Scalar J-coupling

The number of NMR parameters available for measurement is rather small, consisting of the chemical shift, relaxation rates (/1 and lo), scalar (J) couplings, dipolar (D) couplings, cross-relaxation rates (the NOE), and hydrogen exchange rates. All of these have been quantified for many of the amide protons of A131 A, and most of the data suggest the presence of little persistent structure. 

In addition to the chemical shift information, an NMR spectrum may also contain coupling information. The types of couplings frequently present in NMR experiments include scalar (J) couplings between high-abundance nuclei such as protons, dipolar couplings that are important for cross-relaxation processes and the determination of nuclear Over-hauser effect (NOE) (described later in this chapter), and quadrupolar coupling associated with quadrupolar nuclei (/> 1/2). 

Solomon and Bloembergen (21,22) formulated a similar expression for the interaction between the two spins being scalar (J-coupling) rather than... 

In an alternative formulation of the Redfield theory, one expresses the density operator by expansion in a suitable operator basis set and formulates the equation of motion directly in terms of the expectation values of the operators (18,20,50). Consider a system of two nuclear spins with the spin quantum number of 1/2,1, and N, interacting with each other through the scalar J-coupling and dipolar interaction. In an isotropic liquid, the former interaction gives rise to J-split doublets, while the dipolar interaction acts as a relaxation mechanism. For the discussion of such a system, the appropriate sixteen-dimensional basis set can for example consist of the unit operator, E, the operators corresponding to the Cartesian components of the two spins, Ix, ly, Iz, Nx, Ny, Nz and the products of the components of I and the components of N (49). These sixteen operators span the Liouville space for our two-spin system. If we concentrate on the longitudinal relaxation (the relaxation connected to the distribution of populations), the Redfield theory predicts the relaxation to follow a set of three coupled differential equations ... 

Now consider a two-spin system which is scalar (J) coupled, such as the - C pair in chloroform (CHCI3). The 13 C nuclei have two different resonant frequencies depending on whether the attached H nucleus is in the a or the spin state. In the absence of proton... 

Although solid state 2H NMR techniques are also used widely in structural studies, the principal use of these techniques has been to obtain detailed information on reorientational motions in the solid state, and our discussion is focused on this aspect of 2H NMR. As discussed above, the quadrupole interaction is usually the dominant nuclear spin interaction in 2H NMR, and other nuclear spin interactions (e.g. dipole-dipole interaction, CSA and scalar J-coupling) are generally negligible in comparison. For 2H, the quadrupole interaction is typically about 150-250 kHz, whereas the direct dipolar interactions and CSA are typically about 10 kHz and 0.7 kHz (at 11.7 T) respectively. Since the EFG originates from... 

Figure 12 Segment of a polypeptide chain showing the magnitude of the scalar J-couplings used in heteronuclear NMR experiments.

This section summarizes briefly the basis for the temperature dependence of those NMR parameters most widely used for thermal measurements, and references sections in the article in which further discussion and applications are included. The parameters considered are net polarization, chemical shielding (a), T relaxation times, scalar (J) coupling and molecular diffusion coefficients. 

Through-bond interactions scalar (J) coupling via bonding electrons. 

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. 

A two-bond scalar J-coupling between the quadrupolar A1 and spin V2 P nuclei of ca. 30 Hz has been determined successfully by Massiot et aV in inorganic solids, such as berlinite (AIPO4), hydrated AIPO4 and VPI-5 molecular sieve using J-Resolved experiments, and subsequently used to acquire 2D J-HQMC heteronuclear correlation. Similar JpM coupling values have been measured in VPI-5 by Trebosc et who applied the PS REDOR (Frequency... 

Detection of the NMR spectrum of an organic compound in solution with the single pulse experiment, as described above, is quite feasible. However, were the spectrum acquired in this manner, one would notice that the number of peaks in it exceeds the number of unique carbon environments. Most organics contain protons as well as carbon atoms. These protons are coupled to the carbons through the indirect (or scalar) J coupling, resulting in splitting of the carbon resonances of any carbon with attached protons. 

Information on the structure of molecules can be obtained from nuclear pair interactions magnetic dipole-dipole interactions provide distance information, while scalar J couplings allow one to determine dihedral angles. 

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