Magnetic field effects ring-current effect

The method of computation is identical to that utilized for the nucleosides and 5 -nucleotides and is fully described in the first paper of this series [20]. The chemical shifts are calculated as the sum of three contributions the ring current effect/ the local magnetic anisotropy term and the polarization or electric field contribution. For a given conformation taken as reference we suppose all the chemical shifts equal to zero and we report only the variation (AS) of these quantitities as a function of the torsional angle studied/ positive variations corresponding to upfield shifts. 

Figure 15. Ring current effects in benzene. The overlapping of the 2i>, orbitals leads to two current loops, above and below the. s-bond plane of the benzene molecule. Only one loop is represented here. A magnetic field (black arrows) is induced by the ring current (white arrows), changing the shielding constant.

Ring current (Section 15.8) The circulation of tt electrons induced in aromatic rings by an external magnetic field. This effect accounts for the downfield shift of aromatic ring protons in the lH NMR spectrum. 

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). 

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