Survey of 13C Chemical Shifts

The magnetic shielding constant (eq. 1.39) arises from the influence of chemical effects on the Larmor frequency of a nucleus. It is described in terms of three additive contributions (eq. 3.2)  

The diamagnetic term rdla accounts for local electronic circulations around the nucleus induced by the applied field B0. According to the Lenz rule, the resultant intramolecular field opposes B0, so that the nucleus will be shielded. As known from the Lamb formula, eq. 3.3 [9], 

The contribution rN is referred to as the neighbor anisotropy effect. This term accounts for the fields arising from electronic circulations around the atoms surrounding the observed nucleus. It depends on the nature of the neighbor atoms and on molecular geometry. Sometimes, a medium term must be added in eq. (3.2), correcting for solvent and pH effects. 

For nuclei other than hydrogen, such as 13C, the paramagnetic shielding term rpara predominates. It opposes rdla and therefore deshields. According to eq. 3.4 derived by Karplus and Pople [75], 

The connection between 13C shielding and electronic excitation agrees with the fact that carbonyl compounds are most deshielded ((5C 170 ppm, n- it transitions with AE x 7 eV) when compared with alkenes and aromatic compounds ( 5C ss 100-150 ppm, it - it transitions with AE x 8 eV) and alkanes ((5C 50 ppm, a -+o transitions with AE x 10 eV). 

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