Dipolar coupling parameter

Expansion of the Boltzmann Distribution in the Dipolar Coupling Parameter... 

The value of the dipolar coupling parameter hd [Eq. (3.17)] determines the strength of the dipolar interaction, but the width of the distribution of energy barriers is equally important for the dynamic properties. For example, Jonsson... 

The values of the parameters that characterize the runs are in reduced units, r = 1.79 and /i = 1.35. As a consequence, the value of the dipolar coupling parameter y = n p/kgT is smaller than that used by Bossis et al. (0.58 here, vs. 1 and 2) and should lead to a lower dielectric constant e more representative of nonassociated polar liquids. 

Thus, subsequent Fourier transformation in l yields a spinning sideband pattern, with sidebands separated by coR and intensity distribution determined by d. Simulation of the sideband pattern then yields d, the dipolar coupling constant, or in a mobile system undergoing rapid uniaxial motion, <4ff, where de is the motionally averaged dipolar coupling parameter, given for uniaxial motion by... 

It is the interpretation of the parameters Dq that yields information on molecular geometry. The direct dipolar coupling parameter of nuclei i and / of a molecule can be written as an average over all molecular motions, including the vibrations and tumbling of the molecule... 

Figure 3-14 shows simulated SC-2D NMR spectra for the a methyl protons and the 2,3,5,6 ring protons, both of which were calculated as a 7 spin system consisting of the a methyl protons and the 2,3,5,6 ring protons, with effective dipolar coupling parameters of = -3410, = 400, = 0, A32 = 3250, A35 = -400, and... 

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. 

Rotational-echo double-resonance (REDOR)(75,79) is a new solid-state NMR technique which is sensitive to through-space carbon-nitrogen interactions between selectively 13C and 15N-enriched sites separated by up to 5A (20-22). The parameter directly measured in a REDOR experiment is the heteronuclear dipolar coupling constant DCN, which is in itself proportional to the inverse third power of the intemuclear distance, rCN. It is this dependence on (icn)3 which accounts both for REDOR s ability to accurately measure short distances and its insensitivity to longer-range interactions. As a technique which can probe, in detail, intermolecular interactions over a distance range of 5A, REDOR is well suited to studying the distribution of small selectively-labeled molecules in polymer delivery systems. 

The estimated correlation times for the loop domains of the order of 10 4 s are obtained for the suppressed peaks in the [l-13C]amino-acid-labelled bR, including Gly, Ala, and Leu residues as shown in Figure 24C. The loop dynamics can be also examined by measurements of the 13C-1H dipolar couplings by DIPSHIFT experiment in which fluctuations of the Co,-Cp vector result in additional motional averaging as order parameters, in addition to the rotation of Ala methyl groups which scales the dipolar... 

The isotropic chemical shift, the trace of the chemical shift tensor, is one of the basic NMR parameters often measured for both spin-1/2 and quadrupolar nuclei. The CSA can also be measured in non-cubic environments, such as the n3Cd nuclei experience in the chalcopyrite structure of crystalline CdGeAs2 [141] or CdGeP2 [142], and the 31P nuclei in the latter compound [142], Although the isotropic chemical shift can be measured from the NMR spectrum of a static powder because the CSA is zero in many cases because of cubic symmetry of the lattice, improved resolution is obtained by using MAS to remove dipolar couplings. Two particular areas where the isotropic chemical shifts have proven very informative will now be discussed, semiconductor alloys and semiconductor polytypes. 

When performing 2D-NMR experiments one must keep in mind that the second frequency dimension (Fx) is digitized by the number of tx increments. Therefore, it is important to consider the amount of spectral resolution that is needed to resolve the correlations of interest. In the first dimension (F2), the resolution is independent of time relative to F. The only requirement for F2 is that the necessary number of scans is obtained to allow appropriate signal averaging to obtain the desired S/N. These two parameters, the number of scans acquired per tx increment and the total number of tx increments, are what dictate the amount of time required to acquire the full 2D-data matrix. 2D-homo-nuclear spectroscopy can be summarized by three different interactions, namely scalar coupling, dipolar coupling and exchange processes. 

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