/or the side groups attached to them, on a much smaller and more localized scale than the large-scale cooperative motions of chain segments associated with Ta. These motions are usually inferred from the results of measurements using methods such as nuclear magnetic resonance spectroscopy. See... 

Many experimental techniques have been used to examine the detailed structure of perfluorinated polymeric membranes. These include transmission electron microscopy [23], small angle X-ray scattering [24], Infra Red spectroscopy [25,26], neutron diffraction [27], Nuclear Magnetic Resonance [26,28], mechanical and dielectric relaxation [25,29], X-ray diffraction, and transport measurements. All these methods show convincing evidence for the existence of two phases in the perfluorosulfonate and perfluorocarboxylate polymers. One phase has crystallinity and a structure close to that of polytetrafluoroethylene (PTFE), and the other is an aqueous phase containing ionic groups. 

The isothermal time dependence of relaxation and fluctuation due to molecular motions in liquids at equilibrium usually cannot be described by the simple linear exponential function exp(-t/r), where t is the relaxation time. This fact is well known, especially for polymers, from measurements of the time or frequency dependence of the response of the equilibrium liquid to external stimuli such as in mechanical [6], dielectric [7, 33], and light-scattering [15, 34] measurements, and nuclear-magnetic-resonance spectroscopy [14]. The correlation or relaxation function measured usually decays slower than the exponential function and this feature is often referred to as non-exponential decay or non-exponentiality. Since the same molecular motions are responsible for structural recovery, certainly we can expect that the time dependence of the structural-relaxation function under non-equilibrium conditions is also non-exponential. An experiment by Kovacs on structural relaxation involving a more complicated thermal history showed that the structural-relaxation function even far from equilibrium is non-exponential. For example (Fig. 2.7), poly(vinyl acetate) is first subjected to a down-quench from Tq = 40 °C to 10 °C, and then, holding the temperature constant, the sample... 

Local mode relaxation of isolated lignin and its model compounds have been detected by dynamic mechanical measurement, and broad-line nuclear magnetic resonance spectroscopy (b-NMR) [49,53], although this molecular motion has scarcely received attention in recent papers. Transition map of local mode relaxation of various kinds of polymers is found elsewhere [56]. Figure shows second moment of absorption line of b-NMR of DL in powder form. When the relaxation is from the... 

Several spectroscopic techniques, namely, Ultraviolet-Visible Spectroscopy (UV-Vis), Infrared (IR), Nuclear Magnetic Resonance (NMR), etc., have been used for understanding the mechanism of solvent-extraction processes and identification of extracted species. Berthon et al. reviewed the use of NMR techniques in solvent-extraction studies for monoamides, malonamides, picolinamides, and TBP (116, 117). NMR spectroscopy was used as a tool to identify the structural parameters that control selectivity and efficiency of extraction of metal ions. 13C NMR relaxation-time data were used to determine the distances between the carbon atoms of the monoamide ligands and the actinides centers. The II, 2H, and 13C NMR spectra analysis of the solvent organic phases indicated malonamide dimer formation at low concentrations. However, at higher ligand concentrations, micelle formation was observed. NMR studies were also used to understand nitric acid extraction mechanisms. Before obtaining conformational information from 13C relaxation times, the stoichiometries of the... 

Methods such as nuclear magnetic resonance (NMR), electron spectroscopy for chemical analysis (ESCA), electron spin resonance (ESR), infrared (IR), and laser raman spectroscopy could be used in conjunction with rate studies to define mechanisms. Another alternative would be to use fast kinetic techniques such as pressure-jump relaxation, electric field pulse, or stopped flow (Chapter 4), where chemical kinetics are measured and mechanisms can be definitively established. 

Nuclear magnetic resonance (NMR) spectroscopy has been applied to elucidate metal-binding mechanisms to organic ligands mainly by two approaches by measuring the effects of metal complexation on either the relaxation times of H of water molecules solvating the metal cation or on the chemical shifts of NMR-active metal ions (e.g.. Cd, Al, and Pb) (e.g., Connors, 1987 Wilson, 1989 Macomber, 1998). Relatively few and sparse studies have been performed by NMR on metal-HS complexes. A comprehensive and updated review has been provided by Kmgery et al. (2001) on the various applications of NMR spectroscopy to the study of metal-HS interactions. 

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