Lecture 37 Nuclear magnetic resonance - PowerPoint Presentation

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Lecture 37

Nuclear magnetic resonance

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Nuclear magnetic resonance

The use of NMR in chemical research was pioneered by

Herbert S. Gutowski of Department of Chemistry, University of Illinois, who established the relationship between chemical shifts and molecular structures. He also discovered spin-spin coupling.Foundation of magnetic spectroscopy.Proton NMR.

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Circular electric current = magnet

Electrons in

p

,

d, f

orbitals

Electron spinNuclear spin

angular

momentum

charge

magnetic

moment

mass

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Magnet-magnetic-field interaction

high energy

low energy

Classical

Magnetic moment

Magnetic field

Quantum

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Tesla

Nikola Tesla

Public domain image from Wikipedia

kgm

2

/s

C

J

kg

T (Tesla)

1 T = 1 V s / m

2

Field strength in 500 MHz NMR ($0.5M) = 11.7 T

Field strength in 1 GHz NMR ($20M) = 23.5 T

Strongest continuous magnetic field = 45 T

(National High Magnetic Field Lab at Tallahassee, FL)

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Electrons in

p

, d, f orbitals

First-order perturbation theory

Bohr

magneton

9.724×10

−24

J/T

(2

l

+ 1)-fold

degeneracy

(field off)

Zeeman effect (field on)

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Quantum electrodynamics

g

-value

2.002319…

2-fold

degeneracy

(field off)

Electron spin

α

β

ESR or EPR (field on)

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Nuclear

g

-factor

p

roton: 5.586

2-fold

degeneracy

(field off)

Nuclear spin

α

β

NMR (field on)

Nuclear

magneton

1800 times smaller

than Bohr

magneton

P

roton

mass

Negative sign

positive nuclear charge

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Proton NMR

α

β

Sample

Sweep coils

Radio

freq

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Proton NMR spectra

Overall intensity

Groups of peaks

Relative intensities of groups of peaks

Pattern in each group (hyperfine structure)

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Overall intensity

α

β

Intensity of a NMR signal

~ energy of RF radiation absorbed / time

~ Δ

E ×

number of excess

α

spins

~

B

2

/

T

Stronger magnet + lower temperature

excess

α

spins

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Group of peaks: chemical shifts

Resonance freq.

Chemical shift

Resonance freq. of TMS

Si(CH

3

)

4

“ppm”

α

β

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Group of peaks: chemical shifts

Resonance freq.

Chemical shift

Shielding

constant

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Group of peaks: chemical shifts

Shielding

constant

+

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Group of peaks: chemical shifts

Shielding

constant

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Group of peaks: chemical shifts

14 12 10 8 6 4 2 0

δ

-COO

H

-C

H

O

Ar

-

H

ArO

H

RO

H

-C

H

-

-C

H

2

-

RC

H

3

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Relative intensities

C

2H6

O

H

H

2

H

3

OH

CH2

CH3

CH

3

CH

2

OH

RO

H

-C

H

2

-

RC

H

3

4 2 0

δ

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Hyperfine structure

CH

3

CH

2

OH

OH

CH

2

CH3

α

β

α

α

β

β

H

nearby H

Spin-spin coupling:

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Hyperfine structure

CH

3

CH

2

OH

OH

CH

2

CH

3

α

β

α

α

β

β

H

H

Spin-spin coupling:

ββ

α

α

βα, α

β

H

2

α

β

,

β

α

β

β

α

α

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Hyperfine structure

CH

3CH2

OH

OH

CH

2

CH

3

1

1

1

1

2

1

1

3

3

1

1

4

6

4

1

Pascal’s triangle

nearby H

nearby H

2

nearby H

3

nearby H

4

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CH

3

CH2OH

OH

CH

2

CH

3

Q

: Why doesn’t the proton in the OH group cause splitting?

A

: The proton undergoes a rapid exchange with protons in other ethanol or water molecules; its spin is indeterminate in the time scale of spectroscopic transitions; this causes lifetime broadening of spectral line rather than splitting.

?

Hyperfine structure

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CH

3

CH2OH

OH

CH

2

CH

3

Q

: Why is there no spin-spin coupling between the two protons in the CH

2

group?

A

: There

is

spin-spin coupling between them; however, its effect on the peaks is null and undetectable; this is because these protons are chemically and magnetically equivalent.

?

?

Hyperfine structure

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Hyperfine structure

CH

3CH2OH

Triplet

magnetic

Singlet

non-magnetic

no spin-spin coupling

with spin-spin coupling

No change in spacing

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Spin-spin coupling constant

H

H

H

C

H

H

C

C

H

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Spin-spin coupling constant

H

H

Fermi

contact

Fermi

contact

Covalent bond

singlet-coupling

higher energy??

Fermi contact

lower energy!

higher energy??

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Spin-spin coupling constant

H

C

Fermi

contact

Fermi

contact

Covalent

bond

singlet

coupling

H

Covalent

bond

singlet

coupling

Hund

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Spin-spin coupling constant

H

C

C

H

H

C

H

Martin

Karplus

Department of Chemistry

University of Illinois

ILLIAC

Karplus

equation

Image (c) University of Illinois

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Magnetic resonance imaging: MRI

Paul

Lauterbur

(far right)

Department of Chemistry

University of Illinois

Magnetic field gradient

Intensity

~ number of protons (in water) at

x

x

Resonance frequency

~ location (

x

)

Public domain image from Wikipedia

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Summary

We have studied the foundation of magnetic interactions and magnetic spectroscopy.

We have learned the theory of proton NMR as an essential tool for chemical structural analysis.The origins of chemical shifts, hyperfine structures, and spin-spin coupling constants are discussed as well as their relation to molecular structures.