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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

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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