Nuclear Shielding and 1. H Chemical Shifts. 1 H NMR Spectroscopy Nuclear Magnetic Resonance

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1

1HH

NMR Spectroscopy

NMR Spectroscopy Nuclear Magnetic Resonance Nuclear Magnetic Resonance

Nuclear Shielding and

1H Chemical Shifts

What do we mean by "shielding?"

What do we mean by "shielding?"

What do we mean by "chemical shift?"

What do we mean by "chemical shift?"

The electrons surrounding a nucleus affect the effective

magnetic field sensed by the nucleus ShieldingShielding

An external magnetic field An external magnetic field affects the motion of the affects the motion of the electrons in a molecule, electrons in a molecule, inducing a magnetic field inducing a magnetic field within the molecule.

within the molecule. CC HH

H H 00

Shielding Shielding An external magnetic field An external magnetic field affects the motion of the affects the motion of the electrons in a molecule, electrons in a molecule, inducing a magnetic field inducing a magnetic field within the molecule.

within the molecule.

The direction of the induced The direction of the induced magnetic field is opposite to magnetic field is opposite to that of the applied field.

that of the applied field.

C C HH

HH 00

Shielding Shielding The induced field shields the The induced field shields the nuclei (in this case, C and H) nuclei (in this case, C and H) from the applied field.

from the applied field.

A stronger external field is A stronger external field is needed in order for energy needed in order for energy difference between spin states difference between spin states to match energy of

to match energy of rf rf radiation.radiation.

C C HH

HH 00

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Chemical Shift Chemical Shift Chemical shift is a measure of Chemical shift is a measure of the degree to which a nucleus the degree to which a nucleus in a molecule is shielded.

in a molecule is shielded.

Protons in different Protons in different environments are shielded to environments are shielded to greater or lesser degrees;

greater or lesser degrees;

they have different chemical they have different chemical shifts.

shifts.

C C HH

H H 00

Chemical Shift Chemical Shift Chemical shifts (

Chemical shifts (δδ) are) are measured relative to the measured relative to the protons in tetramethylsilane protons in tetramethylsilane (TMS) as a standard.

(TMS) as a standard. SiSi CHCH33 CHCH33

CH CH33 HH33CC

δδ = = position of signal - position of TMS peakposition of signal - position of TMS peak spectrometer frequency

spectrometer frequency x 10x 1066 The reference point is defined by

the position of TMS (δ) as zero ppm(δ)

0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

Chemical shift ( Chemical shift (δδ, ppm), ppm) measured relative to TMS measured relative to TMS

Upfield Upfield Increased shielding Increased shielding Downfield

Downfield Decreased shielding Decreased shielding

(CH (CH33))44Si (TMS)Si (TMS)

0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

Chemical shift ( Chemical shift (δδ, ppm), ppm) δδ 7.28 ppm 7.28 ppm

H H CC

Cl Cl

Cl Cl Cl Cl

Effects of Molecular Structure on

1H Chemical Shifts

protons in different environments experience different degrees of shielding and have different chemical

shifts

The chemical shift is independent of the operating frequency of the spectrometer

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Chemical Shift Chemical Shift

Example: The signal for the proton in chloroform Example: The signal for the proton in chloroform (HCCl

(HCCl33) appears 1456 Hz downfield from TMS at) appears 1456 Hz downfield from TMS at a spectrometer frequency of 200 MHz.

a spectrometer frequency of 200 MHz.

δδ = = position of signal - position of TMS peakposition of signal - position of TMS peak spectrometer frequency

spectrometer frequency x 10x 1066

δδ = = 1456 Hz - 0 Hz1456 Hz - 0 Hz 200 x 10 200 x 1066 Hx Hx

x 10 x 1066

δ = 7.28δ = 7.28

Question

• What is the chemical shift of a proton that appears at 2065 Hz when recorded on a 300 MHz NMR spectrometer?

• A) 6.88 ppm

• B) 0.14 ppm

• C) 2.07 ppm

• D) 1.40 ppm

Electron withdrawal produces NMR signals downfield at

higher frequency (at larger δ values)

Electronegative

Electronegative substituents substituents decreasedecrease the shielding of methyl groups the shielding of methyl groups CH

CH33FF δδ 4.3 ppm 4.3 ppm CH

CH33OCHOCH33 δδ 3.2 ppm 3.2 ppm CH

CH33N(CHN(CH33))22 δδ 2.2 ppm 2.2 ppm CHCH33CHCH33 δδ 0.9 ppm 0.9 ppm CHCH33Si(CHSi(CH33))33 δδ 0.0 ppm 0.0 ppm

Electronegative

Electronegative substituents substituents decreasedecrease the shielding of methyl groups the shielding of methyl groups

CHCH33FF δ 4.3 ppmδ 4.3 ppm least shielded Hleast shielded H CHCH33OOCHCH33 δ 3.2 ppmδ 3.2 ppm

CH

CH33NN(CH(CH33))22 δ 2.2 ppmδ 2.2 ppm CH

CH33CHCH33 δ 0.9 ppmδ 0.9 ppm CH

CH33SiSi(CH(CH33))33 δ 0.0 ppmδ 0.0 ppm most shielded Hmost shielded H

Effect is cumulative Effect is cumulative

CHCHClCl33 δδ 7.3 ppm 7.3 ppm CHCH22ClCl22 δδ 5.3 ppm 5.3 ppm CH

CH33ClCl δδ 3.1 ppm 3.1 ppm

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Question

• Which proton is most shielded?

• A) CHCl

3

• B) CH

2

Cl

2

• C) CHBr

3

• D) CBr

4

Methyl, Methylene, and Methine Methyl, Methylene, and Methine C

CHH33 more shielded than CH more shielded than CH2 2 ;; CCHH22 more shielded than C more shielded than CHH

HH33CC CC C CHH33

CH CH33

HH δ 0.9δ 0.9

δδ 1.6 1.6

δ 0.8δ 0.8 HH33CC CC

C CHH33

CH CH33

CCHH22 δ 0.9δ 0.9

CHCH33 δδ 1.2 1.2

Question 3

• Select the most shielded proton in 1,1,2- trichlorobutane.

• A) 1

• B) 2

• C) 3

• D) 4

Protons attached to sp

Protons attached to sp22 hybridized carbon hybridized carbon are less shielded than those attached are less shielded than those attached

to sp

to sp33 hybridized carbon hybridized carbon

H

H HH

H H H

H H H H H

C C CC

H H H

H H

H HH

CH CH33CHCH33

δ 7.3 ppmδ 7.3 ppm δ 5.3 ppmδ 5.3 ppm δδ 0.9 ppm 0.9 ppm

“Diamagnetic Anisotropy”

The π electrons are less tightly held by the nuclei than are σ electrons; therefore they are more free to move in response to a magnetic field

Causes unusual chemical shifts for hydrogen bonded to carbons that form π bonds

But Protons Attached to sp Hybridized Carbon But Protons Attached to sp Hybridized Carbon

are More Shielded than those Attached are More Shielded than those Attached

to sp

to sp22 Hybridized Carbon Hybridized Carbon

CC CC H H H

H H

H HH

δδ 5.3 5.3

δδ 2.4 2.4

CHCH22OCHOCH33 C

C CC H

H

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Protons Attached to Benzylic and Allylic Protons Attached to Benzylic and Allylic Carbons are Somewhat Less Shielded than Usual Carbons are Somewhat Less Shielded than Usual

δ 1.5δ 1.5 δδ 0.8 0.8 H

H33CC CHCH33

δ 1.2δ 1.2 H

H33CC CHCH22 δδ 2.6 2.6

H

H33C—C—CHCH22——CHCH33

δ 0.9δ 0.9 δδ 0.9 0.9 δδ 1.3 1.3

Question

• Assign the chemical shifts δ 1.6,

• δ 2.2, and δ 4.8 to the appropriate protons of methylene cyclopentane.

• A) x = 1.6; y = 2.2; z = 4.8

• B) x = 4.8; y = 1.6; z = 2.2

• C) x = 1.6; y = 4.8; z = 2.2

• D) x = 2.2; y = 1.6; z = 4.8

Proton Attached to C=O of Aldehyde Proton Attached to C=O of Aldehyde

is Most Deshielded C is Most Deshielded C——HH

δ 2.4δ 2.4

δδ 9.7 9.7

δ 1.1δ 1.1 C C CC

O O

H H HH

CHCH33 H

H33CC

Question

• Assign the chemical shifts δ1.1,

• δ2.4, and δ9.7 to the appropriate protons of 2-methylpropanal.

• A) x = 2.4; y = 1.1; z = 9.7

• B) x = 1.1; y = 9.7; z = 2.4

• C) x = 9.7; y = 2.4; z = 1.1

• D) x = 1.1; y = 2.4; z = 9.7

Chemical Shift Table Chemical Shift Table

Type of proton

Type of proton Chemical shift (Chemical shift (δδ),),

ppmppm Type of protonType of proton Chemical shift (Chemical shift (δδ),), ppmppm

C C H

H RR 0.9-1.80.9-1.8

1.6-2.6 1.6-2.6 CC

H

H CC CC

2.1-2.5 2.1-2.5 CC

H H CC

OO

2.5 C 2.5

C HH CC CC

C C

HH ArAr 2.3-2.82.3-2.8

C C CC H H

4.5-6.5 4.5-6.5

Chemical Shift Table Chemical Shift Table

Type of proton

Type of proton Chemical shift (δChemical shift (δ),),

ppmppm Type of protonType of proton Chemical shift (δChemical shift (δ),), ppmppm

6.5-8.5 6.5-8.5

9-10 9-10

2.2-2.9 2.2-2.9

3.1-4.1 3.1-4.1

C C

HH BrBr 2.7-4.12.7-4.1

3.3-3.7 3.3-3.7 H

H ArAr

C C O O H H

C C HH NRNR

CC H

H ClCl

C C H

H OO

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Chemical Shift Table Chemical Shift Table

Type of proton

Type of proton Chemical shift (Chemical shift (δδ),), ppm ppm

1-31-3 HH NRNR

0.5-5 0.5-5 H

H OROR

6-8 6-8 H

H OArOAr

10-13 10-13 CC

O O H HOO

Characteristic Values of Chemical Shifts

1

H NMR spectrum / Integration

1-bromo-2,2-dimethylpropane

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The area under each signal is proportional to the number of protons that give rise to that signal.

The height of each integration step is proportional to the area under a specific signal.

The integration tells us the relative number of protons that give rise to each signal, not absolute number.

Integration

1

H NMR spectrum / Integration

1-bromo-2,2-dimethylpropane

1 1

4.54.5 (2)(2)

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Figure

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References

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