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Bryce DL. Double-rotation (DOR) NMR spectroscopy: Progress and perspectives. SOLID STATE NUCLEAR MAGNETIC RESONANCE 2024; 130:101923. [PMID: 38471386 DOI: 10.1016/j.ssnmr.2024.101923] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/02/2024] [Revised: 03/05/2024] [Accepted: 03/06/2024] [Indexed: 03/14/2024]
Abstract
Double-rotation (DOR) solid-state NMR spectroscopy is a high-resolution technique developed in the late 1980s. Although multiple-quantum magic-angle spinning (MQMAS) became the most widely used high-resolution method for half-integer spin quadrupoles after 1995, development and application of DOR NMR to a variety of chemical and materials science problems has endured. This Trend article recapitulates the development of DOR NMR, discusses various applications, and describes possible future directions. The main technical limitations specific to DOR NMR are simply related to the size of the double rotor system. The relatively large outer rotor (and thus coil) used for most applications over the past 35 years translates into relatively low rotor spinning frequencies, a low filling factor, and weak radiofrequency powers available for excitation and for proton decoupling. Ongoing developments in NMR instrumentation, including ever-shrinking MAS rotors and spherical NMR rotors, could solve many of these problems and may augur a renaissance for DOR NMR.
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Affiliation(s)
- David L Bryce
- Department of Chemistry and Biomolecular Sciences, Centre for Catalysis Research and Innovation, and Nexus for Quantum Technologies, University of Ottawa, 10 Marie Curie Private, Ottawa, Ontario, K1N 6N5, Canada.
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Yu Y, Keil P, Hansen MR, Edén M. Improved Magnetization Transfers among Quadrupolar Nuclei in Two-Dimensional Homonuclear Correlation NMR Experiments Applied to Inorganic Network Structures. Molecules 2020; 25:molecules25020337. [PMID: 31947638 PMCID: PMC7024165 DOI: 10.3390/molecules25020337] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/22/2019] [Revised: 01/08/2020] [Accepted: 01/09/2020] [Indexed: 11/17/2022] Open
Abstract
We demonstrate that supercycles of previously introduced two-fold symmetry dipolar recoupling schemes may be utilized successfully in homonuclear correlation nuclear magnetic resonance (NMR) spectroscopy for probing proximities among half-integer spin quadrupolar nuclei in network materials undergoing magic-angle-spinning (MAS). These (SR221)M, (SR241)M, and (SR281)M recoupling sequences with M=3 and M=4 offer comparably efficient magnetization transfers in single-quantum–single-quantum (1Q–1Q) correlation NMR experiments under moderately fast MAS conditions, as demonstrated at 14.1 T and 24 kHz MAS in the contexts of 11B NMR on a Na2O–CaO–B2O3–SiO2 glass and 27Al NMR on the open framework aluminophosphate AlPO-CJ19 [(NH4)2Al4(PO4)4HPO4·H2O]. Numerically simulated magnetization transfers in spin–3/2 pairs revealed a progressively enhanced tolerance to resonance offsets and rf-amplitude errors of the recoupling pulses along the series (SR221)M< (SR241)M< (SR281)M for increasing differences in chemical shifts between the two nuclei. Nonetheless, for scenarios of a relatively minor chemical-shift dispersions (≲3 kHz), the (SR221)M supercycles perform best both experimentally and in simulations.
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Affiliation(s)
- Yang Yu
- Department of Materials and Environmental Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden;
| | - Philipp Keil
- Institute for Physical Chemistry, Westfälische Wilhelms-Universität Münster, DE-48 149 Münster, Germany; (P.K.); (M.R.H.)
| | - Michael Ryan Hansen
- Institute for Physical Chemistry, Westfälische Wilhelms-Universität Münster, DE-48 149 Münster, Germany; (P.K.); (M.R.H.)
| | - Mattias Edén
- Department of Materials and Environmental Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden;
- Correspondence:
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Dicaire NM, Perras FA, Bryce DL. 23Na magic-angle spinning and double-rotation NMR study of solid forms of sodium valproate. CAN J CHEM 2014. [DOI: 10.1139/cjc-2013-0442] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Sodium valproate is a pharmaceutical with applications in the treatment of epilepsy, bipolar disorder, and other ailments. Sodium valproate can exist in many hydrated and acid-stabilized forms in the solid state, and it can be difficult to obtain precise structural information about many of these. Here, we present a 13C and 23Na solid-state NMR study of several forms of sodium valproate, only one of which has been previously structurally characterized by single-crystal X-ray diffraction. 23Na magic-angle spinning (MAS), double-rotation (DOR), and multiple-quantum magic-angle spinning (MQMAS) NMR spectra are shown to provide useful information on the number of molecules in the asymmetric unit, the local coordination geometry of the sodium cations, and the presence of amorphous phases. Two previously identified forms are shown to be highly similar, or identical, according to the 23Na NMR data. The utility of carrying out both DOR and MQMAS NMR experiments to identify all crystallographically unique sites is demonstrated. 13C cross-polarization MAS NMR spectra also provide complementary information on the number of molecules in the asymmetric unit and the crystallinity of the sample.
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Affiliation(s)
- Nuiok M. Dicaire
- Department of Chemistry and Centre for Catalysis Research and Innovation, University of Ottawa, 10 Marie Curie Private, Ottawa, ON K1N 6N5, Canada
| | - Frédéric A. Perras
- Department of Chemistry and Centre for Catalysis Research and Innovation, University of Ottawa, 10 Marie Curie Private, Ottawa, ON K1N 6N5, Canada
| | - David L. Bryce
- Department of Chemistry and Centre for Catalysis Research and Innovation, University of Ottawa, 10 Marie Curie Private, Ottawa, ON K1N 6N5, Canada
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Brinkmann A, Edén M. Central-transition double-quantum sideband NMR spectroscopy of half-integer quadrupolar nuclei: estimating internuclear distances and probing clusters within multi-spin networks. Phys Chem Chem Phys 2014; 16:7037-50. [DOI: 10.1039/c4cp00029c] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/15/2023]
Abstract
Clusters within quadrupolar spin networks are probed and internuclear distances between quadrupolar nuclei are estimated by central-transition double-quantum sideband NMR spectroscopy.
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Affiliation(s)
- Andreas Brinkmann
- Measurement Science and Standards
- National Research Council Canada
- Ottawa, Canada
| | - Mattias Edén
- Physical Chemistry Division
- Department of Materials and Environmental Chemistry
- Arrhenius Laboratory
- Stockholm University
- 106 91 Stockholm, Sweden
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Perras FA, Bryce DL. Measuring dipolar and J coupling between quadrupolar nuclei using double-rotation NMR. J Chem Phys 2013; 138:174202. [DOI: 10.1063/1.4802192] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023] Open
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Perras FA, Korobkov I, Bryce DL. NMR crystallography of sodium diphosphates: combining dipolar, shielding, quadrupolar, diffraction, and computational information. CrystEngComm 2013. [DOI: 10.1039/c3ce40875b] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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Perras FA, Korobkov I, Bryce DL. 23Na double-rotation NMR of sodium nucleotides leads to the discovery of a new dCMP hendecahydrate. Phys Chem Chem Phys 2012; 14:4677-81. [PMID: 22389051 DOI: 10.1039/c2cp40273d] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Obtaining definitive information concerning the coordination environment of sodium ions which balance the negative charges found in nucleotides is a challenging task. We show that high resolution 1D and 2D (23)Na NMR spectra of sodium nucleotides obtained in the solid state with the use of double-rotation (DOR) provide valuable structural information. Sensitive spin diffusion homonuclear correlation experiments are used to establish the relative proximities of various pairs of crystallographically distinct Na sites and to assign the spectral resonances. Additionally, the DOR sidebands are simulated to obtain coordination information which is complementary to that obtained using multiple-quantum magic-angle spinning NMR spectra. These experiments led us to discover a new hendecahydrate of deoxycytidine monophosphate (dCMP), the structure of which is confirmed via single-crystal X-ray diffraction. This hydrate crystallizes reproducibly when deuterated water is used exclusively in the preparation process.
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Affiliation(s)
- Frédéric A Perras
- Department of Chemistry and Centre for Catalysis Research and Innovation, University of Ottawa, 10 Marie Curie Private, Ottawa, Ontario, Canada
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Deters H, Eckert H. Characterization of the glass-to-vitroceramic transition in yttrium aluminum borate laser glasses using solid state NMR. SOLID STATE NUCLEAR MAGNETIC RESONANCE 2012; 41:48-59. [PMID: 22244244 DOI: 10.1016/j.ssnmr.2011.11.008] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/11/2011] [Revised: 11/14/2011] [Indexed: 05/31/2023]
Abstract
The crystallization of laser glasses in the system (Y(2)O(3))(0.2){((Al(2)O(3))(x))(B(2)O(3))(0.8-x)} (0.15 ≤ x ≤ 0.40) doped with 0.5 mol% of ytterbium oxide has been investigated by x-ray powder diffraction, and various solid state NMR techniques. The crystallization process has been analyzed in a quantitative fashion by high-resolution solid state (11)B, (27)Al, and (89)Y NMR spectroscopy as well as (11)B{(27)Al} and (27)Al{(11)B} rotational echo double resonance (REDOR) experiments. The homogeneous glasses undergo major phase segregation processes resulting in crystalline Al(5)BO(9) (historically denoted as Al(18)B(4)O(33)), YBO(3), crystalline YAl(3)(BO(3))(4), residual glassy B(2)O(3), and an additional yet not identified crystalline phase ("X-phase").
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Affiliation(s)
- Heinz Deters
- Institut für Physikalische Chemie, Graduate School of Chemistry, Westfälische Wilhelms-Universität Münster, Corrensstrasse 30, D48149 Münster, Germany
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Perras FA, Bryce DL. Removal of sidebands in double-rotation NMR in real time. JOURNAL OF MAGNETIC RESONANCE (SAN DIEGO, CALIF. : 1997) 2011; 211:234-239. [PMID: 21652243 DOI: 10.1016/j.jmr.2011.05.002] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/22/2011] [Revised: 05/04/2011] [Accepted: 05/07/2011] [Indexed: 05/30/2023]
Abstract
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.
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Affiliation(s)
- Frédéric A Perras
- Department of Chemistry and Centre for Catalysis Research and Innovation, University of Ottawa, 10 Marie Curie Private, Ottawa, Ontario, Canada K1N 6N5
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Howes AP, Vedishcheva NM, Samoson A, Hanna JV, Smith ME, Holland D, Dupree R. Boron environments in Pyrex® glass—a high resolution, Double-Rotation NMR and thermodynamic modelling study. Phys Chem Chem Phys 2011; 13:11919-28. [DOI: 10.1039/c1cp20771g] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
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Fisch M, Armbruster T, Rentsch D, Libowitzky E, Pettke T. Crystal-chemistry of mullite-type aluminoborates Al18B4O33 and Al5BO9: A stoichiometry puzzle. J SOLID STATE CHEM 2011. [DOI: 10.1016/j.jssc.2010.10.032] [Citation(s) in RCA: 38] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
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Wang Q, Hu B, Lafon O, Trébosc J, Deng F, Amoureux JP. Double-quantum homonuclear NMR correlation spectroscopy of quadrupolar nuclei subjected to magic-angle spinning and high magnetic field. JOURNAL OF MAGNETIC RESONANCE (SAN DIEGO, CALIF. : 1997) 2009; 200:251-260. [PMID: 19646906 DOI: 10.1016/j.jmr.2009.07.009] [Citation(s) in RCA: 57] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/27/2009] [Revised: 07/06/2009] [Accepted: 07/08/2009] [Indexed: 05/28/2023]
Abstract
We present a new application of the R2(2)(1) symmetry-based dipolar recoupling scheme, for exciting directly double-quantum (2Q) coherences between the central transition of homonuclear half-integer quadrupolar nuclei. With respect to previously published 2Q-recoupling methods (M. Eden, D. Zhou, J. Yu, Chem. Phys. Lett. 431 (2006) 397), the R2(2)(1) sequence is used without pi/2 bracketing pulses and with an original super-cycling. This leads to an improved efficiency (a factor of two for spin-5/2) and to a much higher robustness to radio-frequency field inhomogeneity and resonance offset. The 2Q-coherence excitation performances are demonstrated experimentally by (27)Al NMR experiments on the aluminophosphates berlinite, VPI5, AlPO(4)-14, and AlPO(4)-CJ3. The two-dimensional 2Q-1Q correlation experiments incorporating these recoupling sequences allow the observation of 2Q cross-peaks between central transitions, even at high magnetic field where the difference in offset between octahedral and tetrahedral (27)Al sites exceeds 10 kHz.
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Affiliation(s)
- Q Wang
- UCCS, CNRS-8181, Lille University, Fr-59652, Villeneuve d'Ascq, France
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Edén M. Homonuclear dipolar recoupling of half-integer spin quadrupolar nuclei: techniques and applications. SOLID STATE NUCLEAR MAGNETIC RESONANCE 2009; 36:1-10. [PMID: 19595580 DOI: 10.1016/j.ssnmr.2009.06.005] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/09/2009] [Accepted: 06/16/2009] [Indexed: 05/28/2023]
Abstract
We review recent advances in solid state NMR methodology for recovering homonuclear dipolar interactions among half-integer quadrupolar spins undergoing sample rotation. Existing dipolar recoupling techniques are contrasted, based on (i) the form of their associated dipolar Hamiltonian, (ii) the different experimental conditions necessitating their realization and (iii) their roles as components in multi-dimensional NMR correlation spectroscopy. Various types of structural information accessible from such solid state NMR experimentation is reviewed. Promises and limitations of methodologies targeting homonuclear dipolar couplings between half-integer spins under high-resolution conditions are discussed, with particular focus on the demands set for structural investigations of crystalline as well as structurally disordered (amorphous) inorganic network materials.
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Affiliation(s)
- Mattias Edén
- Physical Chemistry Division, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden.
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Determination of the bond-angle distribution in vitreous B2O3 by 11B double rotation (DOR) NMR spectroscopy. J SOLID STATE CHEM 2009. [DOI: 10.1016/j.jssc.2009.06.025] [Citation(s) in RCA: 39] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
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Hung I, Wong A, Howes AP, Anupõld T, Samoson A, Smith ME, Holland D, Brown SP, Dupree R. Separation of isotropic chemical and second-order quadrupolar shifts by multiple-quantum double rotation NMR. JOURNAL OF MAGNETIC RESONANCE (SAN DIEGO, CALIF. : 1997) 2009; 197:229-236. [PMID: 19201231 DOI: 10.1016/j.jmr.2009.01.005] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/21/2008] [Revised: 01/06/2009] [Accepted: 01/06/2009] [Indexed: 05/27/2023]
Abstract
Using a two-dimensional multiple-quantum (MQ) double rotation (DOR) experiment the contributions of the chemical shift and quadrupolar interaction to isotropic resonance shifts can be completely separated. Spectra were acquired using a three-pulse triple-quantum z-filtered pulse sequence and subsequently sheared along both the nu(1) and nu(2) dimensions. The application of this method is demonstrated for both crystalline (RbNO(3)) and amorphous samples (vitreous B(2)O(3)). The existence of the two rubidium isotopes ((85)Rb and (87)Rb) allows comparison of results for two nuclei with different spins (I=3/2 and 5/2), as well as different dipole and quadrupole moments in a single chemical compound. Being only limited by homogeneous line broadening and sample crystallinity, linewidths of approximately 0.1 and 0.2 ppm can be measured for (87)Rb in the quadrupolar and chemical shift dimensions, enabling highly accurate determination of the isotropic chemical shift and the quadrupolar product, P(Q). For vitreous B(2)O(3), the use of MQDOR allows the chemical shift and electric field gradient distributions to be directly determined-information that is difficult to obtain otherwise due to the presence of second-order quadrupolar broadening.
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Affiliation(s)
- Ivan Hung
- Physics Department, University of Warwick, Coventry, UK
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Brinkmann A, Kentgens APM, Anupõld T, Samoson A. Symmetry-based recoupling in double-rotation NMR spectroscopy. J Chem Phys 2009; 129:174507. [PMID: 19045358 DOI: 10.1063/1.3005395] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
In this contribution, we extend the theory of symmetry-based pulse sequences of types CN(n) (nu) and RN(n) (nu) in magic-angle-spinning nuclear resonance spectroscopy [M. H. Levitt, in Encyclopedia of Nuclear Magnetic Resonance, edited by D. M. Grant and R. K. Harris (Wiley, Chichester, 2002), Vol. 9]. to the case of rotating the sample simultaneously around two different angles with respect to the external magnetic field (double-rotation). We consider the case of spin-1/2 nuclei in general and the case of half-integer quadrupolar nuclei that are subjected to weak radio frequency pulses operating selectively on the central-transition polarizations. The transformation properties of the homonuclear dipolar interactions and J-couplings under central-transition-selective spin rotations are presented. We show that the pulse sequence R2(2) (1)R2(2) (-1) originally developed for homonuclear dipolar recoupling of half-integer quadrupolar nuclei under magic-angle-spinning conditions [M. Eden, D. Zhou, and J. Yu, Chem. Phys. Lett. 431, 397 (2006)] may be used for the same purpose in the case of double rotation, if the radio frequency pulses are synchronized with the outer rotation of the sample. We apply this sequence, sandwiched by central-transition selective 90 degrees pulses, to excite double-quantum coherences in homonuclear spin systems consisting of (23)Na and (27)Al nuclei.
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Affiliation(s)
- Andreas Brinkmann
- Physical Chemistry/Solid State NMR, Institute for Molecules and Materials, Radboud University Nijmegen, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands.
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Thrippleton MJ, Ball TJ, Wimperis S. Satellite transitions acquired in real time by magic angle spinning (STARTMAS): ‘‘Ultrafast’’ high-resolution MAS NMR spectroscopy of spin I=3∕2 nuclei. J Chem Phys 2008; 128:034507. [DOI: 10.1063/1.2813892] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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Lo AYH, Edén M. Efficient symmetry-based homonuclear dipolar recoupling of quadrupolar spins: double-quantum NMR correlations in amorphous solids. Phys Chem Chem Phys 2008; 10:6635-44. [DOI: 10.1039/b808295b] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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Hung I, Wong A, Howes AP, Anupõld T, Past J, Samoson A, Mo X, Wu G, Smith ME, Brown SP, Dupree R. Determination of NMR interaction parameters from double rotation NMR. JOURNAL OF MAGNETIC RESONANCE (SAN DIEGO, CALIF. : 1997) 2007; 188:246-59. [PMID: 17707665 DOI: 10.1016/j.jmr.2007.07.009] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/30/2007] [Revised: 07/18/2007] [Accepted: 07/19/2007] [Indexed: 05/16/2023]
Abstract
It is shown that the anisotropic NMR parameters for half-integer quadrupolar nuclei can be determined using double rotation (DOR) NMR at a single magnetic field with comparable accuracy to multi-field static and MAS experiments. The (17)O nuclei in isotopically enriched l-alanine and OPPh(3) are used as illustrations. The anisotropic NMR parameters are obtained from spectral simulation of the DOR spinning sideband intensities using a computer program written with the GAMMA spin-simulation libraries. Contributions due to the quadrupolar interaction, chemical shift anisotropy, dipolar coupling and J coupling are included in the simulations. In l-alanine the oxygen chemical shift span is 455 +/- 20 ppm and 350 +/- 20 ppm for the O1 and O2 sites, respectively, and the Euler angles are determined to an accuracy of +/- 5-10 degrees . For cases where effects due to heteronuclear J and dipolar coupling are observed, it is possible to determine the angle between the internuclear vector and the principal axis of the electric field gradient (EFG). Thus, the orientation of the major components of both the EFG and chemical shift tensors (i.e., V(33) and delta(33)) in the molecular frame may be obtained from the relative intensity of the split DOR peaks. For OPPh(3) the principal axis of the (17)O EFG is found to be close to the O-P bond, and the (17)O-(31)P one-bond J coupling ((1)J(OP)=161 +/- 2 Hz) is determined to a much higher accuracy than previously.
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Affiliation(s)
- I Hung
- Physics Department, University of Warwick, Coventry, UK
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