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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
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
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
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
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