Solid-state NMR Of Quadrupolar Nuclei Research Articles

On the applicability of cosine-modulated pulses for high-resolution solid-state NMR of quadrupolar nuclei with spin > 3/2

In MQMAS-based high-resolution solid-state NMR experiments of half-integer spin quadrupolar nuclei, the high radiofrequency (RF) field requirement for the MQ excitation and conversion steps with two hard-pulses is often a sensitivity limiting factor in many practical applications. Recently, the use of two cosine-modulated (cos) low-power (lp) pulses, lasting one-rotor period each, was successfully introduced for efficient MQ excitation and conversion of spin-3/2 nuclei with a reduced RF amplitude. In this study, we extend our previous investigations of spin-3/2 nuclei to systems with higher spin values and discuss the applicability of coslp-MQ excitation and conversion in MQMAS and MQ-HETCOR experiments under slow and fast spinning conditions. For the numerical simulations and experiments we used a moderate magnetic field of 14.1 T. Two spin-5/2 nuclei (85Rb and 27Al) are mainly employed with a large variety of CQ values, but we show that the practical set up is also available for higher spin values, such as spin-9/2 with 93Nb in Cs4Nb11O30. We demonstrate for nuclei with spin value larger than 3/2 a preferential use of coslp-MQ acquisition for low-gamma nuclei and/or large CQ values with a much reduced RF-field with respect to that of hard-pulses used with conventional methods.

Solid-state NMR of quadrupolar nuclei for investigations into supported organometallic catalysts: scope and frontiers.

The rational development of heterogeneous catalysts is a prerequisite for improving current catalytic technologies. One of the most successful approaches to achieve this goal is the grafting of organometallic complexes onto a given support, via surface organometallic chemistry (SOMC). In such cases, the immobilization of (transition) metal centres onto an inorganic support affords single-site catalysts with precise tuning of the coordination sphere and thus target-specific catalytic reactivity. In this context, the precise characterization of the surface sites by several spectroscopic techniques in parallel is of paramount importance. Solid-state NMR, in particular for half-integer quadrupolar nuclei, is a method that has increased its scope in the field of catalysis, especially due to recent methodological breakthroughs, occasionally combined with DFT calculations. In this tutorial review, we provide an overview of the most robust and efficient NMR experiments for half-integer quadrupolar nuclei, from simple one-dimensional acquisition under static and MAS conditions up to high-resolution and heteronuclear correlation two-dimensional spectra, as illustrated by selected examples.

Quantitative structure parameters from the NMR spectroscopy of quadrupolar nuclei

Abstract Nuclear magnetic resonance (NMR) spectroscopy is one of the most important characterization tools in chemistry, however, 3/4 of the NMR active nuclei are underutilized due to their quadrupolar nature. This short review centers on the development of methods that use solid-state NMR of quadrupolar nuclei for obtaining quantitative structural information. Namely, techniques using dipolar recoupling as well as the resolution afforded by double-rotation are presented for the measurement of spin–spin coupling between quadrupoles, enabling the measurement of internuclear distances and connectivities. Two-dimensional J-resolved-type experiments are then presented for the measurement of dipolar and J coupling, between spin-1/2 and quadrupolar nuclei as well as in pairs of quadrupolar nuclei. Select examples utilizing these techniques for the extraction of structural information are given. Techniques are then described that enable the fine refinement of crystalline structures using solely the electric field gradient tensor, measured using NMR, as a constraint. These approaches enable the solution of crystal structures, from polycrystalline compounds, that are of comparable quality to those solved using single-crystal diffraction.

Enhancing the central-transition NMR signal of quadrupolar nuclei by spin population transfer using SW-FAM pulse trains with a tangent-shaped sweep profile

Solid-state NMR of quadrupolar nuclei with half-integer spin, such as 25Mg (I=5/2) or 43Ca (I=7/2), suffers from low sensitivity, which may be improved using spin population transfer (SPT) from the satellite transitions. Effecting SPT with good efficiency is especially challenging under static conditions, and several techniques such as double-frequency sweeps (DFS), hyperbolic secant pulses (HS) and frequency-swept fast-amplitude modulated pulses (SW-FAM) have been suggested for achieving the necessary manipulations of the satellite transitions. We here investigate the SPT properties of an SW-FAM sequence with a tangent-shaped profile. The new SW(tan)-FAM pulse train is shown to possess superior SPT performance to the SW(1/τ)-FAM sequence, which hitherto has been considered to be the best FAM method for signal enhancement of static spectra, by both numerical simulations on a 27Al model system, and experimental results on aluminium acetyl acetonate, Al(acac)3. In addition, the CT enhancement of individual crystallites from the polycrystalline sample with a defined angle between principal z-axis of the diagonal Q-tensor and the external field was considered by numerical simulations. In the vicinity of the magic angle θm=54.7°, a region of zero enhancement exists. Use of the SW(tan)-FAM sequence allows extending the frequency sweep further into this region, with beneficial effects for the overall enhancement and the faithfulness of the line shape. In agreement with previously published studies, our numerical simulations on SPT for single crystals again evidence that the enhancement factors for a polycrystalline sample range from zero enhancement to the maximum gain of 2I, with the total enhancement factor of the full powder pattern being the summation of these strongly varying individual factors. This variation is the cause for line shape distortions in SPT-enhanced spectra. At the same time, these findings prove the capability of frequency sweeps (i.e., DFS and SW-FAM) to fully invert the satellite transitions under certain conditions.

Signal Enhancement of the Central-Transition Signal in Solid-State NMR of Quadrupolar Nuclei with Spin I =5/2 Using Fast Amplitude-Modulated (FAM) Pulse Trains: A Computational Study

In solid-state NMR of quadrupolar nuclei with half-integer spin I, a variety of fast amplitude-modulated (FAM) pulse trains have been used to enhance the intensity of the central-transition signal, by transferring spin population from the satellite transitions. Here, we describe the results of extensive numerical calculations using the Simpson program package to evaluate and compare the signal enhancing performance of FAM-I, SW( )-FAM, and SW(1/ )-FAM sequences under both static and MAS conditions for nuclei with I =5/2. The comparative calculations were performed for a two-dimensional grid of the quadrupolar interaction parameters, namely =0.5 ... 4 MHz, =0 ... 1, and a range of MAS spinning speeds, i.e. 0 kHz (static), 2.5 kHz, 5 kHz and 10 kHz. In agreement with previously published experimental data, the best signal enhancement of static spectra is delivered by the SW(1/ )-FAM sequence, whereas under MAS conditions, SW( )-FAM and FAM-I give better results.

Fast amplitude-modulated pulse trains with frequency sweep (SW-FAM) in static NMR of half-integer spin quadrupolar nuclei

In solid-state NMR of quadrupolar nuclei with half-integer spin I, fast amplitude-modulated (FAM) pulse trains have been utilised to enhance the intensity of the central-transition signal, by transferring spin population from the satellite transitions. In this paper, the signal-enhancement performance of the recently introduced SW-FAM pulse train with swept modulation frequency [T. Bräuniger, K. Ramaswamy, P.K. Madhu, Enhancement of the central-transition signal in static and magic-angle-spinning NMR of quadrupolar nuclei by frequency-swept fast amplitude-modulated pulses, Chem. Phys. Lett. 383 (2004) 403–410] is explored in more detail for static spectra. It is shown that by sweeping the modulation frequencies linearly over the pulse pairs (SW(1/τ)-FAM), the shape of the frequency distribution is improved in comparison to the original pulse scheme (SW(τ)-FAM). For static spectra of 27Al (I=5/2), better signal-enhancement performance is found for the SW(1/τ)-FAM sequence, as demonstrated both by experiments and numerical simulations.

Third-order effect in solid-state NMR of quadrupolar nuclei

The theory and experimental observation of the third-order effect in solid-state NMR of quadrupolar nuclei are presented. The third-order effect consists of spherical harmonic terms up to rank l=6 and shifts NMR frequencies between two spin states that are not symmetric such as satellite transitions. Two-dimensional satellite transition magic-angle spinning experiment averages both the first and the second-order quadrupolar interactions making the quantitative measurement of the third-order effect possible. The third-order quadrupolar effect in andalusite has been measured at 11.7 T and its powder patterns are fitted with numerical simulations.

High-resolution solid-state NMR of quadrupolar nuclei

We report the observation of high-resolution solid-state NMR spectra of (23)Na (I = [unk]), (27)Al (I = [unk]) and (51)V (I = [unk]) in various inorganic systems. We show that, contrary to popular belief, relatively high-resolution ( approximately 10 ppm linewidth) spectra may be obtained from quadrupolar systems, in which electric quadrupole coupling constants (e(2)qQ/h) are in the range approximately 1-5 MHz, by means of observation of the ((1/2), -(1/2)) spin transition. The ((1/2), -(1/2)) transition for all nonintegral spin quadrupolar nuclei (I = [unk], [unk], [unk], or [unk]) is only normally broadened by dipolar, chemical shift (or Knight shift) anisotropy or second-order quadrupolar effects, all of which are to a greater or lesser extent averaged under fast magic-angle sample rotation. In the case of (23)Na and (27)Al, high-resolution spectra of (23)NaNO(3) (e(2)qQ/h approximately 300 kHz) and alpha-(27)Al(2)O(3) (e(2)qQ/h approximately 2-3 MHz) are presented; in the case of (51)V(2)O(5) (e(2)qQ/h approximately 800 kHz), rotational echo decays are observed due to the presence of a approximately 10(3)-ppm chemical shift anisotropy. The observation of high-resolution solid-state spectra of systems having spins I = [unk], [unk], and [unk] in asymmetric environments opens up the possibility of examining about two out of three nuclei by solid-state NMR that were previously thought of as "inaccessible" due to the presence of large (a few megahertz) quadrupole coupling constants. Preliminary results for an I = [unk] system, (93)Nb, having e(2)qQ/h approximately 19.5 MHz, are also reported.