Stereoelectronic effects on chemical shifts

We will start with a short discussion of the origin of NMR chemical shift. The observed chemical shift is determined by the net magnetic field felt by each nucleus in a molecule. The net magnetic field at an atom, can be related to the applied magnetic field, using the following relationship  

The diamagnetic contributions come directly from induced magnetic fields of electrons directly bound to the atom, while the non-local term stems from through-space interactions and anisotropic effects of neighboring atoms or functionalities. Paramagnetic contributions are not easily understood within simple models. We will discuss them later. 

The most obvious way for orbital interactions to affect the observed NMR spectra is through delocalization of electron density. As electron density is redistributed via stereoelectronic effects, nuclei will be either shielded or deshielded depending on the specific effects that dominate. As a result, stereoelectronic effects can significantly alter NMR chemical shifts. 

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