The Dipolar-Splitting-Ratio Method - A Convenient Approach to the Analysis of Dipolar-Chemical-Shift NMR Spectra of Static Powder Samples
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Principal axis theorem
Residual dipolar coupling
Magnetic dipole–dipole interaction
Principal value
Coupling constant
Chemical shift
The calculation of residual dipolar coupling effects between (observed) spin-1/2 nuclei and quadrupolar neighbours in magic angle spinning NMR spectra may face complications if the nuclei involved have negative gyromagnetic ratios. A procedure is described with which these cases can be conveniently handled in order to avoid potentially confusing situations. Specific literature examples, involving (29Si, 14N) residual dipolar coupling effects in Si,N-containing compounds, are discussed.
Magic angle spinning
Residual dipolar coupling
Carbon-13 NMR satellite
Magnetic dipole–dipole interaction
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J-coupling
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Measurement of distances from dipolar couplings is essential for structural characterization, refinement and validation using the solid-state nuclear magnetic resonance (ssNMR) spectroscopy. Particularly, knowledge about NH dipolar interactions in biological solids is important for understanding the hydrogen (H)-bonding interactions, molecular geometry and spin dynamics. In this regard, we have proposed a proton-detected two-dimensional (2D) 15N-1H dipolar coupling/1H chemical shift correlation experiment using the C-symmetry based windowless recoupling of chemical shift anisotropy (ROCSA) in combination with the DIPSHIFT pulse-based method for the measurement of short NH distances in the isotopically labeled and naturally abundant biological solids at fast magic angle spinning (MAS) rates (40–70 kHz). Our proposed method results in undistorted recoupled 15N-1H dipolar coupling powder lineshapes that are free from the recoupled 1H CSA contributions under the 15N evolution, a feature that is essential for the measurement of NH distances with improved accuracy (± 500 Hz in terms of the NH dipolar couplings). The pulse sequence developed in the present study is also insensitive to the 1H–1H homonuclear dipolar interactions, relaxation effects owing to its constant-time implementation, and t1-noise from the fluctuations in the MAS.
Magnetic dipole–dipole interaction
Homonuclear molecule
Residual dipolar coupling
Chemical shift
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Magic angle spinning
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The use of NMR to characterize structure and dynamics in solids is becoming increasingly important. However, most spin-active nuclei are beset by anisotropic quadrupolar interactions which induce substantial broadening in NMR spectra of powders, leading to overlapping peaks which obscure crucial information such as quadrupole coupling constants, chemical shifts and J-couplings. Recent advances in this area have made it possible to obtain isotropic NMR spectra from which these data are available. The most powerful of these is multiple-quantum magic-angle spinning (MQMAS), a unique feature of which is that splittings due to J-coupling are, in some cases, amplified. This phenomenon is demonstrated by the direct observation of 1J(63/65Cu, 13C) in K3Cu(CN)4 and 1J( 11B,31P) in (PhO)3P-BH3 in NMR spectra of the quadrupolar nucleus. Theoretical considerations show that quadrupole-induced residual dipolar distortions do not influence MQMAS spectra, but may introduce differential dipolar splittings in single-quantum satellite transitions. In many situations, however, quadrupolar interactions are simply too large to be monitored directly by NMR. While their spectroscopic observation is the domain of nuclear quadrupole resonance, such nuclei can influence the appearance of NMR spectra of nearby, dipolar-coupled nuclei. For small to moderate quadrupolar interactions, first-order perturbation theory has been successful in describing these effects, but larger quadrupolar interactions demand a more rigorous treatment. The NMR lineshape of a spin-1/2 nucleus coupled to a spin-3/2 nuclide is analyzed by employing full-matrix diagonalization of the combined Zeeman-quadrupolar Hamiltonian operator for cases of axial symmetry. A highlight of this treatment is that the effective dipolar coupling constant may be obtained in favourable circumstances. In a bis(tribenzylphosphine)cuprate salt, its precise measurement facilitates an unambiguous determination of the anisotropic 63/65Cu, 31P J tensor. A careful application of this theory to NMR spectra of solid copper(I) cyanide results in direct structural insight which has not been accessible by more conventional means.
Magnetic dipole–dipole interaction
Residual dipolar coupling
Carbon-13 NMR satellite
Magic angle spinning
J-coupling
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Residual dipolar coupling
Magnetic dipole–dipole interaction
Magic angle spinning
Axial symmetry
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Principal axis theorem
Residual dipolar coupling
Magnetic dipole–dipole interaction
Principal value
Coupling constant
Chemical shift
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Biaxial nematic
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We report 133Cs nuclear magnetic resonance (NMR) measurements of magnetic insulator Cs2CuCl4 in the paramagnetic phase (T≥4.2 K) as a function of the orientation of an applied magnetic field with respect to the principal crystalline axes. The magnetic shift tensor is determined. It is found that its principal axes do not coincide with the principal axes of the crystal. The Cu–Cs dipolar interaction tensor is calculated as well. From these, we deduce the full transferred hyperfine tensor for the two inequivalent Cs sites of the unit cell. We find that the tensors are anisotropic, containing non-zero off-diagonal terms, and that the transferred hyperfine coupling between Cu electronic spins and Cs nuclei dominates the NMR shift on both Cs sites.
Principal axis theorem
Principal value
Magnetic dipole–dipole interaction
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Homonuclear molecule
Magnetic dipole–dipole interaction
Heteronuclear molecule
Residual dipolar coupling
Magic angle spinning
Magic angle
Pulse sequence
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Homonuclear molecule
Magnetic dipole–dipole interaction
Residual dipolar coupling
Pulse sequence
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We show that 13C–1H dipolar couplings in fully protonated organic solids can be measured by applying a Symmetry-based Resonance-Echo DOuble-Resonance (S-REDOR) experiment at ultra-fast Magic-Angle Spinning (MAS). The 13C–1H dipolar couplings are recovered by using the R1253 recoupling scheme, while the interference of 1H–1H dipolar couplings are suppressed by the symmetry properties of this sequence and the use of high MAS frequency (65 kHz). The R1253 method is especially advantageous for large 13C–1H dipolar interactions, since the dipolar recoupling time can be incremented by steps as short as one rotor period. This allows a fine sampling for the rising part of the dipolar dephasing curve. We demonstrate experimentally that one-bond 13C–1H dipolar coupling in the order of 22 kHz can be accurately determined. Furthermore, the proposed method allows a rapid evaluation of the dipolar coupling by fitting the S-REDOR dipolar dephasing curve with an analytical expression.
Magnetic dipole–dipole interaction
Residual dipolar coupling
Dephasing
Heteronuclear molecule
Magic angle spinning
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