Journal of Magnetic Resonance, Series A最新文献

A new 1D NMR exchange experiment in the slow-motion regime of spinning solids, with chemically equivalent nuclei exhibiting quadrupole coupling or chemical-shift anisotropy, is proposed. It consists of the usual three-pulse sequence for 2D exchange spectroscopy,P1—t₁—P2—τ_m—P3—t—acquisition, but with the evolution time fixed at one-half a spinning period,t₁= T_R/2, and a mixing time equal to an integer multiple thereof, τ_m= GT_R. The magnetic polarizations associated with the various spinning sidebands are then polarized in alternate directions at the beginning of the mixing time. Dynamic processes during τ_mredistribute the polarizations, resulting in modified sideband patterns during the detection time,t. Experimental results are presented for carbon-13 and deuterium in dimethyl sulfone, which undergoes molecular reorientation in the solid state. The results are compared with simulations which include the effect of reorientation and longitudinal relaxation.

SIMPLTN, a computer program for thesimulation ofpulse andtwo-dimensionalNMR, is described and illustrated. The program is menu-driven and is designed to run as much like a spectrometer as possible. This approach allows a complete density-matrix calculation to be performed, yet still makes the program easy to use. SIMPLTN serves as a learning tool, and allows the design and testing of new pulse NMR methods. A spin system and a pulse program are defined, and then the program calculates an FID. The simulated data can then be transformed, inspected, and plotted using a variety of commercial NMR software packages. Systems of up to six strongly coupled spins, and almost any common liquid-state pulse program, can be simulated. SIMPLTN includes a batch mode in which simulations may be queued, and parameters may be systematically varied. This paper gives an overview of the program, some of the unique theoretical features of the program, and examples of its use.

Relaxation resulting from the modulation of dipolar interaction is commonly used for estimating distances in molecules in solutions. However, for most nuclei with spinI>$\text{1}\text{2}$ the single-quantum-transition relaxation by dipolar interaction is masked by quadrupolar relaxation. In the present study, it is shown that even in systems where single-quantum relaxation times are dominated by quadrupolar interaction, dipolar relaxation can be measured by following the −m[formula]mtransitions. This is demonstrated for⁷Li in the complex [Li–Kryptofix 211]⁺X⁻(X = Cl, Br) dissolved in glycerol at temperatures for which slow motion prevails and no¹H–⁷Li NOE can be observed. The relaxation times that are most important for the assessment of the dipolar interaction of⁷Li are −$\text{1}\text{2}$[formula]$\text{1}\text{2}$ and −[formula][formula][formula]and they are measured by multiple-quantum-filtration techniques. For estimating the quadrupolar interaction, the relaxation times of the populations and those of the transitions ±$\text{1}\text{2}$[formula]±[formula]were measured. The longitudinal and transverse relaxation times of⁶Li as well as the¹H–⁶Li NOE were also measured and, together with the⁷Li measurements, were used to obtain the strengths of dipolar (D) and quadrupolar (χ) interactions. The experimental data were analyzed using several models to describe the motion. The model that gave the best fit and resulted in parameters that were physically meaningful encompassed a whole-body isotropic motion as well as internal anisotropic motion. For this particular model, the following values for the quadrupolar and the dipolar interactions strength were obtained:D(⁷Li)/2π = 6.8 kHz, χ(⁷Li)/2π = 85 kHz andD(⁶Li)/2π = 1.4 kHz, χ(⁶Li)/2π = 2.6 kHz. From the value ofD, an estimate of the average lithium–proton distance was calculated to be 3.3 Å, which is in fair agreement with crystallographic studies. The sizes of the quadrupolar and dipolar interactions were independently confirmed by the⁷Li NMR powder spectra of the complexes that were used for the solution studies.

Analytical solutions are provided for a set of three simultaneous first-order differential equations which describe either cross relaxation among three groupings of spin-$\text{1}\text{2}$ nuclei (regardless of the number of spins within each grouping) or the complete longitudinal relaxation of a system of two spins-$\text{1}\text{2}$, including CSA-dipolar interference terms (which couple the longitudinal spin order to conventional longitudinal magnetizations). In spite of their complexity, the expressions so obtained afford a time savings by a factor of 50 when used in a computer program. The efficiency of the method is illustrated by the fit of experimental data, exhibiting an unusual evolution due to both intra- and intermolecular dipolar couplings.

Double sideband modulation and pulse shaping can be combined to improve the performance of selective pulses in solid-state deuteron NMR. Efficient off-resonance selective inversion of deuterons requires simultaneous irradiation of both spin-1 transitions, i.e., at symmetric offsets from the Larmor frequency (ν₀± Δ). This is best accomplished with double-sideband-modulated pulses. Frequency-selective profiles can be improved by shaping the RF pulses. A variety of commonly used selective pulse shapes are evaluated for their applicability to solid-state deuteron powder patterns. A new pulse shape is introduced; the squared isosceles triangle. Generally, pulses with low bandwidth–duration product, such as the Gaussian envelope, are found to be the most effective for selective inversion in solid-state²H NMR. This is a consequence of the large inhomogeneous quadrupolar interaction and fast, anisotropic spin–spin relaxation.

The shielded-loop resonator is known to have low capacitive sample loss due to perfect balancing. We present a new analysis of the unbalanced driven shielded-loop resonator that calculates the resonance frequencies and also determines some design considerations. The analysis enables us to optimize the use of this resonator. Theory and design considerations are shown to agree with observations in measurements on two coils, with various sizes and frequencies.

A simple broadband radiofrequency pulse sequence for the excitation of multiple-quantum coherences in the presence of fast magic-angle spinning is introduced. This sequence involves back-to-back (BABA) 90° radiofrequency pulse cycles timed to span two rotor periods. It proved to be robust and insensitive to off-resonance effects, to isotropic chemical shifts, and to chemical-shift anisotropies. This is demonstrated on crystalline phosphates, composed ofQ⁽¹⁾,Q⁽²⁾, andQ⁽³⁾groups, which are characterized by a large chemical-shift anisotropy spanning more than 200 ppm. With these experiments,³¹P high-resolution double-quantum NMR spectroscopy is introduced as a tool for the direct investigation of dipolar connectivities between like or differentQ⁽ⁿ⁾units in phosphates.³¹P dipolar connectivities in two samples, Mg₂P₂O₇and MgP₄O₁₁were established. Therefore, double-quantum spectroscopy of this kind has potential for the investigation of disordered solids, for instance, phosphorus glasses.