Post on 19-Jan-2016
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Nuclear Magnetic Resonance (NMR) Spectroscopy
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• Chromatography (Separations)
• Mass Spectrometry
• Infrared (IR) Spectroscopy
• Nuclear Magnetic Resonance (NMR) Spectroscopy
• X-ray Crystallography (visual solid state molecular structure)
Analytical Chemistry
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Molecular Vibrations [IR]
Molecular Rotations [Rotational Spectroscopy]
Nuclear Spin "Flipping" [NMR]
Electronic Excitations [UV/Vis Spectroscopy]
Scattering [X-ray Crystallography]
The Electromagnetic Spectrum
• Identify the environment of hydrogen and carbon atoms
• Identify atom connectivity
• Identify stereochemical relationships
Nuclear Magnetic Resonance (NMR) Spectroscopy
The spin state of a nucleus is affected by an applied magnetic field
Nuclear Magnetic Resonance (NMR) Spectroscopy
α-state
β-state
B0
Add EnergyEnergy released
(& detected)
Nuclear Magnetic Resonance (NMR) Spectroscopy
Effect of Field Strength
An NMR Spectrometer
O
CH3H3C
1H NMR of Acetone
O
OH3CCH3
1H NMR of Methyl Acetate
Electron Density Maps
Electron Shielding
Electron Shielding
0 ppm4681012 2
X H
X = N, O, S
O
HR
O
ORH
R C
H
H
HC C
H
HC C H
HX C
H
H
H
X = N, O, S, halogen
Common NMR Shifts
0 ppm4681012 2
"alkyl" regionnear N,O,S,halogen"olefin" region
"aromatic" (benzene) regionaldehydes
acids
Common NMR Shifts
O
OH3CCH3
O
H3C OCH3
1H NMR of Methyl Acetate
1H NMR of Neopentyl Bromide
1H NMR of Neopentyl Bromide
O
OH3CCH2
CH3
3
2
3
1H NMR of Ethyl Acetate
What are these strange signals?
Integral ratios
Hydrogen nuclei will couple to each other if:
•They are not chemically equivalent
•They are 2 or 3 bonds apart
•Double bonds can cause coupling through 4 bonds
H
H H
H
H H
2 bonds 3 bonds 4 bonds
coupling coupling no coupling
Proton Coupling
Me O CH3
O H HThese 3 hydrogens are identical - 1 signal
These 2 hydrogens will couple to the methyl group
B0
Add to Beff
Identical, no effect on Beff
Subtract from Beff
Higher Beff Lower Beff
Split
CH3 CH3
Proton Coupling
Me O CH3
O H H
These 2 hydrogens are identical - 1 signal
These 3 hydrogens will couple to the methylene group
B0
Add to Beff
Subtract from Beff
Split
CH2Add to Beff
Subtract from Beff
CH2
Proton Coupling
O
OH3CCH2
CH3
3
2
3
1H NMR of Ethyl Acetate
Multiplicity
Determining Hydrogen Atom Relationships
•If the structures are identical – Homotopic (no coupling)
•If the structures are enantiomers – Enantiotopic (no coupling)
•If the structures are diastereomers – Diastereotopic (coupling is possible)
The Substitution Test: For any pair of H’s, substitute each separately with an X and compare the two structures.
H
HClCl
X
HClCl
H
XClCl
identical = homotopic
H
HClMe
X
HClMe
H
XClMe
enantiomers = enantiotopic
H
XClMe
H
XClMe
Determining Hydrogen Atom Relationships
MeMe
HO H
H H
MeMe
HO H
X H
MeMe
HO H
H X
diastereomers = diastereotopic
diastereomers = diastereotopicMe Me
HH
Me Me
HX
Me Me
XH
Determining Hydrogen Atom Relationships
The coupling constant (J) is the distance between two adjacent peaks of a split NMR signal in hertz (Hz)
Coupled protons have the same coupling constant
Coupling Constants
HH
H
H
H
H
6-12 Hz0-3 Hz 12-18 Hz
H
H
H
H
1-3 Hz6-8 Hz 0-1 Hz
(usually not observed)
H
H
Useful Coupling Constants
Olefin Geometry Through Coupling Constants
A Splitting Diagram for a Doublet of Doublets
A Quartet Vs. A Doublet Of Doublets
Dry, ultra-pure ethanol
Ethanol with trace acid
Coupling With “Exchangeable” Protons
Molecular Ion = 74 [C4H10O]
Four Different (But Similar) Compounds
1
9
Unknown #1
1
2
1
6
Unknown #2
2 1
3
4
Unknown #3
1
1
8
Unknown #4
86
89 (5%)
Putting It All Together - Identifying Unknown Compounds
Putting It All Together - Identifying Unknown Compounds
1
36
Putting It All Together - Identifying Unknown Compounds