In NMR spectroscopy, shielding and deshielding describe how the electrons around a nucleus affect the magnetic field it experiences, which in turn influences the chemical shift in the spectrum.
1. Shielding
- Electrons around a nucleus create a small magnetic field that opposes the external magnetic field.
- This reduces the effective magnetic field felt by the nucleus.
- When a nucleus is shielded, it resonates at a lower frequency (appears upfield on the NMR spectrum, to the right).
Example:
- Hydrogen in a CH₃ group (alkyl hydrogen) is surrounded by electrons and is highly shielded. Its signal appears at a lower chemical shift (~0.9 ppm in ¹H NMR).
2. Deshielding
- When electrons are pulled away from a nucleus by electronegative atoms (like O, N, Cl) or π-bonds (double or triple bonds), the nucleus experiences a stronger effective magnetic field.
- This makes the nucleus less shielded, or deshielded, and it resonates at a higher frequency (appears downfield, to the left in the spectrum).
Example:
- Hydrogen attached to a carbon next to oxygen (–CH₂–OH) is deshielded and appears at a higher chemical shift (~3–4 ppm).
- Aromatic hydrogens are also deshielded due to ring currents and appear downfield (~7–8 ppm).
3. Why It Matters
- Shielding and deshielding explain why different hydrogens (or carbons) in a molecule appear at different positions in the NMR spectrum.
- By analyzing these shifts, chemists can identify functional groups and molecular environments.
In Short
Shielding occurs when electrons protect the nucleus, moving its signal upfield (lower ppm).
Deshielding occurs when electrons are pulled away, moving the signal downfield (higher ppm).