Anisotropic electronic shielding

Anisotropic electron charge distributions in bonds or atomic groups can be treated as magnetic point-dipoles88. They can substantially influence the shielding of protons, whereas such effects are much less significant for heavier nuclei. The important features of these effects are ... 

So what about aromatic protons (<56.0-9.5) aldehyde protons (<59.5—9.6), or even protons oh double, nay triple bonds (<52.5-3.1) All these protons are attached to carbons with n bonds, double or triple bonds, or aromatic systems. The electrons in these n bonds generate their own little local magnetic field. This local field is not spherically symmetric — it can shield or deshield protons depending on where the protons are — it s anisotropic. In Fig. 137, the shielding regions have plusses on them, and deshielding regions have minuses. 

Here, a denotes the shielding constant caused by the anisotropic shielding of the external magnetic field by the electron shell around the resonating nuclei. The so-called isotropic chemical shift, 5, is defined as <5 = cr ref—<7, where cr ref is the shielding constant of the nuclei in a reference material, and <5 is defined by the following ... 

These hybridisation variations are caused by anisotropy within the chemical bonds. This is due to the non-homogeneous electronic distribution around bonded atoms to which can be added the effects of small magnetic fields induced by the movement of electrons (Fig. 9.12). Thus, protons on ethylene are deshielded because they are located in an electron-poor plane. Inversely, protons on acetylene that are located in the C-C bond axis are shielded because they are in an electron-rich environment. Signals related to aromatic protons are strongly shifted towards lower fields because, as well as the anisotropic effect, a local field produced by the movement of the aromatic electrons or the ring current is superimposed on the principal field (Fig. 9.12). 

Carbon-13 shifts of alkynes (Table 4.13) [246-250] are found between 60 and 95 ppm. To conclude, alkyne carbons are shielded relative to olefinic but deshielded relative to alkane carbons, also paralleling the behavior of protons in proton NMR. Shielding relative to alkenes is attributed to the higher electronic excitation energy of alkynes which decreases the paramagnetic term according to eq. (3.4), and to the anisotropic effect of the triple bond. An increment system can be used to predict carbon shieldings in alkynes... 

The electrons modify the magnetic field experienced by the nucleus. Chemical shift is caused by simultaneous interactions of a nucleus with surrounding electrons and of the electrons with the static magnetic field B0. The latter induces, via electronic polarization and circulation, a secondary local magnetic field which opposes B0 and therefore shields the nucleus under observation. Considering the nature of distribution of electrons in molecules, particularly in double bonds, it is apparent that this shielding will be spatially anisotropic. This effect is known as chemical shift anisotropy. The chemical shift interaction is described by the Hamiltonian... 

Chemical shift is substantially determined by the electron population surrounding the nucleus in question and shielding it from the applied field. Chemical shifts are therefore used to probe the total electron population. The chemical shift range with protons is so small that aromatic ring currents and other anisotropic influences make such measurements using proton spectra unreliable, but the agreement between calculated and observed 13C chemical shifts is good. 

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