Two-Dimensional Multiple-Quantum MAS NMR of Quadrupolar Nuclei: A Comparison of Methods

Brown, Steven P.; Wimperis, Stephen

doi:10.1006/jmre.1997.1217

Cited by 192 publications

(388 citation statements)

References 15 publications

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“…45 (Note that for the case 35 of an isolated half-integer quadrupolar nucleus experiencing only the quadrupolar interaction, the linewidth is independent of the MAS frequency.) Defining the quadrupolar frequency 46  , hence Ref. 40 states that the maximum line broadening is observed when  Q / r = 1.2-1.3.)…”

mentioning

confidence: 99%

Towards homonuclear J solid-state NMR correlation experiments for half-integer quadrupolar nuclei: experimental and simulated 11B MAS spin-echo dephasing and calculated 2JBB coupling constants for lithium diborate

Barrow

Yates

Feller

et al. 2011

Phys. Chem. Chem. Phys.

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is observed: the B3 (larger C Q ) site dephases more slowly than the B4 site at all investigated MAS frequencies (5 to 20 kHz) at 14.1 T. Increasing the MAS frequency leads to markedly slower dephasing for the B3 site, while there is a much less evident effect for the B4 site. Considering samples at 5, 25, 80 (natural abundance) and 100 % 11 B isotopic 15 abundance, dephasing becomes faster for both sites as the 11 B isotopic abundance increases. The experimental behaviour is rationalised using density matrix simulations for two and three dipolarcoupled 11 B nuclei. The experimentally observed slower dephasing for the larger C Q (B3) site is reproduced in all simulations and is explained by the reintroduction of the dipolar coupling by the so-called "spontaneous quadrupolar-driven recoupling mechanism" having a different dependence 20 on the MAS frequency for different quadrupolar frequencies. Specifically, isolated spin-pair simulations show that the spontaneous quadrupolar-driven recoupling mechanism is most efficient when the quadrupolar frequency is equal to twice the MAS frequency. While for isolated spin-pair simulations, increasing the MAS frequency leads to faster dephasing, agreement with experiment is observed for three-spin simulations which additionally include the homogeneous nature of the 25 homonuclear dipolar coupling network. First-principles calculations, using the GIPAW approach, of the 2 J 11B-11B couplings in lithium diborate, metaborate and triborate are presented: a clear trend is revealed whereby the 2 J 11B-11B couplings increase with increasing B-O-B bond angle and B-B distance. However, the calculated 2 J 11B-11B couplings are small (0.95, 1.20 and 2.65 Hz in lithium diborate), thus explaining why no zero crossing due to J modulation is observed experimentally, 30 even for the sample at 25 % 11 B where significant spin-echo intensity remains out to durations of 200 ms.

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confidence: 99%

Towards homonuclear J solid-state NMR correlation experiments for half-integer quadrupolar nuclei: experimental and simulated 11B MAS spin-echo dephasing and calculated 2JBB coupling constants for lithium diborate

Barrow

Yates

Feller

et al. 2011

Phys. Chem. Chem. Phys.

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“…The excitation pulse width was optimized to maximize the signal intensities. The 35 Cl and 37 Cl Lamor frequencies were 99.9 (58.8) MHz and 83.2 (48.9) MHz, respectively, at 24 (14.1) T. The optimized 35 Cl and 37 Cl pulse widths and repetition delay (pw, RD) used in the 1D measurements were as listed: 35 Cl at 24 T (2.6 μs, 2 s), 37 Cl at 24 T (33 μs, 5 s), and 35 Cl at 14.1 T (15 μs, 2 s) for CaCl2·2H2O; 35 Cl (22 μs, 2 s), and 37 Cl (37 μs, 5 s) at 24 T for tyrosine·HCl; 35 Cl (22 μs, 3 s), and 37 Cl (38 μs, 5 s) at 24 T for L-histidine·HCl·H2O. It should be pointed out that the other combinations of the rf-field strength and pulse width could be used without sacrificing the sensitivity very much.…”

Section: Methodsmentioning

confidence: 99%

“…In this contribution, we demonstrate the half-integer low γ quadrupolar nuclei 35 Cl and 37 Cl NMR at 24 T with a LTS/HTS hybrid magnet. Since chloride is a quite important nucleus in pharmaceutical, organic, and inorganic chemistry, both 35 Cl and 37 Cl have been extensively studied.…”

Section: Introductionmentioning

confidence: 91%

“…Since chloride is a quite important nucleus in pharmaceutical, organic, and inorganic chemistry, both 35 Cl and 37 Cl have been extensively studied. [19][20][21][22][23][24][25][26][27] To demonstrate the advantage of the high magnetic field, we propose a simple theoretical model a) to predict the sensitivity enhancement at higher magnetic fields, and b) to estimate the isotope effect.…”

Section: Introductionmentioning

confidence: 99%

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24 T High-Resolution and -Sensitivity Solid-State NMR Measurements of Low-Gamma Half-Integer Quadrupolar Nuclei 35Cl and 37Cl

Pandey

Hashi²,

Ohki³

et al. 2016

ANAL. SCI.

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Solid-state NMR observations of low-gamma half-integer quadrupolar nuclei, 35 Cl and 37 Cl, were demonstrated using a 24 T hybrid magnet ( 1 H resonance frequency of 1.02 GHz) comprised of the high-temperature (HTS) and low-temperature (LTS) superconductors, and compared with results using a 14.1 T standard NMR magnet. While at 24 T the linewidth is 1.7 times narrower than that at 14.1 T, the gain in the sensitivity is 7.0 times because of enhanced polarization, reduced linewidth, and the use of larger rotor. A simple theoretical model was used to rationalize the sensitivity enhancements. The ratio of 35 Cl and 37 Cl quadrupolar couplings agrees well with the ratio of quadrupolar moments, and no isotopedependent chemical shift has been observed. In addition, the 3QMAS spectrum of 35 Cl is shown to demonstrate the high sensitivity rendered by the 24 T spectrometer.

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“…80 echoes were acquired with a spacing of 2.4 ms, giving a spikelet spacing of 416 Hz in the Fourier-transformed spectrum. For 27 Al, triple-quantum MAS NMR experiments were carried out using a phase-modulated split-t1 shifted-echo pulse sequence, 15 with the final pulse chosen to be selective for the central transition. The scale in the indirect dimension was referenced according to the convention in Ref.…”

Section: Nmr Spectroscopymentioning

confidence: 99%

An NMR crystallographic approach to monitoring cation substitution in the aluminophosphate STA-2

Seymour

Eschenroeder

Wright

et al. 2015

Solid State Nuclear Magnetic Resonance

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The substitution of the divalent cations Mg 2+ and Zn 2+ into the aluminophosphate (AlPO) framework of STA-2 has been studied using an "NMR crystallographic" approach, combining multinuclear solid-state NMR spectroscopy, X-ray diffraction and firstprinciples calculations. Although the AlPO framework itself is inherently neutral, the positive charge of the organocation template in an as-made material is usually balanced either by the coordination to the framework of anions from the synthesis solution, such as OH  or F  , and/or by the substitution of aliovalent cations. However, the exact position and distribution of the substituted cations can be difficult to determine, but can have a significant impact upon the catalytic properties a material exhibits once calcined.   For as-made Mg substituted STA-2, the positive charge of the organocation template is balanced by the substitution of Mg 2+ for Al 3+ and, where required, by hydroxide anions coordinated to the framework. 27 Al MAS NMR spectra show that Al is present in both tetrahedral and five-fold coordination, with the latter dependent on the amount of substituted cations, and confirms the bridging nature of the hydroxyl groups, while high-resolution MQMAS spectra are able to show that Mg appears to preferentially substitute on the Al1 site. This conclusion is also supported by first-principles calculations.The calculations also show that 31 P chemical shifts depend not only on the topologicallydistinct site in the SAT framework, but also on the number of next-nearest-neighbour Mg species, and the exact nature of the coordinated hydroxyls (whether the P atom forms part of a six-membered ring, P(OAl)2OH, where OH bridges between two Al atoms). The calculations demonstrate a strong correlation between the 31 P isotropic chemical shift and the average bond angle. In contrast, for Zn substituted STA-2, both X-ray diffraction and NMR spectroscopy show less preference for substitution onto Al1 or Al2, with both appearing to be present, although that into Al1 appears slightly more favoured.

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Two-Dimensional Multiple-Quantum MAS NMR of Quadrupolar Nuclei: A Comparison of Methods

Cited by 192 publications

References 15 publications

Towards homonuclear J solid-state NMR correlation experiments for half-integer quadrupolar nuclei: experimental and simulated 11B MAS spin-echo dephasing and calculated 2JBB coupling constants for lithium diborate

Towards homonuclear J solid-state NMR correlation experiments for half-integer quadrupolar nuclei: experimental and simulated 11B MAS spin-echo dephasing and calculated 2JBB coupling constants for lithium diborate

24 T High-Resolution and -Sensitivity Solid-State NMR Measurements of Low-Gamma Half-Integer Quadrupolar Nuclei 35Cl and 37Cl

An NMR crystallographic approach to monitoring cation substitution in the aluminophosphate STA-2

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