Chemical through-bond contribution

Integrating over the hysteresis loop between the compression and decompression curves in Figure 19 yields the amount of energy dissipated through the reversible bond formation/dissociation process. Unfortunately, it is not possible to determine the contribution of these transitions to the friction of phosphate films because such a calculation would require knowledge of the number of similar instabilities that occur per sliding distance, which is certainly beyond the limits of first-principles calculations. Nonetheless, the results do indicate that pressure- and shear-induced chemical reactions can contribute to the friction of materials. 

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. 

For CO, its HOMO is the lone pair 5 o orbital and its LUMOs are the unoccupied 2 JT orbitals. Elementary chemical bonding theory describes the interaction with a molecule as CO only through chemical bonding contributions through its HOMO and LUMOs. We will see that this is a rather approximate view of the surface chemical bond, because of the important hybridization that occurs in the adsorbing molecule. 

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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