The Experts below are selected from a list of 279 Experts worldwide ranked by ideXlab platform
Hans J. Jakobsen - One of the best experts on this subject based on the ideXlab platform.
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Crystal Structure Studies by Single-Crystal NMR Spectroscopy. 71Ga and 69Ga Single-Crystal NMR of β-Ga2O3 Twins
Journal of the American Chemical Society, 1998Co-Authors: Thomas Vosegaard, Inger Pihl Byriel, Laurent Binet, Dominique Massiot, Hans J. JakobsenAbstract:71Ga and 69Ga single-crystal NMR spectra of a β-Ga2O3 twin have been analyzed in terms of the 71Ga/69Ga quadrupole coupling and Chemical Shielding Anisotropy (CSA) interactions. Analyses of the magnitudes and orientations of the interaction tensors for the two distinct crystallographic gallium sites in the two twin crystals resulted in accurate values for the parameters describing the quadrupole coupling and CSA tensors. Moreover, it has been possible to determine the actual twin-law of our sample crystal. Along with a recent 71Ga single-crystal NMR investigation of YGG, this work shows that gallium CSA generally has to be taken into account in addition to the dominant quadrupole coupling interaction.
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51V Chemical Shielding and Quadrupole Coupling in Ortho- and Metavanadates from 51V MAS NMR Spectroscopy
Inorganic Chemistry, 1998Co-Authors: J. Skibsted, And Claus J. H. Jacobsen, Hans J. JakobsenAbstract:The combined effect from the 51V quadrupole coupling and Chemical Shielding Anisotropy (CSA) has been characterized for five orthovanadates (Mg3(VO4)2, Zn3(VO4)2, BiVO4, TaVO5, NbVO5) and two metavanadates (RbVO3 and CsVO3) employing 51V magic-angle spinning (MAS) NMR of the central and satellite transitions. Furthermore, five metavanadates studied earlier (MVO3 where M = Li, Na, NH4, Tl, K) have been reinvestigated using rotor-stabilized MAS NMR, which improves the reliability of the spinning sideband (ssb) intensities. The complete manifolds of ssbs from all seven single-quantum transitions, observed in the MAS NMR spectra, have been analyzed using least-squares fitting and numerical error analysis. The optimized data demonstrate that for the metavanadates the magnitudes of the quadrupole and Shielding Anisotropy tensors (i.e., CQ, ηQ, δσ, ησ) and of the Euler angle, which relates the principal element of the two tensors, can be determined with high precision. Somewhat larger error limits are observed f...
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1H Chemical Shielding Anisotropies from Polycrystalline Powders Using MSHOT-3 Based CRAMPS
Journal of magnetic resonance (San Diego Calif. : 1997), 1998Co-Authors: M. Hohwy, Hans J. Jakobsen, Jimmy Tønners Rasmussen, P.v. Bower, Niels Christian NielsenAbstract:Abstract It is demonstrated that combined rotation and multiple-pulse spectroscopy (CRAMPS) based on MSHOT-3 homonuclear multiple-pulse decoupling represents a powerful method for determination of 1 H Chemical Shielding anisotropies from polycrystalline powders. By virtue of high-order dipolar decoupling, large spectral width, resonance offset stability, and the absence of artifacts fromtilted-axis precession, MSHOT-3-based CRAMPS enables straightforward sampling of high-quality spectra. Comparison with explicit calculations, taking the effect of the multiple-pulse sequence into account, shows that the spectra may be simulated and iteratively fitted using standard software for the calculation of magic-angle spinning spectra influenced by Chemical Shielding Anisotropy with the Shielding interaction reduced by the scaling factor of the MSHOT-3 decoupling sequence. The method is demonstrated by experimental determination of 1 H Chemical Shielding anisotropies for adipic acid, Ca(OH) 2 , malonic acid, and KHSO 4 . The data are compared with those determined previously from single-crystal NMR studies.
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27Al Chemical Shielding Anisotropy
Journal of Magnetic Resonance, 1997Co-Authors: Thomas Vosegaard, Hans J. JakobsenAbstract:Abstract Accurate values for the 27 Al Chemical Shielding Anisotropy (CSA) are reported for sapphire (α-Al 2 O 3 ). The values (δ σ = −17.3 ± 0.6 ppm, η σ = 0.03 ± 0.06) are obtained from single-crystal 27 Al NMR and appear to be the first convincing determination of an 27 Al CSA.
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Characterization of the Two Rubidium Sites in Rb2CrO4 by 87Rb Single-Crystal NMR
The Journal of Physical Chemistry B, 1997Co-Authors: Thomas Vosegaard, Inger Pihl Byriel, Hans J. JakobsenAbstract:The two crystallographically distinct sites in Rb2CrO4 are characterized in terms of their 87Rb quadrupole coupling and Chemical Shielding Anisotropy (CSA) interactions employing 87Rb single-crystal NMR. Accurate values for the quadrupole coupling and CSA parameters are reported for both rubidium sites along with the direction cosines describing the orientation of the tensors for the NMR interactions with respect to the Rb2CrO4 crystal frame. The parameters for site 1 represent the first accurate investigation of this site, while the parameters for site 2 compare excellently with those previously reported. Some remarkable similarities are observed between the present parameters for Rb2CrO4 and the 133Cs parameters determined from recent magic-angle spinning and single-crystal 133Cs NMR studies for the isomorphous Cs2CrO4.
Thomas Vosegaard - One of the best experts on this subject based on the ideXlab platform.
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Crystal Structure Studies by Single-Crystal NMR Spectroscopy. 71Ga and 69Ga Single-Crystal NMR of β-Ga2O3 Twins
Journal of the American Chemical Society, 1998Co-Authors: Thomas Vosegaard, Inger Pihl Byriel, Laurent Binet, Dominique Massiot, Hans J. JakobsenAbstract:71Ga and 69Ga single-crystal NMR spectra of a β-Ga2O3 twin have been analyzed in terms of the 71Ga/69Ga quadrupole coupling and Chemical Shielding Anisotropy (CSA) interactions. Analyses of the magnitudes and orientations of the interaction tensors for the two distinct crystallographic gallium sites in the two twin crystals resulted in accurate values for the parameters describing the quadrupole coupling and CSA tensors. Moreover, it has been possible to determine the actual twin-law of our sample crystal. Along with a recent 71Ga single-crystal NMR investigation of YGG, this work shows that gallium CSA generally has to be taken into account in addition to the dominant quadrupole coupling interaction.
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27Al Chemical Shielding Anisotropy
Journal of Magnetic Resonance, 1997Co-Authors: Thomas Vosegaard, Hans J. JakobsenAbstract:Abstract Accurate values for the 27 Al Chemical Shielding Anisotropy (CSA) are reported for sapphire (α-Al 2 O 3 ). The values (δ σ = −17.3 ± 0.6 ppm, η σ = 0.03 ± 0.06) are obtained from single-crystal 27 Al NMR and appear to be the first convincing determination of an 27 Al CSA.
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Characterization of the Two Rubidium Sites in Rb2CrO4 by 87Rb Single-Crystal NMR
The Journal of Physical Chemistry B, 1997Co-Authors: Thomas Vosegaard, Inger Pihl Byriel, Hans J. JakobsenAbstract:The two crystallographically distinct sites in Rb2CrO4 are characterized in terms of their 87Rb quadrupole coupling and Chemical Shielding Anisotropy (CSA) interactions employing 87Rb single-crystal NMR. Accurate values for the quadrupole coupling and CSA parameters are reported for both rubidium sites along with the direction cosines describing the orientation of the tensors for the NMR interactions with respect to the Rb2CrO4 crystal frame. The parameters for site 1 represent the first accurate investigation of this site, while the parameters for site 2 compare excellently with those previously reported. Some remarkable similarities are observed between the present parameters for Rb2CrO4 and the 133Cs parameters determined from recent magic-angle spinning and single-crystal 133Cs NMR studies for the isomorphous Cs2CrO4.
Richard R. Ernst - One of the best experts on this subject based on the ideXlab platform.
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EFFECTS OF DYNAMICS AND ENVIRONMENT ON 15N Chemical Shielding Anisotropy IN PROTEINS. A COMBINATION OF DENSITY FUNCTIONAL THEORY, MOLECULAR DYNAMICS S IMULATION, AND NMR RELAXATION
Journal of the American Chemical Society, 1999Co-Authors: Christoph Scheurer, Nikolai R. Skrynnikov, S. F. Lienin, S. K. Straus, Rafael Brüschweiler, Richard R. ErnstAbstract:The interpretation of nuclear spin relaxation data of biomolecules often requires the accurate knowledge of Chemical Shielding Anisotropy (CSA) tensors, which significantly depend on the environmen...
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Orientation of the Chemical Shielding Anisotropy Tensor of the Carbonate Carbon in Diphenyl Carbonate and Its Consequences for NMR Studies on Polycarbonate
Macromolecules, 1998Co-Authors: P. Robyr, Marcel Utz, Z. Gan, Christoph Scheurer, M. Tomaselli, Ulrich W. Suter, Richard R. ErnstAbstract:The orientation of the Chemical Shielding Anisotropy tensor of the carbonate carbon in diphenyl carbonate was determined by using solid state NMR spectroscopy. The same tensor orientation was obtained from quantum mechanical calculation based on density functional theory. The most shielded direction of the tensor is along the CO double bond. The least shielded direction is for the carbonate plane perpendicular to the CO double bond. Chemical Shielding calculations in diphenyl carbonate fragments with different conformations show that the tensor orientation measured in crystalline diphenyl carbonate also applies to the carbonate groups of bisphenol A polycarbonate. However, this orientation is not identical to that assumed so far in the work on bisphenol A polycarbonate. The consequences of this incorrect assumption in previous studies are examined.
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Quantification of conformational disorder in glassy polycarbonate by two-dimensional nuclear magnetic resonance spectroscopy
Molecular Physics, 1996Co-Authors: M. Tomaselli, P. Robyr, Richard R. Ernst, Beat H. Meier, C. Grob-pisano, Ulrich W. SuterAbstract:We have investigated the localchain conformation in amorphous bisphenol-A polycarbonate at the carbonate–phenylene fragment by a solid-state nuclear magnetic resonance study. Chemical-Shielding Anisotropy (CSA) powder patterns have been analysed, and their sensitivity to local conformation is discussed. CSA–dipole tensor correlation experiments on isotopically enriched samples were analysed in terms of dihedral-angle distributions. The comparison with atomistic simulations allowed for a critical test of the glass model and provided information for the improvement and refinement of the model.
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Molecular dynamics monitored by cross‐correlated cross relaxation of spins quantized along orthogonal axes
The Journal of Chemical Physics, 1992Co-Authors: Rafael Brüschweiler, Richard R. ErnstAbstract:A new approach is proposed for the analysis of molecular dynamics on a wide range of time scales by the measurement of specific cross‐correlated nuclear magnetic resonance cross‐relaxation pathways of spins quantized along orthogonal spin‐lock axes. Cross‐correlated time modulation of (i) Chemical shifts and J couplings, and of (ii) dipolar coupling and Chemical Shielding Anisotropy, respectively, are determined using a sensitive technique, called ortho‐ROESY (orthogonal rotating frames Overhauser effect spectroscopy). The two processes (i) and (ii) can be separated experimentally by their distinct dependence on the spin‐lock radio frequency amplitude. The relaxation rate constant depends on the power spectral density sampled at a frequency close to zero. This renders the experiment attractive for investigations of slowly tumbling (bio)molecules.
Rafael Brüschweiler - One of the best experts on this subject based on the ideXlab platform.
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Reconstruction of interatomic vectors by principle component analysis of nuclear magnetic resonance data in multiple alignments
The Journal of Chemical Physics, 2002Co-Authors: Jean-christophe Hus, Rafael BrüschweilerAbstract:A general method is presented for the reconstruction of interatomic vector orientations from nuclear magnetic resonance (NMR) spectroscopic data of tensor interactions of rank 2, such as dipolar coupling and Chemical Shielding Anisotropy interactions, in solids and partially aligned liquid-state systems. The method, called PRIMA, is based on a principal component analysis of the covariance matrix of the NMR parameters collected for multiple alignments. The five nonzero eigenvalues and their eigenvectors efficiently allow the approximate reconstruction of the vector orientations of the underlying interactions. The method is demonstrated for an isotropic distribution of sample orientations as well as for finite sets of orientations and internuclear vectors encountered in protein systems.
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EFFECTS OF DYNAMICS AND ENVIRONMENT ON 15N Chemical Shielding Anisotropy IN PROTEINS. A COMBINATION OF DENSITY FUNCTIONAL THEORY, MOLECULAR DYNAMICS S IMULATION, AND NMR RELAXATION
Journal of the American Chemical Society, 1999Co-Authors: Christoph Scheurer, Nikolai R. Skrynnikov, S. F. Lienin, S. K. Straus, Rafael Brüschweiler, Richard R. ErnstAbstract:The interpretation of nuclear spin relaxation data of biomolecules often requires the accurate knowledge of Chemical Shielding Anisotropy (CSA) tensors, which significantly depend on the environmen...
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Cross-correlation-induced J coupling
Chemical Physics Letters, 1996Co-Authors: Rafael BrüschweilerAbstract:Abstract A dynamic frequency shift for spin 1 2 systems is described, which originates from dipolar and Chemical Shielding Anisotropy cross-correlation, and which is manifested as an additive contribution to the apparent scalar J coupling constant. Its characteristic dependence on molecular motion and magnetic field strength may serve as a complementary source of dynamic information.
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Molecular dynamics monitored by cross‐correlated cross relaxation of spins quantized along orthogonal axes
The Journal of Chemical Physics, 1992Co-Authors: Rafael Brüschweiler, Richard R. ErnstAbstract:A new approach is proposed for the analysis of molecular dynamics on a wide range of time scales by the measurement of specific cross‐correlated nuclear magnetic resonance cross‐relaxation pathways of spins quantized along orthogonal spin‐lock axes. Cross‐correlated time modulation of (i) Chemical shifts and J couplings, and of (ii) dipolar coupling and Chemical Shielding Anisotropy, respectively, are determined using a sensitive technique, called ortho‐ROESY (orthogonal rotating frames Overhauser effect spectroscopy). The two processes (i) and (ii) can be separated experimentally by their distinct dependence on the spin‐lock radio frequency amplitude. The relaxation rate constant depends on the power spectral density sampled at a frequency close to zero. This renders the experiment attractive for investigations of slowly tumbling (bio)molecules.
J. Skibsted - One of the best experts on this subject based on the ideXlab platform.
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51V Chemical Shielding and Quadrupole Coupling in Ortho- and Metavanadates from 51V MAS NMR Spectroscopy
Inorganic Chemistry, 1998Co-Authors: J. Skibsted, And Claus J. H. Jacobsen, Hans J. JakobsenAbstract:The combined effect from the 51V quadrupole coupling and Chemical Shielding Anisotropy (CSA) has been characterized for five orthovanadates (Mg3(VO4)2, Zn3(VO4)2, BiVO4, TaVO5, NbVO5) and two metavanadates (RbVO3 and CsVO3) employing 51V magic-angle spinning (MAS) NMR of the central and satellite transitions. Furthermore, five metavanadates studied earlier (MVO3 where M = Li, Na, NH4, Tl, K) have been reinvestigated using rotor-stabilized MAS NMR, which improves the reliability of the spinning sideband (ssb) intensities. The complete manifolds of ssbs from all seven single-quantum transitions, observed in the MAS NMR spectra, have been analyzed using least-squares fitting and numerical error analysis. The optimized data demonstrate that for the metavanadates the magnitudes of the quadrupole and Shielding Anisotropy tensors (i.e., CQ, ηQ, δσ, ησ) and of the Euler angle, which relates the principal element of the two tensors, can be determined with high precision. Somewhat larger error limits are observed f...
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Magnitudes and relative orientation of vanadium-51 quadrupole coupling and anisotropic Shielding tensors in metavanadates and potassium vanadium oxide (KV3O8) from vanadium-51 MAS NMR spectra. Sodium-23 quadrupole coupling parameters for .alpha.- and
Journal of the American Chemical Society, 1993Co-Authors: J. Skibsted, Niels Chr. Nielsen, Henrik Bildsøe, Hans J. JakobsenAbstract:51 V MAS NMR spectra, exhibiting the complete manifold of spinning sidebands (ssb's) from all seven 51 V (I=7/2) single-quantum transitions, have been obtained for a series of metavanadates (MVO 3 , M=Li, Na, K, NH 4 , Tl) and for KV 3 O 8 employing spectral windows up to 2 MHz. These MAS NMR spectra are influenced by interactions from both the 51 V quadrupole coupling and Chemical Shielding Anisotropy. Analysis of the spectra has been performed in terms of the two interactions employing a newly developed method requiring simulations and iterative fitting of the calculated to the experimental intensities of the spinning sidebands
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51V MAS NMR spectroscopy: determination of quadrupole and anisotropic Shielding tensors, including the relative orientation of their principal-axis systems
Chemical Physics Letters, 1992Co-Authors: J. Skibsted, Niels Chr. Nielsen, Henrik Bildsøe, Hans J. JakobsenAbstract:Abstract 51 V MAS NMR spectra exhibiting the complete manifold of spinning sidebands (SSBs) from all seven 51 V ( I =7/2) transitions have been observed for NH 4 VO 3 and V 2 O 5 . These spectra are influenced by interactions from both 51 V quadrupole coupling and Chemical-Shielding Anisotropy. A simulation and iterative-fitting program which allows the first determination of quadrupole and anisotropic Shielding tensors, including the relative orientation of their principal-axis systems, from MAS NMR spectra is introduced. The 51 V parameters determined from the SSB intensities for NH 4 VO 3 and V 2 O 5 are compared with literature data from static NMR spectra.