Total Spin-lattice Relaxation Research Articles

The characterization of asphaltene behavior in some aromatic solvents by dynamic nuclear polarization technique

Dynamic nuclear polarization (DNP) studies of asphaltene/some hydrocarbons solutions were performed at 1.53mT. The low field EPR spectrum of Asphaltene/Mesitylene sample was recorded. The DNP parameters such as enhancement factor, nuclear−electron coupling constant, saturation factor, leakage factor, the parameter for the relative importance of scalar and translational dipolar interaction and the total spin-lattice relaxation rate were determined. The effect of irradiation with neutrons from the Ra-Be source to the DNP parameters was investigated. Dissolved molecular oxygen effect on the enhancement factor for each sample was examined experimentally. We also make some interpretations depending on the results obtained for the interactions between nuclei of the solvent and electrons delocalized on the asphaltene.

Water and Backbone Dynamics in a Hydrated Protein

Rotational immobilization of proteins permits characterization of the internal peptide and water molecule dynamics by magnetic relaxation dispersion spectroscopy. Using different experimental approaches, we have extended measurements of the magnetic field dependence of the proton-spin-lattice-relaxation rate by one decade from 0.01 to 300 MHz for 1H and showed that the underlying dynamics driving the protein 1H spin-lattice relaxation is preserved over 4.5 decades in frequency. This extension is critical to understanding the role of 1H 2O in the total proton-spin-relaxation process. The fact that the protein-proton-relaxation-dispersion profile is a power law in frequency with constant coefficient and exponent over nearly 5 decades indicates that the characteristics of the native protein structural fluctuations that cause proton nuclear spin-lattice relaxation are remarkably constant over this wide frequency and length-scale interval. Comparison of protein-proton-spin-lattice-relaxation rate constants in protein gels equilibrated with 2H 2O rather than 1H 2O shows that water protons make an important contribution to the total spin-lattice relaxation in the middle of this frequency range for hydrated proteins because of water molecule dynamics in the time range of tens of ns. This water contribution is with the motion of relatively rare, long-lived, and perhaps buried water molecules constrained by the confinement. The presence of water molecule reorientational dynamics in the tens of ns range that are sufficient to affect the spin-lattice relaxation driven by 1H dipole-dipole fluctuations should make the local dielectric properties in the protein frequency dependent in a regime relevant to catalytically important kinetic barriers to conformational rearrangements.

Nuclear Spin–Lattice Relaxation in Transition Metal Alloys and Intermetallics

General analytical formulae describing the various contributions to the total spin–lattice relaxation rate in metallic materials (single crystals or powders) are presented. The formulae are obtained in the tight-binding approximation and can be easily used for any point-group symmetry. The cubic (Oh), hexagonal (D3h), and tetragonal (D2d) symmetries are considered in detail.

Lineshapes of a Spin-12 Nucleus with Scalar Coupling to a Quadrupolar Nucleus Subject to Random Field Relaxation

Abstract The lineshapes in solution of a spin- 1 2 nucleus with scalar coupling to one quadrupolar nucleus which is subject to random-field relaxation have been calculated using a relaxation matrix treatment. For the case of the quadrupolar nucleus havingI= 1, a lineshape expression has been derived. ForI> 1, lineshapes have been calculated numerically. As an example of the use of these calculations, the13C NMR spectra of the hexacyanocobaltate(III) anion in aqueous solution were recorded at various temperatures and these spectra were simulated invoking contributions from both quadrupolar relaxation and spin–rotation relaxation to the total spin–lattice relaxation rate of the59Co nucleus. At 360 K, spin–rotation relaxation constitutes 28% of the total59Co nuclear relaxation rate.

13C spin-lattice relaxation in benzophenone and its isotopomers

In order to analyze deuterium effects on 13C spin-lattice relaxation time (T 1) in benzophenone (BPN), five BPN isotopomers were prepared and their 13C NMR spectra recorded. In BPN all the phenyl carbon atoms, except the quarternary one, are predominantly relaxed by the dipole-dipole mechanism due to C-H dipole-dipole relaxation. Other contributions to the total spin-lattice relaxation times of individual carbon atoms are discussed as well. Deuteration affects also the magnitude of spin-lattice relaxation times. An increase in T 1 by cca. 550% at the ortho (C 2) and para (C 4) carbon atoms in perdeuterated BPN has been observed.

Nuclear spin-lattice relaxation in transition metal compounds with local tetragonal symmetry

We report on the various contributions to the total spin-lattice relaxation rate in metallic materials with local tetragonal symmetry. The analytical formulae are given in the tight-binding approximation. The calculations show the relation between various partial electron densities of states and corresponding contributions to the relaxation rates. The presented formulae can be used to compare theoretically calculated electron band structure parameters with those obtained from NMR spin-lattice relaxation time measurements.

Phosphorus 31P Spin–lattice relaxation times as a tool for the determination of the structure of 1,3‐dithianes with P‐containing substituents at the anomeric carbon atom

AbstractRelaxation times, T, due to the dipolar mechanism of relaxation of 31P nuclei were determined for eight pairs of 1,3‐dithianes with P‐containing substituents at the anomeric carbon atom. The separation of dipole–dipole from the total spin–lattice relaxation could be applied for the unambiguous assignment of configuration of the compounds under investigation. The T relaxation times are smaller for compounds with phosphorus located axially (cis). This observation is discussed in terms of internal rotation around the C‐2 bond and the distances between the phosphorus and axial H‐4,6 protons.

Nuclear-magnetic-resonance evidence for a change in local electronic structure accompanying hydrogen pairing in yttrium-hydrogen solid solutions.

Measurements of the proton spin-lattice relaxation (SLR) rate in yttrium-hydrogen solid solutions ({alpha}-YH{sub {ital x}}) show that the hydrogen-pairing phenomenon, which occurs in the temperature range 170--300 K, is accompanied by a continuous, strong (22%) decrease in the Korringa parameter, {ital R}{sub 1{ital e}}/{ital T}, with decreasing temperature, where {ital R}{sub 1{ital e}} is the conduction-electron contribution to the total SLR rate. This shows that a substantial (37%) change occurs in the product of the hyperfine field at the proton site and the local {ital d}-band density of states at the Fermi level, {ital N}{sub {ital d}}({ital E}{sub {ital F}}), when hydrogens become paired. Similar behavior has not been detected in the corresponding Sc-H and Lu-H systems in which pairing also occurs.

Anisotropic molecular reorientation of liquid benzene revisited. A study using 13C magnetic relaxation through chemical shift anisotropy and spin rotation

We report on 13C magnetic relaxation in liquid benzene (C6H6 and C6D6) at 293 K at resonance frequencies of 22.63 and 100.6 MHz. The total spin–lattice relaxation rates are separated into contributions from 13C–1H and 13C–2D dipolar, spin–rotation (SR), and chemical shift anisotropy (CSA) interactions. As the principal axis of the 13C chemical shift tensor is collinear with the sixfold axis of benzene, CSA relaxation provides information on the tumbling motion of benzene, hitherto not available from other NMR experiments. The resulting correlation time τ⊥ = 1.63 ps at 20 °C indicates a faster tumbling motion, and hence, a smaller anisotropy of benzene reorientation than usually quoted in the literature. This result is qualitatively confirmed by an analysis of 13C relaxation by SR interaction, and is in fair agreement with recent results obtained in Raman line shape measurements. It is, however, at variance with the figures deduced from Rayleigh scattering experiments. The activation energy for the tumbling motion is found to be 9.8 kJ mol−1, which is markedly larger than the activation energy of 7.7 kJ mol−1 obtained from 2D relaxation data for the motion of in-plane axes.

Applications of the intermolecular nuclear Overhauser effect. I. Determination of differential solvent effects: the hydration of pyridine

The intermolecular relaxation rates of the pyridine-water system have been obtained by the measurement of the total spin lattice relaxation rate and the intermolecular nuclear Overhauser effect between pyridine and water, for each pyridine proton. The advantages of this method for the determination of the intermolecular relaxation rates are discussed, and the method is compared with alternative methods. The results indicate that there is a varying degree of hydration about the pyridine molecule, with the nitrogen being the preferred site of water interaction. It is necessary to interpret the results in terms of solute-solute as well as solute-solvent interactions. A model is proposed which takes account of both types of interaction, and is considered in terms of previously proposed models of pyridine-water interactions.

有機セレン化学において｀７７´ＳｅＮＭＲから得られる情報

Recent development in 77Se NMR spectroscopy is reviewed and the information available therefrom is critically discussed. After brief discussion on the technical problems inherent in the measurement, an NMR chart is given which illustrates relationship between structures and chemical shifts having the range as wide as 2, 000 ppm. Spin-spin coupling constants pertaining to 77Se can be informative of stereochemistry. Contribution of dipolar interaction between nuclear spins to the total spin-lattice relaxation is negligible in 77Se NMR. Spin rotation and/or chemical shift anisotropy are shown to be important here. Recent high research activities in organoselenium chemistry will be strongly aided by this new methodology.

The shape of the NMR line for polysulphide oligomer vulcanizates at low temperatures

Broad band NMR is used in this work to study the shape of the lines produced by polysulphide vulcanizates made with various curing agents. The amount and type of the added inorganic oxidant has been found to affect the shape and width of the NMR absorption line at low temperatures. The experimentally detected complex shape found on using Na 2Cr 2O 7 or MnO 2 as curing agents is probably explained by the presence of weakly reacting groups of atoms in the polymer. Paramagnetic Cr 3+ and Mn 2+ admixtures present in the curing agents have been found to affect the spin-lattice relaxation time, T 1, changes. The impact of these admixtures on the total spin-lattice relaxation time at low temperatures has been theoretically evaluated.

Spin–lattice relaxation in liquid ethylene chloride

The proton spin – lattice relaxation time of ethylene chloride in deuterated ethylene chloride was determined as a function of concentration and temperature over the temperature range 230 to 390 °K. It was found that the activation energy for the total spin–lattice relaxation as well as for the intra-molecular contribution to the total spin–lattice relaxation was 2.0 kcal/mole and was nearly equal to the activation energy for viscous flow, viz. 2.3 kcal/mole.

Proton NMR Relaxation Effects. Cross-Relaxation Processes in Pure Liquids

Spin–lattice relaxation in a liquid containing protons in two different molecular environments can proceed by three different routes. There is a route to relaxation associated with each of the environments resulting from interactions between protons within that same environment. The third route to relaxation results from interaction between protons in different environments and is called cross relaxation. In such a system, the total spin–lattice relaxation decay determined by pulsed NMR will be the sum of two exponentials. Three relaxation rate constants are derived from such data. When cross relaxation arises entirely from dipole–dipole interaction in the extreme narrowing limit, these rate constants may be associated with the three routes to relaxation. (The cross-relaxation rate can always be identified, regardless of the mechanism.) To illustrate such relaxation behavior and the methods for obtaining three independent relaxation rates, 1-phenylpropyne was studied as a function of temperature.