Shielding and Deshielding effects

Why don’t all protons resonate at the same frequency? In a ¹H NMR spectrum each peak represents a different kind of chemical environment, where the protons have different resonance frequencies. In a molecule, each nucleus experiences a different magnetic field due to the surrounding electrons of other atoms. The magnetic fields of the surrounding atoms can oppose the magnetic field that is applied during an NMR experiment and the protons become shielded - this reduces their resonance frequency and thus their chemical shift.

Where elements such as carbon have shielding effects more electronegative elements such as fluorine, chlorine and oxygen have deshielding effects. With protons that are in the proximity of these groups, the electron cloud is more polarised towards the electronegative elements, and the proton becomes less shielded from the applied magnetic field contributing to a greater chemical shift. The chemical shift of protons change for different functional groups due these shielding effects. These effects can be attributed to the extent of the polarisation of electrons due to the various functional groups.

The proton environments with the largest chemical shift values are the ones that are more deshielded, this effect is useful in deducing NMR structures as it separates different proton resonances into distinct peaks over a wide range allowing for effective elucidation.