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Nuclear Magnetic Resonance SpectroscopyPart-2
Prepared By
Dr. Khalid Ahmad Shadid
Islamic University in MadinahDepartment of Chemistry
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Chemical Shifts
Chemical shift: The relative energy of resonance of a nucleus resulting from its local environment
NMR spectra show applied field strength increasing from left to right. Left part is downfield and right part is upfield
Nuclei that absorb on upfield side are strongly shielded Chart calibrated versus a reference TMS, set as 0.00
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CHEMICAL SHIFTالكيميائية اإلزاحة
امتصاص عندها يحدث التي الترددات لتعيين تستعمل . المقابل التردد بين الفرق بقياس نقوم الراديو اشعة
إلحداث المقابل والتردد الدراسة تحت البروتون لرنيينسايلين التترامثل مرجع في بروتون رنيين
TMS
Si
CHEMICAL SHIFT
015
ppm
IntensityAll protons absorb in this region
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1. Electronegativity
CH3OH
CH3F
CH3Cl
CH3BrCH3I
(CH3)4C(CH3)4Si
CH3-XElectroneg-ativity of X
Chemical Shift ()
4.03.5
3.1
2.82.5
2.1
1.8
4.263.47
3.05
2.68
2.16
0.86
0.00
O
CH3
3.9 ppm
0.9 ppm
H3C
• Proton signals range from 0 to 12• Different types of proton will occur at different chemical shifts• The magnetic field experienced by a proton is influenced by various
structural factors:
Compound CH4 CH3Cl CH2Cl2 CHCl3
d / ppm 0.23 3.05 5.30 7.27
Factors Influencing Chemical Shifts
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2. Hybridization of adjacent atoms.
RCH3, R2CH2, R3CH
R2C=CHR, R2C=CH2
RCHO
R2C=C(R)CHR2
RC CH
Allylic
Type of Hydrogen(R = alkyl)
Name ofHydrogen
Chemical Shift ()
Alkyl
Acetylenic
Vinylic
Aldehydic
0.8 - 1.7
1.6 - 2.6
4.6 - 5.7
9.5-10.1
2.0 - 3.0
Factors Influencing Chemical Shifts
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3. Hydrogen Bonding Effects Protons involved in H-bonding (-OH or -NH) are observed
over a large range of chemical shift values (d 0.5 - 5 ppm) since H-bonding effects are solvation, acidity, concentration and temperature dependent
Factors Influencing Chemical Shifts
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Factors Influencing Chemical Shifts
Magnetic anisotropy: "non-uniform magnetic field“ Electrons in p systems (e.g. aromatics, alkenes, alkynes,
carbonyls etc.) interact with the B0 which induces a magnetic field that causes the anisotropy
As a result, the nearby protons will experience 3 fields: the applied field, the shielding field of the valence electrons and the field due to the p system
Depending on the position of the proton in this third field, it can be either shielded (smaller d) or deshielded (larger d)
4. Magnetic anisotropy Effect:
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A carbon-carbon triple bond shields an acetylenic hydrogen and shifts its signal to lower frequency (to the right) to a smaller value.
A carbon-carbon double bond deshields vinylic hydrogens and shifts their signal to higher frequency (to the left) to a larger value.
RCH3
R2C=CH2
RC CH
Type of H Name
Alkyl
VinylicAcetylenic
0.8- 1.0
4.6 - 5.72.0 - 3.0
Chemical Shift ()
Factors Influencing Chemical Shifts
4. Magnetic anisotropy Effect: or Diamagnetic effects of bonds
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Figure 13.9 A magnetic field induced in the p bonds of a carbon-carbon triple bond shields an acetylenic hydrogen and shifts its signal upfield: The p electrons in a triple bond circulate around the bond axis to produce a magnetic field directly opposing the applied magnetic field
Factors Influencing Chemical Shifts
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Figure 13.10 A magnetic field induced in the p bond of a carbon-carbon double bond deshields vinylic hydrogens and shifts their signal downfield.
Factors Influencing Chemical Shifts
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The magnetic field B0 induced by circulation of the p electrons (ring Current) in an aromatic ring deshields the hydrogens of the aromatic ring and shifts their signal downfield.
B0
B0
In aromatic rings: The "ring current" generates a local magnetic field which opposes B0
However, on the periphery of the ring, the flux lines are in the direction of B0
Thus, protons attached to the aromatic ring "feel" a larger magnetic field than protons elsewhere in the molecule
Aromatic protons will exhibit a downfield shift (7 - 8 ppm)
Factors Influencing Chemical Shifts
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Chemical Shift - 1H-NMR
Figure 13.8 Average ranges of chemical shifts of representative types of hydrogens.
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ChemicalShifts1H-NMR
RCH2 OR
(CH3 )4Si
ArCH3
RCH3
RC CH
RCCH3
ROHRCH2 OH
ArCH2 R
O
O
RCH2 RR3 CH
R2 NH
RCCH2R
R2 C=CRCHR2
R2 C=CHR
RCH
O
RCOH
O
RCH2 ClRCH2 BrRCH2 I
RCH2 F
ArHO
O
R2 C=CH2
RCOCH3
RCOCH2R
ArOH
9.5-10.1
3.7-3.9
3.4-3.6
Type of Hydrogen
0 (by definition)
Type of Hydrogen
Chemical Shift ()
1.6-2.62.0-3.0
0.8-1.01.2-1.41.4-1.7
2.1-2.3
0.5-6.0
2.2-2.6
3.4-4.0
Chemical Shift ()
3.3-4.0
2.2-2.52.3-2.8
0.5-5.0
4.6-5.05.0-5.7
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4.1-4.73.1-3.3
3.6-3.84.4-4.5
6.5-8.5
4.5-4.7
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Integration of 1H NMR Absorptions: Proton Counting
The relative intensity of a signal (integrated area) is proportional to the number of protons causing the signal
For narrow peaks, the heights are the same as the areas and can be measured with a ruler
Example: in methyl 2,2-dimethylpropanoate integral ratio is 3:9 or 1:3
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Integration is used to deduce the structure. The area under the peaks gives a ratio of the number of H for each signal
Measure the height of each trace and derive a whole number ratio
Integration of 1H NMR Absorptions: Proton Counting
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Good Luck
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