Half-integer Quadrupolar Nuclei In Solids Research Articles

Through-space 11 B-27 Al correlation: Influence of the recoupling channel.

Through-space heteronuclear correlation (D-HETCOR) experiments based on heteronuclear multiple-quantum correlation (D-HMQC) and refocused insensitive nuclei enhanced by polarization transfer (D-RINEPT) sequences have been proven to be useful approaches for the detection of the spatial proximity between half-integer quadrupolar nuclei in solids under magic-angle spinning (MAS) conditions. The corresponding pulse sequences employ coherence transfers mediated by heteronuclear dipolar interactions, which are reintroduced under MAS by radiofrequency irradiation of only one of the two correlated nuclei. We investigate herein using numerical simulations of spin dynamics and solid-state NMR experiments on magnesium aluminoborate glass how the choice of the channel to which the heteronuclear dipolar recoupling is applied affects the transfer efficiency of D-HMQC and D-RINEPT sequences between 11 B and 27 Al nuclei. Experimental results show that maximum transfer efficiency is achieved when the recoupling scheme is applied to the channel, for which the spin magnetization is parallel to the B0 axis in average.

Efficient transfer of DNP-enhanced 1 H magnetization to half-integer quadrupolar nuclei in solids at moderate spinning rate.

We show herein how the proton magnetization enhanced by dynamic nuclear polarization (DNP) can be efficiently transferred at moderate magic-angle spinning (MAS) frequencies to half-integer quadrupolar nuclei, S≥3/2, using the Dipolar-mediated Refocused Insensitive Nuclei Enhanced by Polarization Transfer (D-RINEPT) technique, in which a symmetry-based recoupling scheme built from adiabatic inversion 1 H pulses reintroduces the 1 H-S dipolar couplings, while suppressing the 1 H-1 H ones. The use of adiabatic pulses also improves the robustness to offsets and radiofrequency (rf)-field inhomogeneity. Furthermore, the efficiency of the polarization transfer is further improved by using 1 H composite pulses and continuous-wave irradiations between the recoupling blocks, as well as by manipulating the S satellite transitions during the first recoupling block. Furthermore, in the case of large 1 H-S dipolar couplings, the D-RINEPT variant with two pulses on the quadrupolar channel results in an improved transfer efficiency. We compare here the performances of this new adiabatic scheme with those of its parent version with single π pulses, as well as with those of PRESTO and CPMAS transfers. This comparison is performed using simulations as well as DNP-enhanced 27 Al, 95 Mo, and 17 O NMR experiments on isotopically unmodified γ-alumina, hydrated titania-supported MoO3 , Mg(OH)2 , and l-histidine·HCl·H2 O. The introduced RINEPT method outperforms the existing methods, both in terms of efficiency and robustness to rf-field inhomogeneity and offset.

Accelerating high-resolution NMR of half-integer quadrupolar nuclei in solids: SPAM-MQMAS and SPAM-STMAS

In solid-state NMR, multiple-quantum MAS (MQMAS) and satellite-transition MAS (STMAS) experiments are well-established techniques to obtain high-resolution spectra of half-integer quadrupolar nuclei. In 2004 and 2005, a soft-pulse-added-mixing (SPAM) concept was introduced by Gan and Amoureux to enhance the S/N ratio of MQMAS and STMAS experiments. Despite their robustness and simplicity, SPAM approaches have not yet been widely applied. Here, we further exploit SPAM concepts for sensitivity enhancement upon acquisition of two-dimensional MQMAS and STMAS spectra and also establish a general procedure upon implementation of SPAM-MQMAS and SPAM-STMAS NMR. Its effectiveness and ease in experimental setup are demonstrated using simulations and experiments performed on I ​= ​3/2 (23Na, 87Rb), 5/2 (27Al, 85Rb) and 9/2 (93Nb) nuclei with a variety of quadrupolar coupling constants (CQ). Compared to the conventional z-filter methods, sensitivity enhancements in between 2 and 4 are achievable with SPAM. We recommend to use SPAM with a ratio of 4:1 for the number of echoes and antiechoes to safely maximize the sensitivity and resolution simultaneously. In addition, a comparison of the experimental approaches is made in the context of SPAM-MQMAS and SPAM-STMAS NMR with respect to repetition delay and spinning frequency, aiming to discuss the precautions upon making a judicious choice of high-resolution NMR methods of half-integer quadrupolar nuclei.

Second-order quadrupolar line shapes under molecular dynamics: An additional transition in the extremely fast regime

NMR spectroscopy is a powerful tool for probing molecular dynamics. For the classic case of two-site exchange, NMR spectra go through the transition from exchange broadening through coalescence and then motional narrowing as the exchange rate increases passing through the difference between the resonance frequencies of the two sites. For central-transition spectra of half-integer quadrupolar nuclei in solids, line shape change due to molecular dynamics occurs in two stages. The first stage occurs when the exchange rate is comparable to the second-order quadrupolar interaction. The second spectral transition comes at a faster exchange rate which approaches the Larmor frequency and generally reduces the isotropic quadrupolar shift. Such a two-stage transition phenomenon is unique to half-integer quadrupolar nuclei. A quantum mechanical formalism in full Liouville space is presented to explain the physical origin of the two-stage phenomenon and for use in spectral simulations. Variable-temperature 17O NMR of solid NaNO3 in which the NO3− ion undergoes 3-fold jumps confirms the two-stage transition process. The spectra of NaNO3 acquired in the temperature range of 173–413K agree well with simulations using the quantum mechanical formalism. The rate constants for the 3-fold NO3− ion jumps span eight orders of magnitude (102–1010s−1) covering both transitions of the dynamic 17O line shape.

Removal of sidebands in double-rotation NMR in real time

Double-rotation (DOR) is the only technique generally capable of yielding high-resolution NMR spectra of half-integer quadrupolar nuclei in one dimension for solids without the need for sophisticated coherence pathway selection. Unfortunately, due to the low outer rotor spinning frequencies currently available, the spectra often contain a large number of spinning sidebands which may overlap with the resonances of interest. We implement a simple, robust, and easy to use family of pulse sequences, which in practice are fully analogous to the ‘total suppression of sidebands’ (TOSS) sequences, to suppress all sidebands arising from the spinning of the outer rotor in DOR experiments. By removing the rotor phase dependence of the evolution of the sidebands, the sidebands destructively interfere with one another during the course of signal averaging to yield ‘solution-like’ spectra of half-integer quadrupolar nuclei in solids. Advantages and shortcomings of the method compared to other DOR sideband suppression methods are explored with the aid of simulations.

HMQC and refocused-INEPT experiments involving half-integer quadrupolar nuclei in solids

Hetero-nuclear coherence transfers in HMQC and refocused-INEPT experiments involving half-integer quadrupolar nuclei in solids are analyzed. 1D and 2D schemes are considered under MAS for the general case of multi-spin systems SI n ( n ⩽ 4), where S is an observed nucleus. These results are also discussed in the context of high-resolution schemes featuring MQMAS or STMAS. The theoretical predictions are verified experimentally in a series of 1D and 2D experiments performed at 9.4 and 18.8 T.

Feasibility of the overtone NMR for half-integer quadrupolar nuclei

We present a theoretical description and computer simulations for the overtone NMR of half-integer quadrupolar nuclei in solids. It is shown that the overtone NMR has the ability to produce high-resolution solid NMR spectra for the quadrupolar nuclei under magic angle spinning conditions. We demonstrate that when polycrystalline 23Na 2MoO 4 is placed in the static magnetic field of 7.05 T, the direct detection of the overtone NMR is possible. It becomes, however, apparent that indirect detection schemes are required to obtain the usual NMR spectra that have separated resonance lines corresponding to chemically non-equivalent sites.

Molecular dynamics of half-integer quadrupolar nuclei studied by QCPMG solid-state NMR experiments on static and rotating samples. Theory and simulations

Simulations of QCPMG NMR type experiments have been used to explore dynamic processes of half-integer quadrupolar nuclei in solids. By setting up a theoretical approach that is well suited for efficient numerical simulations the QCPMG type experiments have been analyzed regarding the effect of the magnitude of the EFG- and CSA-tensors, the spin-quantum number, different dynamical processes and MAS. Compared to the QE experiment the QCPMG experiment offers not only intensity gain by an order of magnitude and changes in overall lineshape as a function of the kinetic rate constant but the lineshape of the individual spin-echo sidebands is also very sensitive towards dynamics. Hereby a visual identification of the dynamics is obtained. In common for all the simulations the spin-echo sidebands are narrow in the slow ( k ⩽ 10 2 Hz) and the fast ( k ⩾ 10 7 Hz) dynamic regime whereas they are broadened in the intermediate regime 10 3 ⩽ k ⩽ 10 7 Hz. The maximum intensity of the spin-echo sidebands for two-site jumps is highly dependent on the type of anisotropic interactions involved and the type of QCPMG experiment. Hence, in the fast limit the maximum intensity was 140% of the initial intensity when significant CSA was present or under the QCPMG-MAS experiment compared to 89 or 71% for the static experiment influenced by the quadrupolar interaction only. For 3-, 4-, and 6-site jumps the maximum intensity in the fast limit reached up to 339% of the intensity in the static limit.

Enhancing sensitivity or resolution of homonuclear correlation experiment for half-integer quadrupolar nuclei

We have recently introduced double-quantum homonuclear correlation NMR experiment for half-integer quadrupolar nuclei in solids, which was based on rotary resonance recoupling [J. Chem. Phys. 120 (2004) 2835]. In this contribution we show on two 23Na ( I = 3/2) containing samples, Na 2SO 4 and Na 2HPO 4, that the efficiency of the experiment can be substantially enhanced by adding rotor assisted population transfer (RAPT) and Carr–Purcell–Meiboom–Gill (CPMG) sequences to it. We also present an upgraded two-dimensional experiment, in which double- and six-quantum coherences are correlated during t 1 evolution period, yielding a high-resolution isotropic spectrum along an indirectly detected dimension. The sensitivity of the upgraded experiment is, however, greatly reduced compared to the sensitivity of the original experiment, so that its application is feasible only when RAPT and CPMG can be used as well.

Signal enhancement of NMR spectra of half-integer quadrupolar nuclei in solids using hyperbolic secant pulses

Signal enhancement of the central NMR transition of quadrupolar nuclei with half-integer spin is achieved by applying a hyperbolic-secant π-inversion (sechinv) pulse to the satellite transitions. In combination with the quadrupolar Carr–Purcell Meiboom–Gill (QCPMG) pulse train, the sechinv pulse provides an additional signal enhancement of up to 2.7 for spin 3/2 nuclei in powdered solids. The 87Rb NMR enhancements realized using this experiment are compared to those obtained employing rotor-assisted population transfer (RAPT) and double frequency sweep (DFS) pulse experiments on magic angle spinning (MAS) powdered samples. In every case, the sequence employing sechinv pulses gives greater enhancements than the previously reported methods.

Double-quantum homonuclear correlation magic angle sample spinning nuclear magnetic resonance spectroscopy of dipolar-coupled quadrupolar nuclei.

A double-quantum homonuclear correlation nuclear magnetic resonance experiment for dipolar-coupled half-integer quadrupolar nuclei in solids is presented. The experiment is based on rotary resonance dipolar recoupling and uses bracketed spin-lock pulses to excite double-quantum coherence and later to convert it to the zero-quantum one. A central-transition-selective pi pulse at the beginning of the t1 evolution period differentiates coherence transfer pathways of double-quantum coherences arising from coupled spins and from a single spin, so that the latter can be efficiently filtered out by phase cycling. The experiment was tested on an aluminophosphate molecular sieve AlPO4-14, a material with a variety of aluminum quadrupolar coupling constants, isotropic chemical shifts and homonuclear distances. In a two-dimensional spectrum aluminum dipolar couplings with internuclear distances between 2.9 and 5.5 A were resolved. Although the experiment requires an application of weak radio-frequency fields, frequency offsets did not affect its performance crucially.

Detecting proximities between quadrupolar nuclei by double-quantum NMR.

A robust, easy to optimize, and efficient homonuclear correlation NMR experiment for half-integer quadrupolar nuclei in solids is described and has been experimentally tested on anhydrous Na2HPO4-(23Na, S= 3/2 nucleus) and as-synthesized AlPO4-14 (27Al, S= 5/2 nucleus), an aluminophosphate molecular sieve.

Double-quantum-filtered nuclear magnetic resonance spectroscopy applied to quadrupolar nuclei in solids

A double-quantum filtered nuclear magnetic resonance experiment is applied to half-integer quadrupolar nuclei in solids. Filtering through a double-quantum coherence excited in a dipolar-coupled two-spin system allows spatial correlations between spins to be determined. Theoretical and experimental investigations have been carried out to optimize the excitation of double-quantum coherence between the central transitions of two neighboring nuclei while minimizing double-quantum excitation on each individual spin. The presence of the large quadrupolar coupling in the systems under study allows multiple-quantum excitation via the dipolar coupling between two spins even under conditions of magic-angle spinning, without the need for a recoupling sequence. Two-dimensional double-quantum-filtered correlation spectra have been recorded on Na23 (I=32) for Na2ZrO3 which contains three distinct Na23 sites.

Multi-rank nuclear magnetic resonance studies of half-integer quadrupolar nuclei in solids by three-dimensional dynamic-angle correlation spectroscopy

The present work introduces a new three-dimensional nuclear magnetic resonance (3D NMR) experiment for the analysis of half-integer quadrupolar nuclei in solids. The method is based on the multi-rank expansion of the high-field NMR Hamiltonian governing the central transition of these spins in terms of irreducible spherical tensor elements. This approach leads to a temporal spin evolution given by an isotropic term characteristic of each chemical site, as well as by second- and fourth-rank anisotropies depending on the principal values and relative orientations of the shielding and quadrupolar interactions. A method for extracting the 3D spectral distribution correlating these three frequency components is presented, based on the acquisition of dynamic-angle spinning NMR signals collected as a function of different initial and final spinning axes. Computational and instrumental details involved in the acquisition of these 3D dynamic-angle correlation spectroscopy (DACSY) data are discussed, and applications of the DACSY methodology to the analysis of different rubidium salts are illustrated. The new type of chemical information that this experiment can provide and its relation to other NMR techniques that have been recently developed for the analysis of quadrupolar nuclei in solids are also discussed.

First-Satellite Spectroscopy, a New Method for Quadrupolar Spins

A new spectroscopic method that detects only the ±1 12 (double-arrow) ±12 transitions (i.e., first satellites) of half-integer quadrupolar nuclei in solids is presented. The technique is based on a central-transition signal enhancement method that makes use of adiabatic radiofrequency sweeps. A plot of the enhanced central-transition signal amplitude versus the stop frequency of the sweep is an integrated version of the first-satellite spectrum. To find the quadrupole parameters, one can fit this integrated spectrum directly or differentiate the data to reveal the traditional spectrum (of the first satellites). The technique is demonstrated for27Al in Al2O3and93Nb in LiNbO3. First-satellite spectroscopy is more sensitive than other methods for studying wide, quadrupolar broadened spectra. The technique is easier to perform experimentally. Since the resulting spectra contain only the innermost satellites, the experimental data are considerably simpler and easier to interpret in cases of two or more inequivalent sites.

NMR Characterization of Half-integer Quadrupolar Nuclei in Solids

Abstract This article describes a few aspects of NMR characterization of half-integer quadrupolar nuclei in powder samples which are either static or rotating around one fixed axis (M.A.S. and V.A.S. techniques). The simultaneous occurence of C.S.A. and quadrupole interactions with possible noncoincident tensors is discussed. Two experimental limitations are taken into accout: the sample spinning speed and the RF-amplifier power