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

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

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

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

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 CH_{2 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

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

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

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)

(9) (9)