Nuclear Magnetic Resonance (NMR) Spectroscopy QuickTime™ and a decompressor are needed to see this picture. QuickTime™ and a decompressor are needed to see this picture. QuickTime™ and a decompressor are needed to see this picture. QuickTime™ and a decompressor are needed to see this picture.
41
Embed
Nuclear Magnetic Resonance (NMR) Spectroscopy. Chromatography (Separations) Mass Spectrometry Infrared (IR) Spectroscopy Nuclear Magnetic Resonance (NMR)
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Nuclear Magnetic Resonance (NMR) Spectroscopy
QuickTime™ and a decompressor
are needed to see this picture.
QuickTime™ and a decompressor
are needed to see this picture.
QuickTime™ and a decompressor
are needed to see this picture.
QuickTime™ and a decompressor
are needed to see this picture.
• Chromatography (Separations)
• Mass Spectrometry
• Infrared (IR) Spectroscopy
• Nuclear Magnetic Resonance (NMR) Spectroscopy
• X-ray Crystallography (visual solid state molecular structure)
Analytical Chemistry
QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.
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