Journal of Magnetic Resonance, Series A最新文献

Organic solids have extensive proton–proton dipolar interactions, and their¹H NMR linewidths are very large even with magic-angle spinning at moderate speeds. Recently it has been shown that substantial narrowing of the proton linewidths of organic solids can be achieved by using single-pulse excitation with acquisition delay or spin echo [S. Ding and C. A. McDowell,J. Magn. Reson. A111, 212 (1994); 115, 141 (1995); 117, 171 (1995)]. This interesting line-narrowing phenomenon has been further examined through the study of several amino acids, their deuterated analogs, and some aromatic compounds. The results confirm that narrow proton peaks are observed with long acquisition delay, and the peaks appear in the appropriate chemical-shift ranges for organic protons (0–10 ppm with respect to tetramethylsilane). However, except for some special cases, the observed peaks cannot be assigned to individual types of protons based on chemical-shift considerations only. To explore the reason for the line narrowing, the effect of acquisition delay on the¹⁹F linewidth of CaF₂was also studied and compared with that on the¹H linewidths of organic solids. It is suggested that the broad proton peak in an organic solid is a superposition of numerous transitions. These transitions have different linewidths, and the narrow peaks in the spectrum remain observable with long acquisition delays.

Proton NMR longitudinal and transverse relaxation times of water-saturated powder packs have been measured. The powders were a series of synthetic calcium silicates with known concentrations of iron or manganese paramagnetic ions. The rate of water proton relaxation has been found to be linearly proportional to the concentration of paramagnetic ion. The constant of proportionality is used to determine the electron relaxation time of ions at the fluid–solid interface. A substantial relaxivity is found in the absence of paramagnetic ions. Thus the oxide surface itself is an unexpectedly good relaxer of fluid-borne nuclear spins. The results answer some long-standing questions connected with the NMR properties of fluid-saturated sedimentary rocks.

The theory of ESR hyperfine-structure exchange narrowing is developed. The kinetic equations describing the nonequilibrium magnetization dynamics of hyperfine components of ESR spectra are derived by using the nonequilibrium statistical-operator method. Numerical solution of these general equations is applied to the simulation of the experimental spectra of (Cd, Mn)Te crystals with a Mn²⁺ion content of between 0.05 and 0.5%. The hyperfine-splitting constant, homogeneous ESR linewidth, and mean-square value of the exchange fluctuation frequency are obtained from the fitting of the spectra for different concentrations of manganese ions. The concentration dependence of the latter quantity is discussed in relation to the spatial dependence of magnetic-ion exchange interaction and the gap value in the band structure of semimagnetic semiconductors.

The intensities of the Hahn spin echoes produced by a series of 90° pulses applied to a sample in large static field gradients are calculated. Static gradients are important because they can be very much larger than pulsed field gradients and echoes can be collected without recovery time problems. One application is for imaging using the stray field of a magnet (STRAFI). The high strength of the gradients in the stray field allows the imaging of solids and other systems with very broad lines. In an imaging pulse sequence consisting of pulses with phases 90^°_x—(90^°_y)_n, the echoes (calculated in the absence of spin relaxation) have relative intensities of 1:[formula]:[formula]:1$\text{1}\text{8}$:1$\text{1}\text{8}$, etc., whereas a 90^°_x—(90^°_x)_npulse sequence produces echoes which have relative intensities of 1:$\text{1}\text{2}$:−$\text{1}\text{2}$:−$\text{3}\text{8}$:$\text{3}\text{8}$, etc. A simple way of calculating these intensities is presented, using an irreducible tensor approach that has been termed the superspin formalism. These numbers are for infinitely short pulses on resonance and without relaxation or diffusion, but the extension to real systems is also discussed. In particular, it is shown that the attenuation down the echo train is due to a varying contribution from bothT₁andT₂processes, as well as diffusion. With longT₁’s and slow diffusion, the decay of the echoes approximatesT₂.

A new pulsed-field-gradient spin-echo NMR spectrometer is reported which is specially designed for the measurement of slow and anisotropic self-diffusion by providing an intense and homogeneous field gradient of 12 T m⁻¹by a quadrupole coil and a high-quality current stabilizer, and also providing a mechanism by which both the sample cell and the quadrupole coil are rotated from the axis of the external field. Diffusion anisotropy is measured by a combination of coil rotation and magic-angle orientation of the sample. The theoretical basis for the method of measuring diffusion anisotropy, instrumentation details, and several sets of test data are given. The experimental results are reported for the rotation pattern of the diffusion coefficient of a smectic Ad liquid crystal of 4-octyloxy-N-(4-cyanobenzylidene)aniline. The component parallel to the layer normal is shown to be 2.1 times larger than the component within the layer.