Chemical shift anisotropy, carbon nucleus

NMR STUDIES of ADSORBED CARBON MONOXIDE. In the n.m.r. investigation of molecules adsorbed on solid supported metals, such as palladium on silica or platinum on alumina, both the chemical shift and line width of the observed nucleus in the adsorbate are drastically affected by the physical properties of the sample. The chemical shift anisotropy of a nucleus in a molecule... 

The chemical shift anisotropies (CSA) of nuclei in different structural environments [8] vary from about 30 ppm for CH2 carbons to about 200 ppm for aromatic carbons. Unlike the dipolar interactions between and spins, the strength of the CSA depends on the strength of the applied magnetic field Bq, because the strength of the local magnetic field Bq experienced by a nucleus depends on the strength of the electronic currents in the vicinity... 

The chemical shift of a nuclear spin is a tensorial quantity. Its value depends on the orientation of the electronic distribution about the nucleus with respect to the external magnetic field. In a liquid, due to the rapid molecular motions, this interaction is averaged to zero and the observed chemical shift is the trace of the tensor. In contrast, in a powder, in the absence of motions, all the orientations have the same probability and the signal obtained for each carbon is the sum of the elementary chemical shifts corresponding to the different orientations. When local motions occur in the bulk below 7g, they usually induce a partial averaging of the chemical shift anisotropy. 

So if this all sounds a bit bleak, what s the good news Well, strangely, there is quite a lot. For a start, let s not forget that had the 13C nucleus been the predominant carbon isotope, the development of the whole NMR technique itself would have been held back massively and possibly even totally overlooked as proton spectra would have been too complex to interpret. Whimsical speculation aside, chemical shift prediction is far more reliable for 13C than it is for proton NMR and there are chemical shift databases available to help you that are actually very useful (see Chapter 14). This is because 13 C shifts are less prone to the effects of molecular anisotropy than proton shifts as carbon atoms are more internal to a molecule than the protons and also because as the carbon chemical shifts are spread across approximately 200 ppm of the field (as opposed to the approx. 13 ppm of the proton spectrum), the effects are proportionately less dramatic. This large range of chemical shifts also means that it is relatively unlikely that two 13C nuclei are exactly coincident, though it does happen. 

To a good approximation, three terms dominate the C-NMR chemical shifts diamagnetic, paramagnetic, and anisotropy shielding terms [57]. Lamb developed a theoretical expression for the diamagnetic term that focuses on the electron density at a specific nucleus [58]. Therefore, one would expect the carbon s hybridization to have a large chemical shift effect. Obviously, electron deficiency in a carbocation will profoundly effect the chemical shift [59-62]. 

In general, the factors that affect the chemical shifts of carbons are the same as for protons (i.e. electron density around the nucleus in question, and anisotropy effects). Carbon chemical shifts can be readily calculated from tables of shift effects found in many texts. However, unlike protons attached to sp carbons, sp carbons attached to sp carbons exhibit only a small shift difference. There are also few good substituent parameters available for calculating the chemical shifts of alkene carbons bearing polar groups, unlike the calculation of NMR chemical shifts near polar groups. However, in systems where resonance is present, some predictions can be made of relative shift differences in the carbons (see Figure 1). 

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