Relaxation measurements - interpretation

Another application of the knowledge of is to employ it for the interpretation of another relaxation measurement in the same system, an approach referred to as the dual spin probe teclmique. A rather old, but... 

NMR has proven to be a valuable tool for formation evaluation by well logging, downhole fluid analysis and laboratory rock characterization. It gives a direct measure of porosity as the response is only from the fluids in the pore space of the rock. The relaxation time distribution correlates with the pore size distribution. This correlation makes it possible to estimate permeability and irreducible water saturation. When more than one fluid is present in the rock, the fluids can be identified based on the difference in the fluid diffusivity in addition to relaxation times. Interpretation of NMR responses has been greatly advanced with the ability to display two distributions simultaneously. 

The dynamic RIS model of polymer chains is applied to the interpretation of nuclear magnetic relaxation measurements of local chain dynamics. According to the proposed model, the relaxation times Tlc and T1H may be related to the chemical structure of a specific polymer. 

Studies on the kinetic behaviour of nucleoside and nucleotide complexes are less common than those on structural aspects. This arises because of the rapid rates of the formation and dissociation reactions, requiring NMR or temperature-jump relaxation measurements. The number of species that can coexist in solution also hinders interpretation. The earlier kinetic studies have been reviewed by Frey and Stuehr.127 Two important biological reactions of the nucleotides are phosphoryl and nucleotidyl group transfers. Both reactions are catalytic nucleophilic reactions and they both require the presence of a divalent metal ion, in particular Mg2+. Consequently, one of the main interests has been in understanding the catalytic mechanism of the metal ion involvement. This has mainly involved studies on related non-enzymic reactions.128... 

Abstract We use Nuclear Magnetic Resonance relaxometry (i.e. the frequency variation of the NMR relaxation rates) of quadrupolar nucleus ( Na) and H Pulsed Gradient Spin Echo NMR to determine the mobility of the counterions and the water molecules within aqueous dispersions of clays. The local ordering of isotropic dilute clay dispersions is investigated by NMR relaxometry. In contrast, the NMR spectra of the quadrupolar nucleus and the anisotropy of the water self-diffusion tensor clearly exhibit the occurrence of nematic ordering in dense aqueous dispersions. Multi-scale numerical models exploiting molecular orbital quantum calculations, Grand Canonical Monte Carlo simulations, Molecular and Brownian Dynamics are used to interpret the measured water mobility and the ionic quadrupolar relaxation measurements. 

In composite systems, 2H NMR is particularly suited to investigate interfacial properties. Indeed, isolated nuclei are observed, which potentially allows spatially selective information to be obtained. It has been used to investigate polymer chain mobility at the polymer-filler interface, mainly in filled silicon (in particular PDMS) networks. The chain mobility differs considerably at the polymer-filler interface, and this may be interpreted in terms of an adsorbed polymer layer at the filler surface. T1 relaxation measurements allowed to determine the fraction of chain units involved in the adsorption layer, or equivalently, the thickness of the layer [75, 76, 77]. The molecular mobility and the thickness of the adsorption layer are very sensitive to the type of filler surface [78]. 

Dielectric relaxation measurements on some amines (aniline, N,N-di-methylaniline, benzidine. ..) display two relaxation times a longer one (of the order of 20—30.10-12 sec) attributed to overall molecular reorientation, and a shorter one (of the order of 1.10-12 sec) which may be interpreted as arising from an intramolecular process and has been attributed to nitrogen inversion 19-aa>, although this attribution is not unequivocal (rotation about the N—C bond may also contribute to the observed relaxation). 

Figure 16 shows relationships between the number of introduced side chains and relaxation rigidity (G,) at 900 s for carboxymethylated wood binding various metal ions [341. Wood specimens were prepared from Japanese linden Tilia japonica Smik.). Carboxymethylation and the introduction of metal ions was the same procedure as mentioned in the previous section [32,33]. Stress relaxation measurements were carried out in an aqueous solution at 30°C. The relaxational property of carboxymethylated wood without metal ions is first discussed. For carboxymethylated wood (a broken line in Fig. 16), Gf (900) decreases with an increase in the number of introduced side chain. This rapid decrease appears to be caused by two factors. One is the effect of sodium hydroxide (NaOH). Young s modulus of wood treated with an aqueous solution of NaOH decreases remarkably under wet conditions, especially at concentrations above 10% NaOH [35]. The other factor is the electrostatic repulsion of ionized carboxymethyl groups in carboxymethylated wood, as mentioned in the above section [291. For example, conformation of polypeptide is influenced by the ionization of the side chains, and the structural change of the helix-coil transition has been interpreted as a reversible transformation. Theoretical treatment of the transformation has been reported to explain the mechanism [23-25, 36-43]. The conformation of component molecules in wood, however, cannot change markedly by ionization in comparison with soluble polyelectrolytes in water, because carboxymethylated wood is not dissolved in water. Only space among the main chains is expanded by the electrostatic repulsion due to negatively charged side chains. For these reasons, G (900) of carboxymethylated wood decreases with an increase in the number of introduced side chains.

We interpret these results by assuming that the high values for the heats of adsorption pertain to specifically active centers, such as lattice holes (7). Dielectric relaxation measurements (4) and KMR studies (2) substantiate such conclusions. 

Zawodzinski et al. [58] have reported NMR relaxation measurements on water in Nafion membranes. In contrast with proton NMR relaxation studies, which are difficult to interpret because of various inseparable contributions to the observed relaxation rates, a direct relationship often exists between the observed relaxation rate and rotational dynamics of the deuteron-bearing species. The time scale probed by such measurements is in the pico- to nanosecond range, and thus very short-range motions are probed. In a membrane equilibrated with saturated water vapor, a Ti on the order of 0.2s was observed. This relaxation rate for D2O in the membrane is only higher by a factor of two than that in liquid D2O, indicating a bulk water-like mobility within the pore at high membrane hydration levels. The relaxation rate increases (i.e., local water motion in the membrane becomes slower) as the water to ion-exchange site ratio decreases. 

A variety of publications are dealing with T1 relaxation measurements in combination with NOE experiments to investigate the reorientational dynamics of ionic liquids in solution (and sometimes also in the solid state) [17-22], The data can then be interpreted concerning inter-ionic interactions and phase transitions. 

It has recently become more widely appreciated that the presence of rotational diffusional anisotropy in proteins and other macromolecules can have a significant affect on the interpretation of NMR relaxation data in terms of molecular motion. Andrec et al. used a Bayesian statistical method for the detection and quantification of rotational diffusion anisotropy from NMR relaxation data. Sturz and Dolle examined the reorientational motion of toluene in neat liquid by using relaxation measurements. The relaxation rates were analyzed by rotational diffusion models. Chen et al measured self-diffusion coefficients for fluid hydrogen and fluid deuterium at pressures up to 200 MPa and in the temperature range 171-372 K by the spin echo method. The diffusion coefficients D were described by the rough sphere (RHS) model invoking the rotation translational coupling parameter A = 1. 

The superconducting fulleride (NH3) aK2C6o has been investigated using SQUID magnetometry and NMR in two differently doped samples. NMR relaxation measurements vahdate the interpretation of the spin susceptibility in terms of density of states and rule out the presence of strong antiferromagnetic correlations in the Fermi liquid. 

Other strategies for investigating protein folding have also emerged. Vug-meyster et al. interpret N Ri rho relaxation measurements and show that... 

One of the principal advantages of CPMAS experiments is that resolution in the solid state allows individual-carbon relaxation experiments to be performed. If a sufficient number of unique resonances exist, the results can be interpreted in terms of rigid-body and local motions (e.g., methyl rotation, segmental modes in polymers, etc.) (1,2). This presents a distinct advantage over the more common proton relaxation measurements, in which efficient spin diffusion usually results in averaging of relaxation behavior over the ensemble of protons to yield a single relaxation time for all protons. This makes interpretation of the data in terms of unique motions difficult. 

When the blends were examined using dielectric relaxation measurements which probe the dipole relaxation spectrum, values of P were found to be much lower (0.1-0.22). This was interpreted as indicating the development of heterogeneity at the molecular level caused probably by the crystalhzation of the PEEK component. 

The following discussion of solution equilibria has been divided arbitrarily into ionic and molecular sections, with that on ionic solutions sub-divided into ion-solvent and ion-ion interactions. Obviously there is overlap, and experimentally it is necessary to make measurements over a range of concentrations and extrapolate to infinite dilution, in order to determine the relative proportions of species and forces present. At the risk of appearing trite, it must be emphasised that it is highly desirable whenever possible to use relaxation measurements as well as spin-spin couplings when interpreting chemical shifts. 

Under conditions where no phase separation or percolation can occur, termination of the square pulse is followed by a double exponential decay (t / and T. y) of the birefringence. The forward and reverse relaxations are found to be symmetrical, i.e., t, = t, and An, = Art-i within experimental error [41]. In the faster process the induced dipoles of the droplets (individually and as constituents of clusters) rapidly collapse, the shape of the droplets reverts to spherical, and/or the droplets randomize in their orientation. If the field-free fast relaxation is interpreted as the ellipsoid-to-sphere structural relaxation of the droplets, the bending modulus k of the surfactant monolayer can be estimated from the measured t / [49]. Depending on the polydispersity assumed, the values found in the range k = 0.4-1.0 kT are consistent with those obtained from the static birefringence A o [7,9]. 

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