Spin-echo NMR Technique Research Articles

NMR measurements on obliquely evaporated Co-Cr films

The distribution of the hyperfine fields or the resonance frequencies in metals and alloys obtained by NMR measurements have been known for a long time. Recently, new experimental data have been published about thin films for studying their chemical inhomogeneities. An example is the study on sputtered and evaporated Co-Cr layers. In this paper we report on the compositional distribution of co-evaporated Co-Cr films by using the Co spin-echo NMR technique. For comparison single source evaporated samples of Co-Cr and pure Co as well as two alloyed ribbons (“bulk” samples) have also been measured. Based on the NMR results the local Cr concentration of the ferromagnetic and less ferromagnetic regions are determined. In comparison the data from the co-evaporated films, even at low substrate temperature, have clearly shown the presence of a process-induced compositional separation. This is in qualitative agreement with the magnetic properties of the samples.

Pulsed-gradient spin-echo NMR measurements of the diffusion coefficients of ketones in poly(methyl methacrylate)

The self-diffusion coefficients of acetone (ACT), 2-butanone (MEK), 2-pentanone (MPK), 2-hexanone (MBK), 3-methyl-2-butanone (MiPK), 4-methyl-2-pentanone (MiBK), and 3,3-dimethyl-2-butanone (MtBK), mixed in various proportions with poly(methyl methacrylate) (PMMA; MW ca. 60 000) have been determined by the use of the pulsed-gradient spin-echo NMR technique. The self-diffusion coefficients of the neat solvents decrease in the order ACT>MEK>MPK>MiPK>MBK>MiBK>MtBK, according to both their increasing overall molecular length and geometric cross section

Dipolar 31P NMR spectroscopy of crystalline inorganic phosphorus compounds

The ability of the 90°− t 1−180° pulse sequence to produce accurate dipole-dipole coupling information in solids is investigated. To this end, the experimental 31P spin echo decays are measured for eighteen crystalline phosphides and phosphorus chalcogenides and compared with simulations, based on the known internuclear distances in these compounds. The experimental results are generally found accurate in compounds where the dominant contribution to the dipole-dipole coupling arises from nuclei in structurally inequivalent sites with large chemical shift anisotropies. For this situation, the quantum mechanical “flip-flop” term in the dipolar Hamiltonian is suppressed and the dipole-dipole coupling is entirely heteronuclear in character. All of those compounds that do not obey this condition show accelerated spin echo decays due to a fractional contribution of the flip-flop term and possibly incomplete refocusing of chemical shift terms on the time scale of the experiment. The results confirm on an empirical basis that the spin echo NMR technique can provide accurate dipole-dipole coupling information (and thus distance distributions) in disordered solids and glasses.

The temperature and density dependence of the self-diffusion coefficient ofn-hexadecane

Self-diffusion coefficients for n-hexadecane have been measured by the NMR spin-echo technique and are reported for the temperature range 25–75°C at pressures up to 300 MPa. The results have been used to test a recently described correlation scheme for self-diffusion coefficients of n-alkanes.

Diffraction-like effects in NMR diffusion studies of fluids in porous solids

THE transport of fluids in porous media is of importance in a wide range of areas, such as oil recovery, heterogeneous catalysis and biological perfusion. The pulsed gradient spin-echo (PGSE) NMR technique has been used for many years to characterize diffusion and flow in such systems1–3. The analogy between NMR measurements in a field gradient and diffraction has been pointed out in the context of NMR imaging4 and, more recently, diffraction-like effects in the PGSE experiment have been discussed for diffusion in both impermeable5 and connected6 structures. The gradient pulse area plays the role of a wavevector, q, which can probe the structure in which the fluid diffuses. Here we report experimental confirmation of these predicted effects from proton NMR studies of a water-saturated, orientationally disordered, loosely packed array of monodisperse polystyrene spheres. The PGSE-NMR experiments may thus be used to provide an indirect, averaged image of the internal structure of porous solids at a resolution higher than that achievable with conventional NMR imaging. This is particularly advantageous for measurements on large samples, as the resolution available with the PGSE method depends only on the available range of gradient pulse amplitude and duration and is unconstrained by the factors determining resolution in conventional NMR imaging.

Solvent Self Diffusion Coefficients and 109Ag NMR Chemical Shifts in Silver Nitrate/Acetonitrile/Water Mixtures – Correlation with Macroscopic Transport Properties –

AbstractWe used the 1H NMR spin echo technique to determine the self diffusion coefficients of acetonitrile and of water in acetonitrile/water/silver nitrate mixtures. Both diffusion coefficients decrease strongly with increasing salt concentration. By comparison with earlier determined electric conductivity and viscosity data we find, that the ratio of molar conductivity and water self diffusion coefficient is constant, as well as the product of the viscosity and the acetonitrile self diffusion coefficient. The 109Ag NMR chemical shifts in these mixtures show, that silver ions are only solvated by acetonitrile.

The temperature and density dependences of the self-diffusion coefficient and the shear viscosity of liquid trichloromethane

Self-diffusion coefficients for trichloromethane are reported for the temperature range 5–75°C at pressures up to 400 MPa, having been measured by both diaphragm cell and NMR spin-echo techniques. Shear viscosities, determined with a falling-body viscometer, are reported in the range 5–50°C and up to 300 MPa. The results are analysed with the aid of models derived from computer simulation molecular dynamics data.

Hyperfine Fields at51V in Heusler Alloys Co2T1-xVxGa (T=Ti, Cr, Mn, Fe) and Estimation of Magnetic Moments of the Constituent Atoms

The hyperfine fields H hf in the ferromagnetic alloys Co 2 T 1- x V x Ga ( T =Ti, Cr, Mn, Fe) were measured by NMR spin-echo technique at 4.2 K as a function of x . H hf (V) on V impurity at b sites ( x ≃0) was determined to be -41, -40±2, -15 and -38±3 kOe for T =Ti, Cr, Mn and Fe, respectively. We estimated from these values, assuming V to be non magnetic, the magnetic moments of the atoms in Co 2 CrGa and Co 2 FeGa with the use of the measured saturation moments. It was found that for the two alloys the same moments were obtained also from H hf (Mn) on Mn impurity instead of V by assuming that the magnetic moment of Mn impurity in these alloys is the same as the Mn moment in Co 2 MnGa. The estimated moments of Cr and Fe were found to be consistent with H hf (Co).

Temperature and Pressure Dependence of Self Diffusion in Octamethylcyclotetrasiloxane and Hexamethylcyclotrisilazane

Abstract The pressure dependence of the self diffusion coefficient D for octamethylcyclotetrasiloxane and hexamethylcyclotrisilazane has been determined by the NMR spin echo technique with pulsed magnetic field gradients at pressures up to 200 MPa and at temperatures between 490 K and 290 K. The data extend partially into the deeply supercooled range. The isobaric temperature dependence of these data is quantitatively described by the empirical Vogel-Fulcher-Tammann equation. For both substances the melting pressure curves were determined in addition.

Rapid determination of water droplet size distributions by PFG-NMR

In 1972 Packer and Rees described a method for the determination of droplet size distributions in emulsions using the pulsed field gradient (PFG) NMR spin—echo technique. This paper describes an improved method, based on this earlier work, which is directed at the routine determination of water droplet size distributions of water-in-oil emulsions. The improvement is based on the recognition that a set of echo attenuation values ( R) as a function of the field gradient pulse width, obtained under conditions where R is independent of the time allowed for diffusion, contains all the necessary information on the water droplet size distribution. The parameters of a log-normal distribution of water droplet sizes are easily obtained from a small number of measurements which are then compared with a matrix of sets of theoretical echo attenuations, representing different combinations of these parameters, using a simple computer program. The method is described and examples of results are shown. The effect of temperature on the measurements is discussed.

A pulsed field gradient spin-echo nuclear magnetic resonance study of self-diffusion in isotropic micellar solutions containing potassium n-alkanoates and tetramethylsilane

Using the pulsed field gradient spin-echo proton NMR technique, self-diffusion coefficients have been determined for isotropic solutions of potassium n-octanoate and potassium n-dodecanoate/tetramethylsilane in deuterium oxide at 22°C. The results suggest unrestricted translational diffusion associated with dynamic equilibria among monomers and micalles leading to changes in the size of the aggregates with an increase in surfactant concentration. From the diffusion data, rough estimates have been made of the mean aggregation number ( n ) at different surfactant concentrations. Small, perhaps irregular micelles, with n values of 3–19 for potassium n-octanoate and larger, more spherical, micelles with up to at least ca. 55 monomers per micelle for the more hydrophobic potassium n-dodecanoate/TMS in 2H 2O system were estimated. Evidence is presented for the solubilization of TMS as a pool in the micelle core thereby inducing an increase in the micelle size and n along with a lower self-diffusion coefficient. Some indication of the size of the TMS pool, which increases with potassium n-dodecanoate concentration above the c.m.c., is afforded from the micelle self-diffusion data. Geometric and dynamic factors are considered which lead to the interpretation of the experimental diffusion parameters in terms of the most appropriate model of micellization.

Self-diffusion of toluene in polystyrene solutions

Abstract : The pulsed-gradient spin-echo (PGSE) NMR technique was used to acquire a broad range of solvent self-diffusion coefficients for the toluene/ polystyrene (MW 270,000) system. Self-diffusion coefficients were determined for samples having polymer weight-fractions ranging from 0.04 to 0.90 over temperature range of 25-115 C. Several different approaches to interpretation of the diffusion data have been considered. The temperature dependence of the data exhibits Arrhenius behavior with energies of activation that increase with concentration from 2.6 to 16.2 kcal/mole. Several theories based on the assumption that the solvent diffusion is slowed by the polymer obstructing its diffusion path are also used to interpret this data. Our results support the hypothesis th the normalized diffusion coefficient is relatively independent of thermodynamic parameters. The data are in qualitative agreement with a standard curve for normalized diffusion coefficients. This approach shows potential as a tool for predicting diffusion rates over a wide concentration range for a variety of polymer solvent systems. The final approach used to interpret this data is that of the free volume theory proposed by Vrentas and Duda. The agreement between the free volume theory and experimental data is improved considerably if two of the needed parameters are adjusted to give optimum fits to the experimental diffusion data.

ChemInform Abstract: Spin Echo Self‐Diffusion Measurements in Molten Salts.

AbstractThe NMR spin echo technique using pulsed gradients is applied for the first time to the measurement of self‐diffusion coefficients in molten salts.

An NMR spin-echo study of self-diffusion in xenon

An experimental investigation has been made of the self-diffusion coefficient in xenon, using the NMR spin-echo technique. This experiment in xenon is the first systematic investigation of self-diffusion in a monatomic gas over a wide range of density and temperature. The data have been obtained using an NMR high-pressure probe in a superconducting magnet. The experimental results are compared with the results of computer simulations on systems of hard spheres and on systems of Lennard-Jones molecules.

ChemInform Abstract: Spin Echo NMR of Cobalt Zeolite Catalysts: Control of Particle Size and Structure.

AbstractExperiments carried out at low radio frequency power on 59Co ferromagnetic particles with zero magnetic field clearly show the spin echo NMR (SENMR) technique to be especially useful in the study of structures and sizes of metal particles generated in zeolite catalysts.

Nonuniform local magnetism in Al-Mn quasicrystals.

The distribution of local magnetism in Al-Mn quasicrystal alloys has been studied by NMR spin-echo techniques. The $^{55}\mathrm{Mn}$ resonance is found to be close to the position observed in non-magnetic orthorhombic ${\mathrm{Al}}_{6}$Mn and $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Al}}_{73}{\mathrm{Si}}_{10}{\mathrm{Mn}}_{17}$ despite large paramagnetic susceptibilities in the quasicrystals. Analysis of spectral intensities shows that the paramagnetism is due to a class of Mn sites having local environments which do not occur in the crystals.

NMR spin-echo study of self-diffusion in xenon and ethene

The NMR spin-echo technique has been used in an experimental study of the density dependence of the self-diffusion coefficient in gases and liquids. The experiment in xenon is the first systematic investigation of self-diffusion in a monatomic gas over a wide range of densities and temperatures. By using an improved NMR probe we have also succeeded in extending earlier high density measurements in ethene down to 5 Amagat. Comparison is made with the molecular dynamic results found by several authors.

NMR Study of Size Effect in Ferromagnetic Ni Metal

The hyperfine field ( H h f ) distribution and the nuclear spin-lattice relaxation time ( T 1 ) of 61 Ni in metallic Ni fine particle of 30∼800 A in average size were measured in the temperature range 4.2 K∼0.4 K by NMR spin echo technique. Below 200 A H h f has a wide distribution toward the lower field side indicating a predominant decrease of the magnetization and/or increasing of s-d mixing near the surface region of the particles. T 1 decreases rapidly with decreasing size keeping the relation of T 1 T =const. T 1 increases with external field. The relaxation behavior is qualitatively exaplained by the Weger mechanism through the indirect interaction between nuclear spin and conduction electrons via the spin wave where the spin wave stiffness constant decreases with decreasing size.

Self-diffusion in compressed supercritical toluene-d8

The self-diffusion coefficients of supercritical toluene-d/sub 8/ were measured by using the NMR spin-echo technique for temperatures from 300 to 450/sup 0/C at constant pressures of 100, 500, and 1000 bar. The results at 100 bar are compared to the predictions for toluene diffusion by using various empirical correlation methods for estimation of the diffusion coefficients in the supercritical region. 26 references, 2 figures, 2 tables.

Li self-diffusion in pure and doped β-LiAl using pulsed-field gradient NMR

The self-diffusion coefficient of 7Li in pure and doped β-LiAl has been measured over the temperature range 297–478 K using a pulsed-field-gradient NMR spin-echo technique. The lithium self diffusion coefficients in these alloys show Arrhenius behavior, D (Li) = D 0 (Li) exp( −(E) k BT ) , within this observed temperature range. The diffusion constant D 0 (Li) and the associated activation energy 〈 E〉 decrease with increasing lithium content. The results can be understood if two types of vacancy point defects are considered: vacancies partially bound to, or associated with, immobile antistructure defects, and vacancies which otherwise are free. A vacancy diffusion mechanism based on two mean jump times for these two types of vacancies yields a quantitative description of the Li diffusion process. The diffusion coefficients of Li in LiAl alloys doped with Ag and In have also been determined, and it was found that the results are closely related to the corresponding defect states.