Pseudo dipolar coupling

Note that this has identical spin operators to those involving in Equation [31]. This means that the spectra of anisotropic systems depend on (2D y + / ), and that these two interactions cannot be determined separately from the spectra. For this reason, Jfj is often referred to a pseudo-dipolar coupling. 

The direct dipolar coupling, and the anisotropic component of the indirect spin-spin interaction in the direction of the magnetic field, appear in the Hamiltonian in the same form. The splittings that arise in NMR spectra are the sum of these two quantities, called the experimental anisotropic coupling Ty= 2Dyzz + 7 )- It is therefore important to know experimentally, or to evaluate theoretically, the values of the pseudo-dipolar coupling in order to accurately determine Dy z, which is related... 

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. 

Perhaps the most detailed studies of both the isotropic and anisotropic exchange interactions have been made for InP, where the anisotropic (pseudo-dipolar) part of the 31P-115In exchange coupling was shown to reduce the measured 31P second moments from theoretical values calculated from dipolar couplings alone [255, 256]. Double and triple resonance 31P MAS-NMR experiments have also provided much information as a result of the existence of the two nearly identical isotopes of... 

One of the NMR experiments most commonly used to measure the CaHa dipolar coupling is a J-modulated pseudo-three-dimensional version of the constant time 13C-HSQC [48, 49]. The J coupling values are extracted by using time-domain fitting in the third dimension. Since spectral overlap is very common in 13C-HSQC of proteins, sometimes only a few... 

Bloembergen and Rowland (78) have also shown that associated with the exchange interaction is a pseudo-dipolar interaction, which as the name implies, has the same functional form as the dipolar interaction. This interaction arises from the presence of the electron-coupled nuclear spin interaction and the dipole-dipole interaction and its magnitude is dependent on the relative amount of p- or d-character of the electronic wave functions in the solid. 

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... 

RDCs belong to the so-called anisotropic NMR parameters which cannot be observed in isotropically averaged samples as, for example, is the case in liquids. Besides RDCs, a number of other anisotropic parameters can be used for structure elucidation, like residual chemical shift anisotropy, residual quad-rupolar couplings for spin-1 nuclei, or pseudo-contact shifts in paramagnetic samples. Here, we will focus on RDCs where we give a brief introduction into the dipolar interaction, then into the averaging effects with the description by the alignment tensor and concepts to deal with the flexibility of molecules. For the other anisotropic NMR parameters, we refer the reader to ref 19 for an introduction and to refs. 6-8 for a detailed description. 

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