Shaped pulse excitation in multi-quantum magic-angle spinning spectroscopy of half-integer quadrupole spin systems

Sensitivity-enhanced multiple-quantum MAS NMR for half-integer spin quadrupolar nuclei using WURST-amplitude shaped pulses

Two-dimensional multiple-quantum MAS (MQMAS) NMR spectroscopy is one of the most widely used solid-state NMR techniques for resolving multiple overlapping central-transition lineshapes for half-integer spin quadrupolar nuclei. In particular when relying on nutation-driven MQ coherence transfers, this technique suffers from low sensitivity that can only be improved by increasing the rf-amplitude of the involved radio-frequency (rf) pulses, which are therefore typically operated at the rf-limit. In such situations, frequently encountered for the three-pulse z-filtered and split-t₁ shifted-echo MQMAS NMR sequences, we introduce the advantages of rf-pulses with smoothly truncated amplitude profiles, which we have termed WURST-Amplitude Shaped Pulses (WASPs). When considering the NMR spectrometer hardware, we demonstrate that WASPs feature more suitable properties in comparison to conventional rectangular pulses, enabling a substantial reduction of voltage reflections and transient effects under identical rf-conditions. By employing extensive numerical simulations and experimental validation, we further show that WASPs intrinsically possess a higher potential for nutation-based 3Q excitation involving spin-3/2 and 3Q and 5Q excitation for spin-5/2 quadrupolar nuclei, specifically when large nutation frequencies are available. The concept of smoothly truncating rf-amplitudes is also extended to Fast Amplitude Modulation (FAM) pulses, normally incorporated for rotor-driven 1Q conversion. We additionally evaluate the potential of employing WASPs with peak rf-amplitudes beyond the rf-limit for conventional rectangular rf-pulses.

Improved quantitation in 3QMAS of spin 5/2 nuclei by RF power modulation of FAM-II

High-resolution NMR of quadrupolar I = 5/2 nuclei using triple-quantum magic angle spinning (3QMAS) techniques can provide more accurate quantitative information on sites with small quadrupolar coupling constants by changing the pulse strength in addition to the pulse length in the FAM-II multiple-quantum conversion sequence. These effects are illustrated using (27)Al NMR of yttrium aluminium garnet and andalusite.

Enhancing MQMAS sensitivity using signals from multiple coherence transfer pathways

Multiple-quantum magic-angle spinning experiment removes second-order quadrupolar broadening from the central-transition of half-integer quadrupolar nuclei. This paper presents a novel scheme to enhance the sensitivity of MQMAS using signals from multiple coherence transfer pathways. The enhancement can be obtained in two ways. The first method uses the multiplex phase cycling to acquire MQMAS spectra from various coherence transfer pathways simultaneously. An addition of spectra collected with no extra time enhances the efficiency of the experiment. The second method, soft-pulse-added-mixing, is designed based on a complete alias of coherence transfer pathways. By properly fixing the soft-pulse phase, signals from various coherence transfer pathways can add constructively resulting higher signal intensities. The two methods are demonstrated for sensitivity enhancement with samples of spin-3/2 and 5/2.

Effects of pulse strength, width, and sample spinning speed on the spectral spin diffusion of multiquantum coherences of spin-32 quadrupolar nuclei

The effects of radio-frequency pulse strength, width, and sample spinning speed on the spin-diffusion spectrum of half-integer quadrupolar spins in solids have been studied by theoretical, numerical, and experimental investigations. It is revealed that the line shape of the cross peaks changes nonmonotonically with respect to the change of pulse strength, pulse width, or sample spinning speed. It is also found that the sample spinning speed has much more pronounced influence on the spin diffusion spectral line shape. In many cases of practical importance, the effect of sample spinning must be included in spectral analysis, in contrast to the practice of previous studies. Moreover, this effect can be exploited to further improve the precision in the determination of relative orientation of the electric-field gradient tensors of the exchange partners.

Signal enhancement in 5QMAS spectra of spin-5/2 quadrupolar nuclei

5QMAS experiments on spin-5/2 systems display a low sensitivity compared with their 3QMAS counterparts. Nevertheless, the superior resolution of 5QMAS over 3QMAS makes these experiments a favorable choice for many materials. We report an enhancement scheme for the 5QMAS experiment, using an improved five-quantum excitation pulse scheme combined with a FAM-II conversion pulse. The results are verified experimentally on a polycrystalline sample of gamma-(27)Al(2)O(3), showing an enhancement factor of 2.4 over the simple two-pulse (CW) 5QMAS scheme. Numerical computations of the efficiency parameter epsilon support these results.

Efficient triple-quantum excitation in modified RIACT MQMAS NMR for I=3/2 nuclei

A new approach involving the creation of triple-quantum (TQ) coherences from both TQ and central transitions (CT) is investigated, in order to enhance the efficiency of triple-quantum excitation for I=3/2 nuclei. The RIACT excitation scheme, a soft pi/2 and hard spin-locking pulse, is shown to induce both adiabatic coherence transfer between CT and TQ coherences and TQ nutation. By combining the RIACT scheme with the presaturation of the satellite transitions, a significant improvement in the TQ excitation can be achieved mainly through enhanced CT polarization via the RIACT mechanism, in particular for nuclei with moderate to large quadrupole coupling constants (> or = 2.0 MHz). There also exists a nontrivial contribution from the TQ transition, which depends on the size of the quadrupole interaction.

Procedures for labeling the high-resolution axis of two-dimensional MQ-MAS NMR spectra of half-integer quadrupole spins

The increasing development and application of the multiple-quantum MAS NMR for half-integer quadrupole spins has led to various RF pulse sequences for improving the excitation of multiple-quantum coherences and their conversion to single-quantum coherences. As a result, several conventions for labeling the Fl dimension of a 2D MQ-MAS spectrum appear in the literature. The corresponding relations for extracting the isotropic chemical shift, the quadrupole coupling constant, and the asymmetry parameter from experimental data are not always provided. We analyze these various conventions systematically and propose a new one, similar to that introduced by J.-P. Amoureux and C. Fernandez (2000, Solid State NMR 10, 339-343). These various conventions are illustrated with 27Al (I = 5/2) nuclei in aluminum acetylacetonate Al(CH3COCHCOCH3)3. Another experimental problem often met, the aliasing of peaks in the 2D spectrum, is analyzed and illustrated with 27Al (I = 5/2) in NH4Y zeolite and 23Na (I = 3/2) in sodium pyrophosphate Na4P2O7.

Population and coherence transfer induced by double frequency sweeps in half-integer quadrupolar spin systems

We have recently shown that the sensitivity of single- and multiple-quantum NMR experiments of half-integer (N/2) quadrupolar nuclei can be increased significantly by introducing so-called double frequency sweeps (DFS) in various pulse schemes. These sweeps consist of two sidebands generated by an amplitude modulation of the RF carrier. Using a time-dependent amplitude modulation the sidebands can be swept through a certain frequency range. Inspired by the work of Vega and Naor (J. Chem. Phys. 75, 75 (1981)), this is used to manipulate +/-(m - 1) <--> +/-m (3/2 < or = m < or = N/2) satellite transitions in half-integer spin systems simultaneously. For (23)Na (I = 3/2) and (27)Al (I = 5/2) spins in single crystals it proved possible to transfer the populations of the outer +/-m spin levels to the inner +/-1/2 spin levels. A detailed analysis shows that the efficiency of this process is a function of the adiabaticity with which the various spin transitions are passed during the sweep. In powders these sweep parameters have to be optimized to satisfy the appropriate conditions for a maximum of spins in the powder distribution. The effects of sweep rate, sweep range, and RF field strength are investigated both numerically and experimentally. Using a DFS as a preparation period leads to significantly enhanced central transition powder spectra under both static and MAS conditions, compared to single pulse excitation. DFSs prove to be very efficient tools not only for population transfer, but also for coherence transfer. This can be exploited for the multiple- to single-quantum transfer in MQMAS experiments. It is demonstrated, theoretically and experimentally, that DFSs are capable of transferring both quintuple-quantum and triple-quantum coherence into single-quantum coherence in I = 5/2 spin systems. This leads to a significant enhancement in signal-to-noise ratio and strongly reduces the RF power requirement compared to pulsed MQMAS experiments, thus extending their applicability. This is demonstrated by (27)Al 3QMAS experiments on 9Al(2)O(3). 2B(2)O(3) and the mineral andalusite. In the latter compound, Al experiences a quadrupolar-coupling constant of 15.3 MHz in one of the sites. Finally a 5QMAS spectrum on 9Al(2)O(3). 2B(2)O(3) demonstrates the sensitivity enhancement of this experiment using a double frequency sweep.

Pure absorption-mode spectra using a modulated RF mixing period in MQMAS experiments

Different approaches to obtain pure absorption-mode lineshapes in MQMAS experiments employing a train of 180 degrees phase-alternating pulses for the multiple-quantum to single-quantum mixing period are investigated. Four pulse sequences, which achieve this by using either the shifted-echo approach or the hypercomplex approach with symmetric coherence transfer pathways, are presented and their improved lineshape- and sensitivity-performance is experimentally demonstrated by (87)Rb MQMAS of RbNO(3). Compared to the original modulated-rf mixing sequence, sensitivity enhancements by factors up to 1.3 are obtained with the sequences described here.

Very fast MAS and MQMAS NMR studies of the spectroscopically challenging minerals kyanite and andalusite on 400, 500, and 800 MHz spectrometers

The well-characterized minerals kyanite and andalusite have long presented great challenges in using solid state 27Al NMR to determine the isotropic chemical shift deltaCS, quadrupole coupling constant e2qQ/h, and asymmetry parameter eta for each of the inequivalent aluminum sites in these minerals. Indeed, these minerals have frequently been used to test advances in instrumentation. Recent advances in magnet technology (up to 18.8 T = 800 MHz 1H) and in MAS probe technology (spinning up to 35 kHz and considerably stronger rf) and refinements of the two-dimensional, multiple quantum magic angle spinning (MQMAS) technique suggested that these developments could be profitably used to study kyanite and andalusite by solid state 27Al NMR. The benefit of being able to study kyanite both by MAS and MQMAS techniques on 400, 500, and 800 MHz spectrometers is demonstrated. The two octahedral aluminum sites with the largest (and nearly equal) e2qQ/h values give overlapping 1D MAS or 2D 3QMAS signals at all three field strengths. Nevertheless, quantitatively accurate 3Q signal intensities at 9.4 T for all four octahedral aluminum sites (with e2qQ/h values up to 10 MHz) allow more detailed analysis. Even if the 3Q signal intensities are not quantitative, their isotropic shifts provide an approach (if accurate e2qQ/h and eta values are available) other than deconvolution of the MAS spectrum for calculating deltaCS values. For andalusite, 34 kHz MAS on the 800 MHz spectrometer significantly narrows the extremely broad signal for the octahedral aluminum, and only slight difficulties are encountered in quantitating the relative amounts of AlO5 and AlO6 present. Even with e2qQ/h = 15.3 MHz, the octahedral aluminum in andalusite gives a signal in a MQMAS experiment, albeit of reduced intensity. As appropriate, we discuss some of the benefits and limitations of these advances in instrumentation and of different experimental approaches for studying non-integral spin quadrupolar nuclei in solids.

Multiple-quantum MAS NMR spectroscopy of spin-3/2 quadrupolar spin systems using shaped pulses

An investigation of the effect of shaping the pulses in multiple-quantum magic-angle nuclear magnetic resonance experiments was carried out on polycrystalline samples containing spin-32 quadrupolar nuclear spins. Both theoretical analysis and numerical simulations show that there exist types of shaped pulses that are more advantageous than the usual rectangular pulses in exciting multiple-quantum coherences and converting these into single-quantum coherences or zero-quantum coherences (in z-filtering cases). The sensitivities can be enhanced without loss of resolution or increasing the RF offset dependences. Moreover, properly shaped pulses can moderate the requirements on the RF power and sample spinning speeds. Especially important for application purposes is the finding that lattice site quantification can also be improved by the use of certain shaped pulses.