Lecture 37 Nuclear magnetic resonance
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Transcript of Lecture 37 Nuclear magnetic resonance
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Lecture 37Nuclear 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 orbitalsElectron spinNuclear spin
angular momentum
charge
magneticmoment
mass
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Magnet-magnetic-field interactionhigh energy
low energy
Classical
Magnetic moment
Magnetic field
Quantum
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Tesla
Nikola TeslaPublic domain image from Wikipedia
kgm2/s
C
J
kg
T (Tesla)
1 T = 1 V s / m2
Field strength in 500 MHz NMR ($0.5M) = 11.7 TField 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 orbitalsFirst-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-value2.002319…
2-fold degeneracy
(field off)
Electron spin
α
βESR or EPR (field on)
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Nuclear g-factorproton: 5.586
2-fold degeneracy
(field off)
Nuclear spin
α
β
NMR (field on)
Nuclear magneton 1800 times smaller
than Bohr magnetonProtonmass
Negative signpositive nuclear charge
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Proton NMR
α
β
Sample
Sweep coils
Radio freq
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Proton NMR spectra(1) Overall intensity(2) Groups of peaks(3) Relative intensities of groups of peaks(4) 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~ ΔE × number of excess α spins~ B2 / T
Stronger magnet + lower temperature
excess α spins
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Group of peaks: chemical shiftsResonance freq.
Chemical shift
Resonance freq. of TMSSi(CH3)4
“ppm”
α
β
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Group of peaks: chemical shiftsResonance freq.
Chemical shift
Shieldingconstant
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Group of peaks: chemical shifts
Shieldingconstant
+
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Group of peaks: chemical shiftsShieldingconstant
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Group of peaks: chemical shifts
14 12 10 8 6 4 2 0 δ
-COOH-CHO
Ar-HArOH
ROH-CH-
-CH2-
RCH3
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Relative intensitiesC2H6O
H H2 H3OH CH2
CH3
CH3CH2OH
ROH-CH2-
RCH3
4 2 0 δ
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Hyperfine structure
CH3CH2OH
OH CH2 CH3
α
βα
α
β
β
H nearby HSpin-spin coupling:
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Hyperfine structure
CH3CH2OH
OH CH2 CH3
α
βα
α
β
β
H HSpin-spin coupling:
ββ
ααβα, αβ
H2
αβ, βαββ
αα
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Hyperfine structure
CH3CH2OH
OH CH2 CH3
1
1 1
1 2 1
1 3 3 1
1 4 6 4 1
Pascal’s triangle
nearby H
nearby H2
nearby H3
nearby H4
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CH3CH2OH
OH CH2 CH3
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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CH3CH2OH
OH CH2 CH3
Q: Why is there no spin-spin coupling between the two protons in the CH2 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
CH3CH2OHTripletmagnetic
Singletnon-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 constantH H
Fermicontact
Fermicontact
Covalent bondsinglet-coupling
higher energy??Fermi contactlower energy!
higher energy??
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Spin-spin coupling constant
H C
Fermicontact
Fermicontact
Covalentbond
singletcoupling
H
Covalentbond
singletcoupling
Hund
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Spin-spin coupling constant
H C C H
H
C H
Martin KarplusDepartment 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.