Quadrupolar coupling

MQCs are not excited uniformly and the efficiency with which the various orders of MQC are excited depends specifically on the parameters of the spin system (dipolar couplings, scalar couplings, quadrupolar couplings, chemical shifts) in the spin system and the choice of the preparation time t. Many researchers have co-added spectra acquired with different preparation times to ensure that all transitions are observed with reasonable intensity. A number of broadband excitation techniques have been developed,13-15 where the value of t in the preparation sequence has been varied either in a pseudo-random or systematic fashion to achieve a more uniform excitation in the multiple quantum domain. An experimental search method has been used to optimise the delays in the preparation period of the MQ excitation sequence16 and Wimperis17 used average Hamiltonian theory to propose... 

NMR-SIM is very powerful simulation tool based on a density matrix approach and is designed to make low demands on computer resources. The spin system parameters consist of a small basic set of spin parameters chemical shift, weak/strong scalar J coupling, dipolar coupling, quadrupolar coupling, longitudinal and transverse relaxation time. Currently the calculated coherence transfer processes are limited to polarization transfer and cross polarization. 

Relaxation in Coupled Spin Systems. - The ability to transfer coherences between different nuclei in homo- and heteronuclear spin systems is one of the cornerstones of NMR. The transfer of polarization from I = (1/2) to enhance the signal of low-gyromagnetic ratio S = (1/2) nuclei has become routine. In solution state the majority of these methods employ free-precession type techniques such as INEPT where polarization transfer from one spin to another is mediated via the scalar coupling. The majority of these techniques are designed for spin-half systems and have shown particular utility in the study of structure and dynamics of biomolecules labeled with C and N. The use of INEPT for achieving polarization transfer in systems of scalar coupled quadrupolar nuclei has also been investigated. ... 

The residual dipolar coupling also manifests itself in NMR spectra of dipolar-coupled quadrupolar spin pairs.Wu and Yamada also observed residual dipolar coupling attributed to the N- B dipolar interaction in the NMR spectra of MAS samples of triethanolamine borate. More recently, the effect of the N- B, "B- B, and Mn- Mn residual dipolar interactions on the B and Mn NMR line shapes acquired through MQMAS were also demonstrated. ... 

As mentioned in Sec. LA, interactions other than isotropic chemical shifts and scalar couplings dominate the NMR of solids. If a high-resolution spectrum is desired, these other effects must be deliberately removed molecular motion alone will not average them during the course of the measurement. Alternatively, experiments have been designed which isolate these interactions—dipolar coupling, quadrupolar coupling (in appropriate nuclei), and chemical-shift anisotropy—and exploit their useful features to provide information about the system under study. 

Champmartin D and Rubini P 1996 Determination of the 0-17 quadrupolar coupling constant and of the C-13 chemical shielding tensor anisotropy of the CO groups of pentane-2,4-dione and beta-diketonate complexes in solution. NMR relaxation study/norg. Chem. 35 179-83... 

The signal from H(3) is broadened because N-H(3) spin coupling is not completely washed out by quadrupolar relaxation of the nitrogen atom (68MI41701>. This broadening niay be useful in assigning signals in the spectrum and it is usually reduced in solvents of... 

The moment of inertia 1 determines the rotational constant 0 = h /IT, which is the parameter that controls the strength of quantum effects. The other parameter of the model, which is the quadrupolar coupling constant K, can be conveniently taken as the energy and temperature scale. We can thus reduce all quantities related to energies by K, and define, e.g., the dimensionless temperature = k T/K, energy = E/K, and rotational... 

Instead of measuring only the time-dependent dipolar interaction via NOE, it is also possible to determine dipolar couplings directly if the solute molecule is partially aligned in so-called alignment media. The most important resulting anisotropic parameters are RDCs, but residual quadrupolar couplings (RQCs), residual chemical shift anisotropy (RCSA) and pseudo-contact shifts (PCSs) can also be used for structure determination if applicable. 

In the solid, dynamics occurring within the kHz frequency scale can be examined by line-shape analysis of 2H or 13C (or 15N) NMR spectra by respective quadrupolar and CSA interactions, isotropic peaks16,59-62 or dipolar couplings based on dipolar chemical shift correlation experiments.63-65 In the former, tyrosine or phenylalanine dynamics of Leu-enkephalin are examined at frequencies of 103-104 Hz by 2H NMR of deuterated samples and at 1.3 x 102 Hz by 13C CPMAS, respectively.60-62 In the latter, dipolar interactions between the 1H-1H and 1H-13C (or 3H-15N) pairs are determined by a 2D-MAS SLF technique such as wide-line separation (WISE)63 and dipolar chemical shift separation (DIP-SHIFT)64,65 or Lee-Goldburg CP (LGCP) NMR,66 respectively. In the WISE experiment, the XH wide-line spectrum of the blend polymers consists of a rather featureless superposition of components with different dipolar widths which can be separated in the second frequency dimension and related to structural units according to their 13C chemical shifts.63... 

Fundamental constants (Cx), spatial tensors in the principal axis frame ((fi3 m,)F), and spin tensors (Tjm) for chemical shielding (a), J coupling (J), dipole-dipole (IS), and quadrupolar coupling (Q) nuclear spin interactions (for more detailed definition of symbols refer to [50])... 

Separation of the Quadrupolar Couplings from Other Interactions. 163... 

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