Chemical through-space contribution

The external magnetic field Bq induces currents in bonds, lone pairs and inner shell orbitals of our molecular systems, and these currents are the source of the chemical shifts. Adopting the picture of localized bond orbitals, through-space contributions are caused by charge distributions of bonds that are not directly connected to the nucleus under consideration. Provided that the charge distributions are sufficiently far away so that their electron density in the vicinity of the nucleus can be neglected, this type of interaction can be approximated by the McConnell equation ... 

From these results it was concluded that hydrogen bonds are dominated by polarization, but in general the proton chemical shift is influenced by through-space contributions (see previous chapter) as well. Since through-space contributions affect the anisotropy even more than the chemical shift mean value, no good correlations between distances and proton CS anisotropies are observed. 

A semi-empirical scheme is reintroduced for the conformational analysis of organic nitro compounds. In this scheme, proton chemical shifts are regarded as functions of short-range electrostatic contributions, through-space effects, and ring-currents, all of which are dependent on distances between atoms. 

In some cases it has also been observed that a remote substituent in molecules in which direct n transmission mechanisms are prohibited can have a very strong influence on the nuclear shielding for example, the Sn substituent chemical shift (SCS) for derivative [11] (relative to parent molecule [12], which is due to both polar-field and residual contribution where the latter is probably due to through-bond and/or through space electron delocalization , is very sensitive to polar substituent influences, like that derived from iodide groups. 

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