Indirect coupling interaction

It should now be evident that the experimental tensor may be expressed as the sum of an isotropic term resulting from the contact interaction, and a tensor resulting from dipolar interactions and any indirect coupling via the orbital angular momentum. This may be written in the form of an equation ... 

The main NMR interactions in solution of interest to chemists are the chemical shift relative to some stated standard (6), the indirect coupling constant (7) and the relaxation times T1 (spin-lattice) T2 (spin-spin related to the line width) and T p, the relaxation time in the rotating frame. In the case of solids and oriented samples both the direct dipole-dipole and the electric quadrupole interactions assume greater importance. We shall confine our attention in this chapter to diamagnetic compounds so that we may neglect nuclear interactions with electron spins. 

While the terminology scalar relaxation of the first kind concerns J modulation (J spin spin or scalar or indirect coupling constant) by exchange phenomena, the usual example of the second kind is a spin 1/2 nucleus (7), J coupled to a fast relaxing quadrupolar nucleus (S) with relaxation times Tj and Tf (and spin number /s)- The relevant interaction is of the form... 

The canonical spin glass consists of a noble metal (Au, Ag, Cu, or Pt) diluted with a transition metal ion, such as Fe or Mn. The magnetic interaction in such systems is mediated by the conduction electrons, leading to an indirect exchange interaction—the RKKY (Ruderman and Kittel [70], Kasuya [71], and Yosida [72]) interaction, whose coupling constant J R) oscillates strongly with distance r between the spins as... 

These factors were examined recently by McLachlan et al. (1960), who extended the molecular-orbital theory of McConnell and coworkers. They found that the coupling involves only excited states of the (i-electrons and that, amongst other factors, the value of Q depends upon the excited triplet states of the a--bonds. The theory developed (McLachlan et al., 1960) is equally applicable to 13C hyperfine coupling and provides a measure of the importance of the direct interaction from spin density on the carbon atom and the indirect coupling from spin density on adjacent carbons acting via the C—C a-bonds. The theoretically estimated values are + 41 and —14 G respectively, whilst experiment suggests +41 and — 6-9 G for Qc. 

Less well-defined but particularly important in terms of the function of non-neuronal cells are so-called receptor-operated channels [6,7]. By definition these are channels in the plasma membrane which open in response to hormone-receptor interaction without a change in membrane potential. The mechanism of their opening may either be by a direct coupling of receptor (possibly via a G protein) with the channel, or by an indirect coupling via the generation of an intracellular messenger such as cAMP or the putative messenger, inositol 1,3,4,5-tetrakisphosphate. 

In many cases, the spectra recorded for the condensed phase are similar to those recorded in the liquid phase, but they usually contain a wider range of information than is available in liquid NMR spectroscopy. The solid state represents the best environment for the investigation of intermolecular interactions. Analysis of the tensorial nature of the chemical shifts provides subtle structural information. Strategies based on dipolar recoupling and /-coupling indicate a number of ways in which direct and indirect coupling constants can be measured, to yield direct structural constraints. This approach, combined with advanced theoretical calculations, traces new trends in structural studies of the condensed matter. 

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