What is shielding and deshielding in NMR? Give one Example.

In nuclear magnetic resonance (NMR) spectroscopy, shielding and deshielding refer to the phenomenon of how electron density around a nucleus affects its chemical shift value in an NMR spectrum. NMR is a powerful analytical technique used to determine the structure of organic molecules and study their interactions.

1. Shielding: When a nucleus is surrounded by electron density (electrons from nearby atoms), it experiences a localized magnetic field created by the circulating electrons. This additional magnetic field opposes the external magnetic field applied during NMR experiments. As a result, the effective magnetic field experienced by the nucleus is slightly reduced, leading to a downfield shift (toward higher chemical shift values) in the NMR spectrum. This phenomenon is known as "shielding."

2. Deshielding: On the other hand, if a nucleus is in an environment with lower electron density around it, there is less opposition to the external magnetic field. As a result, the effective magnetic field experienced by the nucleus is enhanced, leading to an upfield shift (toward lower chemical shift values) in the NMR spectrum. This phenomenon is referred to as "deshielding."

Example:
Consider the proton (1H) NMR spectrum of two different organic compounds, acetone and chloroform. Acetone contains a carbonyl group (C=O), which is highly electronegative and pulls electron density away from the nearby hydrogen atoms, leading to deshielding. Chloroform, on the other hand, has hydrogen atoms attached to three chlorine atoms, and the electron-withdrawing nature of chlorine causes the hydrogen atoms to be shielded due to the presence of more electron density around them.

In the acetone spectrum, the methyl (CH3) protons adjacent to the carbonyl group will appear at a higher chemical shift (upfield) due to deshielding caused by the electron-withdrawing carbonyl group. In the chloroform spectrum, the hydrogen atoms on the methyl groups will appear at a lower chemical shift (downfield) due to shielding from the surrounding electron-rich chlorine atoms.

In summary, shielding and deshielding effects in NMR spectroscopy are a consequence of the electron distribution around a nucleus, leading to shifts in the observed resonance frequencies in the NMR spectrum.