Through-space Scalar Coupling

Through-space and Hydrogen Bond Scalar Couplings 

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Significant through-space scalar couplings have been observed only in rare cases and in very close spatial proximity and usually 19F is involved in such couplings102-105, e.g., in the following fluorinated polycyclic aromatic systems ... 

The direct contribution (c3)dir arises from the through-space dipolar coupling of the nuclear magnetic moments and, as expected, it decreases as the internuclear distance increases with increasing v, because of its R l dependence. The second contribution (C3)ec is the axial component of the tensorial electron-coupled spin-spin interaction the scalar part of this interaction is given by the value of C4. In the v = 0 level the direct contribution is estimated by English and Zorn [51] to be 1.15 kHz, and the electron-coupled part is -0.23 kHz. 

Other basic 2-D experiments include J spectroscopy, which displays scalar-coupled multiplets along the second dimension, and nuclear Overhauser effect spectroscopy (NOESY, pronounced nosy ), which exploits through-space dipolar couplings to provide conformational information. In the NOESY experiment, protons are selectively irradiated as data are acquired, and the effect of neighboring nuclei is monitored by their mutual enhancement. NOESY is one example of an exchange experiment, in which magnetization is transferred from spin to spin. 

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 ... 

A ID NMR experiment provides information on the chemical shift and spin-spin coupling fine structure of the individual resonances in the spectrum. Double or multiple pulse irradiation experiments provide additional data on through bond scalar connectivities or through space dipolar connectivities, which relate to resonance assignments, conformational state and dynamics of the molecules under investigation. 

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