Related ideology regarding chemical shifts

1. Suraj C. (MPA)
FACTORS AFFECTING CHEMICAL SHIFT
Presented by: Rino M Paul Re-Edited by: Suraj C.
Presented to: Ritu Vivek Kimbahune
TETRAMETHYLSILANE(TMS):
•TMS is added to the sample as internal standard.
•TMS protons are all identical, highly shielded providing a single sharp peak always isolatedfrom peaks of interest.
•The TMS was assigned d = 0.00.Organic protons absorb downfield (to the left) of the TMSsignal.
•TMS is inert , highly soluble in organic liquids and easily removed from samples bydistillation.
FACTORS AFFECTING CHEMICAL SHIFT:
Four major factors account for the resonance positions (on the ppm scale) of most protons:
•Deshielding by electronegative elementsInductive effect by electronegative groups
•s-character (hybridization effect)
•Magnetic Anisotropic effect (magnetic fields usually due to π-bonded electrons in themolecule)
•Hydrogen bonding.
1.DESHIELDING BY ELECTRONEGATIVE ELEMENTS:
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2. Suraj C. (MPA)
S-Character:
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3. Suraj C. (MPA)
MAGNETIC ANISOTROPIC FIELDS:
•The presence of a nearby pi bond or pi system greatly affects the chemical shift.
•Induced magnetic fields due to the π- electrons have greatest effect.
Aromatic protons δ= 7-8 ppm
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4. Suraj C. (MPA)
Vinyl (Olefinic) protons, δ = 5-6 ppm :
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5. Suraj C. (MPA)
Acetylene protons δ≈ 2.5 ppm
Aldehyde proton δ= 9-10ppm:
Electronegative oxygen atom
HYDROGEN BONDING O-H and N-H Signals:
• The chemical shift depends on how much hydrogen bonding is taking place (observed in high concentrated solutions).
• Hydrogen bonding lengthens the O-H bond and reduces the valence electron density around the proton it is deshielded and shifted downfield in the NMR spectrum.
• Alcohols vary in chemical shift from 0.5 ppm (free OH) to about 5.0 ppm (lots of H bonding).
SOME MORE EXTREME EXAMPLES:
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6. Suraj C. (MPA)
• Carboxylic acids have strong hydrogen bonding – they form dimers.
• Resonance, electronegativity of oxygen and the formation of hydrogen bonding withdraw electron cloud from the acid protons.
• Thus, protons attached to carboxylic acids are the least shielded protons and have a chemical shift of 10-12 ppm.
• In methyl salicylate, which has strong internal hydrogen bonding, the NMR
absorption for O-H is at about 14 ppm, (highly downfield)
Summary:
Chemical shift:The difference in frequency between the sample and
the standard over the operation frequency.
BOTTOM LINE:
The more the deshielding more downfield.(higher frequency)
The more the shielding more upfield.(lower frequency) RCOOHRCOOH
δ
δ
δ
δ
δ
δ
I
OOOHCH3
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7. Suraj C. (MPA)
TYPICAL VALUES:
Resources and references:
Textbook: Principles of Instrumental Analysis, Skoog, Holler, Nieman
Recommended further reading:
“Principles of instrumental analysis, 5th ed. by Skoog, Holler, Nieman” Chapter 19.
Extra resources are available on the internet.
Relevant web sites
http://www.chemguide.co.uk/analysismenu.html
*****THANK YOU*****
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