Journal of Magnetic Resonance, Series A最新文献

A new crossed-loop resonator (CLR) structure is described that uses two orthogonal resonators to isolate the EPR signal from the microwave source. Resonators of this type are usually referred to as bimodal, which is descriptive of cavity or distributed element resonators. However, it is more useful to think of the new resonator as two virtually independent lumped-element resonators that have a common sample volume where the loops of the two resonators meet orthogonally. The first resonator excites the spins, and the second resonator (or resonant section of the CLR) acts like an antenna that detects only the signal caused by the spin system. In this manner, the very phenomenom that is being studied is used to separate the desired EPR signal from the microwave source and performs the function of the circulator. The signal coupled into the second resonator is due to the spin system. The phase noise of the source is reflected from the first resonator back toward the source and is efficiently isolated from the second resonator and the EPR signal. This resonator structure eliminates the need for the circulator, simplifies the spectrometer circuit, virtually eliminates source phase noise in the detected signal, and allows dispersion spectra to be measured with the same high signal-to-noise (S/N) as absorption spectra. In addition, for pulse experiments, this resonator greatly decreases the dead time of the instrument and allows measurement of a portion of the signal previously inaccessible with other resonators. The CLR is rugged, easy to tune, does not have to be critically coupled, and is easily adjusted to maintain source isolation when the sample is changed.

The prevailing majority of EPR spectra in solid samples concern the randomly oriented paramagnetic particles in solids (polycrystalline samples, powders, glasses, ceramics, frozen solutions). In these cases it is necessary to provide numerical simulation or nonlinear optimization of the spin-Hamiltonian parameters to obtain the relevant physical results. The presence of various paramagnetic particles in the same sample often complicates the situation. This paper deals with the numerical solution to these problems. Illustrative examples of EPR spectroscopy of radiation-induced paramagnetic centers in glasses and EPR of Cu(II) as a spin label in the study of the thermal evolution of SiO₂gel preparation are also presented.

A³¹P and¹⁵N NMR study of cyclotriphosphazenes 1–4 and polyphosphazene 5 has been carried out including solvent and substituent effects.¹⁵N chemical shifts and³¹P,¹⁵N coupling constants are readily obtained through 1D and 2D³¹P,¹⁵N HMQC correlation spectroscopy on natural-abundance samples. This methodology combined with spectral simulation also yields¹⁵N-induced isotope shifts.³¹P and¹⁵N are both relatively insensitive to changes in solvent polarity. Chemical shifts are reasonably explained in terms of electronic density variations, and³¹P,¹⁵N couplings are rationalized by considering their absolute sign, which depends on the substituent bonded to phosphorus.¹⁵N proved to be more sensitive to structural modifications than³¹P. Therefore, the availability of¹⁵N NMR data on cyclotriphosphazenes can be considered a valuable tool in the design of model compounds for polyphosphazene synthesis.

Combined rotation and multiple-pulse experiments (CRAMPS) are analyzed for the conditions of fast magic-angle spinning, where interference effects between both averaging techniques must be considered. Based on this analysis, basic multiple-pulse sequences are adapted to these conditions. As a new aspect compared with conventional CRAMPS experiments, high-speed MAS also plays an important role in the averaging of residual dipolar contributions, that is, high-order correction terms of the dipolar Hamiltonian. The efficiency in doing this was investigated by sequences for scaling the dipolar Hamiltonian and by proper homonuclear decoupling sequences. The resolution of these preliminary experiments approaches that of conventional CRAMPS experiments, whereas the demands with respect to spectrometer hardware and tuning are much lower.

In this work, a novel approach to the usage of an autocorrelation function in order to improve signal-to-noise ratio (SNR) is presented. This method avoids the usual problems entailed by standard autocorrelation function-based approaches to nonstationary signals such as NMR signals. The Cadzow autocorrelation matrix approach to transient data is often not suitable for time-domain signal analysis; in fact, it does not maintain the Hankel structure of the prediction matrix, which is mandatory for many linear-prediction (LP) applications. The approach presented here conserves the Hankel structure of the prediction matrix and, moreover, does not change the frequency and linewidth parameters of the signal components. Furthermore, the proposed autocorrelation-like function permits a weighting of the individual components according to theirT₂decay constant. This property opens new possibilities for retrieving signal parameters by LP procedures. These new procedures are applied to simulated 2D signals and 1D NMR measurements of phosphorus metabolites in frog muscle.

A slice-selection method developed within the frame of the sequence forindirectmagicangle in therotatingframe (I-MARF) imaging of solids is proposed. The pulse sequence is based on the Lee–Goldburg sequence, adapted to imaging by a dedicated gradient setup, and it is left unchanged, except for the insertion of a dc “selection-plane magnetic field.” The selection-plane field makes the magic-angle coherent-averaging condition spatially varying and delimits the selected slice to only the sample portion where the residual dipolar coupling has been preserved.

Expressions for the effective diffusion times in multiple-gradient pulse diffusion measurements in liquid-saturated porous media are derived. This is necessary since the diffusion coefficient is time dependent. The results are needed for quantitative interpretation of the results of PFG diffusion measurements using the multiple-alternating-gradient schemes which are preferred for the majority of porous systems. The essence of the calculation and the results for “square” and “sine” pulse shapes have appeared previously, but without derivation. The details of this somewhat technical calculation may be needed by users.