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Presentation on shielding and deshielded effect
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Group: 5thGroup members: Sadaf Jan, Hafsa Iqbal, Habiba Zahid, Nosheen
& Aiman Shahzadi
Department: Chemistry
Semester: BS-8th
Subject: Molecular Spectroscopy
Assignment topic: Shielding and Deshielding
Submitted to: Maam Taskeen
Date: 9-May-2023
Shielding effects in NMR spectroscopy
Definition:
, Under the influence of applied magnetic field Ho, electrons surroundings nucleus
generate secondary magnetic field called induce magnetic field, which opposes
applied magnetic field in the region of nucleus. Thus the nucleus experience a
weaker magnetic field than applied magnetic field Ho and is said to be shielded .This
type of shelding is called diamagnetic shielding and its effect is termed as shielding
effect.
Explanation:
▪︎In NMR spectroscopy when the electronic density is high around the nucleus, the
opposing magnetic field of that electrons is also bigger and hence shielding is
greater.
▪︎In shielded nuclei chemical shift is observe at low frequency and up fielded.
▪︎ In shielded nuclei peak will be shifted towards right hand side in NMR spectra.
Example
Here it is possible to take into consideration the CH3 and OH proton.
1. The hydrogen atoms are shieled in CH3 and experience little applied magnetic field
and that's why required low frequency for transition and give upfield chemical shift.
In OH group hydrogen atom is deshielded because electronegative oxygen attract
electron away from hydrogen and make it deshielded. It experience applied
magnetic field and require high frequency for transition and give downfield chemical
shift.
De-shielding in NMR spectroscopy:
• Definition
When the electron density around the nuleus falls, the opposing magnetic field B 0
gets smaller, and the nucleus feels more of the external magnetic filed .
Or
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