Download Shielding vs. Deshielding

Shielding vs. Deshielding
 If a nuclei sees a smaller total magnetic field, it is
said to be shielded.
 When a nuclei is shielded, its NMR frequency is
shifted upfield  lower chemical shift
 Shielded  lower
 Deshielded  higher
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Chem 3500 - NMR spectroscopy
Mesomeric effect: definition
 The mesomeric effect or resonance effect in
chemistry is a property of the substituents or
functional groups in a chemical compound. The
effect is used in a qualitative way and describes
the electron withdrawing or releasing properties
of the substituents based on relevant resonance
structures and is symbolized by the letter M. The
mesomeric effect is negative (-M) when the
substituent is an electron-withdrawing group
and the effect is positive (+M) when based on
resonance the substituent is an electron releasing
group.
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Chem 3500 - NMR spectroscopy
Mesomeric effect: Examples
5.29 ppm
-M: electron-withdrawing
effect
6.52
deshielded
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Chem 3500 - NMR spectroscopy
+M: electron-releasing effect
3.74
shielded
Mesomeric effect: releasing group
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Chem 3500 - NMR spectroscopy
Mesomeric effect: withdrawing
groups
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Chem 3500 - NMR spectroscopy
Magnetic anisotropy of bonds
 Chemical bonds are anisotropic in space: they
have different properties along different axis
 When a chemical bond is placed in a external
magnetic field, it will generate a local field
around itself that will also be anisotropic.
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Chem 3500 - NMR spectroscopy
Example: C-C bond
B0
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Chem 3500 - NMR spectroscopy
C
C
Blocal
Example: benzene ring
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Chem 3500 - NMR spectroscopy
More examples
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Chem 3500 - NMR spectroscopy
Predicting 1H chemical shifts:
Shoolery’s rule
 Several factors can influence the 1H chemical
shift: carbon substitution, electronegativity,
mesomeric effect,…
 Those effects are additive
 If methane is used as the base compound, the
contribution of each of substituent can be added
up following the empirical Shoolery’s rule:
0.23
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Chem 3500 - NMR spectroscopy
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Chem 3500 - NMR spectroscopy
Alkenes
 The presence of a double bond affects the protons
on the allylic carbon atoms by shifting their
chemical shift up (deshielding)
CH2
H
1.5
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Chem 3500 - NMR spectroscopy
H
2.1
Tobey-Simon Rule
 The Tobey-Simon rule is an empirical system to
predict the chemical shifts of protons attached to
double bonds
δ 5.28
gem
cis
Htrans
H
Hcis
gem
R
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Chem 3500 - NMR spectroscopy
trans
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Chem 3500 - NMR spectroscopy
Aromatic compounds
 Substituent has a similar effect on the chemical
shifts of the aromatic ring than on alkenes.
 Results from the inductive and resonance effects.
 Prediction possible:
δ 7.27
Si
But it does NOT work when 2 substituents are
ortho to each other…
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Chem 3500 - NMR spectroscopy
Substituent Parameters for Aromatic
Proton
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Chem 3500 - NMR spectroscopy
Protons on Heteroatoms (N,S,O)
 Chemical shifts are very sensitive to pH,
temperature, solvent and concentration
 Typical ranges are:
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Acids
RCOOH
10.5-12 ppm
Phenols
ArOH
4.0-7.0 ppm
Alcohols
ROH
0.5-5.0 ppm
Amines
RNH2
0.5-5.0 ppm
Amides
RCONH2
5.0-8.0 ppm
Enols
CH=CH-OH
>15 ppm
Chem 3500 - NMR spectroscopy