Journal of Magnetic Resonance, Series A最新文献

The possibility of retrieving parameters for the combined effect of quadrupole coupling and small chemical-shielding anisotropies from single-crystal NMR of the central transition for half-integer quadrupolar nuclei has been successfully investigated.⁸⁷Rb single-crystal NMR of RbClO₄and Rb₂SO₄serve as two properly chosen and illustrative examples. Thereby, discrepancies between some of the⁸⁷Rb parameters recently reported for these two interactions in powders of RbClO₄and Rb₂SO₄, and determined from MAS and static powder NMR, have been clarified. The orientations of the crystallographic axes for the two rubidium crystals have been predicted by the NMR results and verified independently by XRD analysis.

NMR measurement of diffusion is a well-established tool for investigating the microstructure of heterogeneous materials. Since an applied magnetic-field gradient is an essential element of the pulse sequences usually used, any magnetic-field gradients present within the sample may interfere with the NMR measurement, in some cases making it impracticable. Here, a method that uses the internal gradients themselves to provide information about the heterogeneous structure, in materials where these gradients are strong, is described. The technique is demonstrated using a range of solvents in packs of different sizes of glass beads.

The velocity of water flowing through a Bentheimer sandstone core has been measured by NMR-imaging techniques. The localized pixel values of velocity indicate a random distribution centered around the mean value corresponding to Darcy's law. When the same flow state is repeated, the velocity map changes but the general characteristics of the velocity distribution remain unchanged. The random nature of the irreproducibility of the flow maps has led us to propose a stochastic theory of flow in porous rocks. The theory leads to a Gaussian velocity distribution which approximates well to the data. Also predicted is a linear relationship between flow variance and mean fluid flow through the rock, the Mansfield–Issa equation, originally proposed as an empirical relationship.

In a preceding paper [P. Mansfield and B. Issa,J. Magn. Reson. A122, 137–148 (1996)], a stochastic model of fluid flow in porous rocks based upon the experimental observation of water flow through a Bentheimer sandstone core was proposed. The flow maps were measured by NMR-imaging techniques. The stochastic theory led to a Gaussian velocity distribution with a mean value in accord with Darcy's law. Also predicted was a linear relationship between flow variance and mean fluid flow through rock, the Mansfield–Issa equation, originally proposed as an empirical relationship. In the present work a flow coupling mechanism between voxels is proposed. Examination of the flow coupling between isolated voxel pairs leads to a complementary explanation of the Gaussian velocity distribution, and also gives further details of the Mansfield–Issa equation. These details lead to a new expression for the connectivity, 〈C〉, between voxels with an experimental value of 〈C〉 = 5.64 × 10⁻⁹for Bentheimer sandstone.

Single-crystal rotational data of CW-EPR and two-pulse field-swept ESE collected at room and low temperatures lead to the spin-Hamiltonian parametersg_z= 2.2596,g_y= 2.0647,g_x= 2.0529,A_z= 151.2 × 10⁻⁴cm⁻¹,A_y= 2.5 × 10⁻⁴cm⁻¹, andA_x= 42.2 × 10⁻⁴cm⁻¹which are temperature independent. The parameters and Cu²⁺ion site in the crystal unit cell are very similar to those in the triglycine sulfate crystal. Electron-spin-echo measurements of the spin–lattice relaxation timeT₁in the temperature range 4.2–90 K gave identical results for inversion- and saturation-recovery methods, indicating a negligible spectral-diffusion contribution. TheT₁decreases from 275 μs at 15 K to 1.7 μs at 83 K and the temperature dependence is governed by the direct (one-phonon) process at low temperatures and two-phonon Raman process at higher temperatures. A new analytical form of theI₈transport integral of the Raman term is proposed. The best fit to the theoretical expression was found for the Debye temperature ϑ_D= 168 ± 3 K which is comparable to ϑ_D= 171 K calculated from the elastic constants of the crystal.