Journal of Magnetic Resonance, Series A最新文献

Proton to phosphorus-31 cross polarization via adiabatic demagnetization in the rotating frame (ADRF-CP) has been used, in conjunction with a surface coil, to detect monohydrogen phosphate (acid phosphate) ions in the presence of a large background of nonprotonated phosphate (orthophosphate) ions in porcine bone and synthetic calcium phosphates. Transient oscillations were observed in the transfer of polarization between the proton dipolar and phosphorus Zeeman nuclear-spin reservoirs at short times after the initiation of thermal contact. The oscillations were observed in all samples, including bone. Orthophosphate suppression was achieved by detecting the signal when the orthophosphate oscillation was passing through zero, and by adjusting the phosphorus RF field to achieve optimal cross polarization with the proton local fields of the acid phosphate ions. ADRF-CP techniques deposit less RF power than traditional spin-lock CP techniques, and are hence compatible within vivoapplication. As the ratio of the protonated to nonprotonated phosphate can be used as a marker for bone-mineral maturity, ADRF-CP spectroscopy creates the possibility of characterizing bone-mineral dynamicsin vivoby solid-state NMR.

A new strategy is described for analyzing the NMR spectra of molecules dissolved in liquid-crystalline phases. The multiple-quantum spectra which involve a change in the magnetic quantum numbermbyN− 1, whereNis the maximum possible, are recorded and analyzed automatically starting from a parameter set having all the spectral parameters set to zero. The results of such an analysis are then used as the starting parameters in an analysis of the single-quantum spectrum using an automatic analysis procedure. The method is illustrated by analyses of the proton spectra of bromobenzene (five spins) and ethynylbenzene and naphthaquinone (six spins) dissolved in nematic solvents.

The microwave-induced delayed phosphorescence (MIDP) responses of several molecules in their excited triplet state are analyzed globally by least-squares minimization of the residuals, χ². The analysis considers not only the initial amplitudes of the MIDP responses which has been customary in the past, but also all data points (up to about 20,000) contained in a data set consisting of all three zero-field transitions and a range of delay times. It is shown that global fitting of the datacannotbe carried out successfully without including both sublevel decay to the ground singlet state and spin–lattice relaxation (SLR). Thus a complete data set containing at least two of the zero-field transitions when analyzed globally yields both sublevel decay constants and the SLR rate constants. Results are reported for tryptophan (Trp), 5-hydroxytryptophan (5-Htrp), the anion of 5-Htrp obtained by ionization of the hydroxyl proton, and 7-azatryptophan (7-Atrp). It is found that theT_zsublevel (zis the out-of-plane axis) decays principally by SLR even at about 1.2 K;k_z= 0 within experimental error (≈±0.01 s⁻¹) except for 5-Htrp anion, where it is measurable and is the smallest of thekvalues. For each triplet state, except for 5-Htrp anion, SLR is least efficient between theT_xandT_ysublevels. This relaxation process requires an angular momentum component alongz. Since thezcomponent of spin–orbit coupling is the weakest one in³(π, π*) states, the van Vleck relaxation process is suggested for Trp, 5-Htrp, and 7-Atrp. For the 5-Htrp anion, the most efficient SLR occurs betweenT_xandT_y, suggesting a different mechanism, possibly direct phonon modulation of the zero-field splittings.

The dependence of the NMR signal from turbulent flow in echo-planar images on flow rate has been investigated for pipe flow, for Reynolds’ numbers up to 6300. The short imaging time possible using echo-planar images essentially “freezes” the random motion of turbulent fluids and permits the study of individual eddies and intermittency. By use of appropriate approximations, the magnitude signal can be related to the fluid velocity shear in one direction. Sequential images of pipe flow have been analyzed to characterize the temporal and spatial characteristics of pipe flow at, below, and above the onset of turbulence. These experiments show that echo-planar imaging may have unique advantages for the spatial mapping and temporal characterization of some turbulent flows.

A pulse-sequence optimization procedure that uses a gradient-search method with analytical derivatives is demonstrated. It can be implemented for arbitrarily complex spin-system Hamiltonians and arbitrary pulse sequences. The computational method presented is time-efficient and more accurate than difference methods. It is applied to the optimization of a pulse sequence proposed by K. V. Schenker, D. Suter, and A. Pines [J. Magn. Reson.73, 99 (1987)] for deuterium decoupling in oriented phases. The optimized sequence is tested experimentally on pentadeuterobenzene dissolved in nematic phase.

A new model is presented for nuclear-spin relaxation in paramagnetic transition metal complexes in solution, allowing the electron-spin relaxation to be outside the Redfield limit. The novel feature is that the transient zero-field splitting (ZFS), modulated by distortions of the solvation shell, is allowed to be of rhombic rather than cylindrical symmetry. The model, which assumes that the static ZFS is absent, is applicable to aqueous solutions of transition metal ions. The magnetic-field dependence of the proton spin–lattice relaxation rate enhancement in aqueous solution has been investigated, and calculations are presented for anS= 1 system such as Ni²⁺(aq), using different degrees of rhombicity of the ZFS and different motional conditions (Redfield limit and slow-motion regime). The new model is also applied to fit the previously reported data for the field dependence of proton relaxation in aqueous solution of Ni(ClO₄)₂at low pH [J. Kowalewski, T. Larsson, and P.-O. Westlund,J. Magn. Reson.74, 56 (1987)]. The inclusion of rhombicity, motivated by recent theoretical work, provides a model which performs as well as the earlierad hocmodel.

The diffusion of water in industrial-grade sodium zeolite 4A powder beds has been visualized using broad-line gradient-echo magnetic-resonance imaging. Profiles of the equilibration of hydrated beds contacting dehydrated beds have been compared with hydrated beds contacting beds hydrated with D₂O in order to compare hydration-gradient-driven water-transport diffusion with self-diffusion. Although the equilibration rates are very different in the two cases, they are both in agreement with a model of coupled intraparticle liquid and interparticle vapor diffusion extended to incorporate the tracer experiments. The extended model is further tested by studies at elevated temperature, reduced hydration levels, and in ammonium-substituted material.