Analytical Chemistry
• Chromatography (Separations)
• Mass Spectrometry
• Infrared (IR) Spectroscopy
• Nuclear Magnetic Resonance (NMR) Spectroscopy
• X-ray Crystallography (visual solid state molecular structure)
The Electromagnetic Spectrum
Scattering [X-ray Crystallography]
Q uickTim e™ and a TI FF ( Uncom pr essed) decom pr essor ar e needed t o see t his pict ur e.
Electronic Excitations [UV/Vis Spectroscopy]
Molecular Vibrations [IR]
Molecular Rotations [Rotational Spectroscopy]
Nuclear Spin "Flipping" [NMR]
Nuclear Magnetic Resonance (NMR)
Spectroscopy
• 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
-state
Add Energy
B0
-state
Energy released
(& detected)
Effect of Field Strength
An NMR Spectrometer
1H
NMR of Acetone
O
H3C
CH3
1H
NMR of Methyl Acetate
O
CH3
H3C
O
Electron Density Maps
Electron Shielding
Electron Shielding
Common NMR Shifts
X
H
X = N, O, S
H
H
X
C
H
H
X = N, O, S, halogen
O
H
O
H
H
R
R
R
H
C
H
H
O
12
C
C
H
10
8
6
C
4
2
C
H
0 ppm
Common NMR Shifts
aldehydes "aromatic" (benzene)
region "olefin" region
acids
12
10
8
6
near N,O,S,
"alkyl" region
halogen
4
2
0 ppm
1H
NMR of Methyl Acetate
O
O
H3C
H3C
O
O
CH3
CH3
1H
NMR of Neopentyl Bromide
1H
NMR of Neopentyl Bromide
1H
NMR of Ethyl Acetate
3
Integral
ratios
O
CH2
H3C
O
CH3
3
2
What are these strange signals?
Proton Coupling
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
These 2 hydrogens will couple to the methyl group
These 3 hydrogens are identical - 1 signal
O
Me
H H
O
CH3
Higher Beff
B0
Split
Identical, no
effect on Beff
Add to Beff
Subtract from Beff
Lower Beff
CH3
CH3
Proton Coupling
These 2 hydrogens are identical - 1 signal
O
Me
H H
O
CH3
These 3 hydrogens
will couple to the
methylene group
Split
CH2
B0
Add to Beff
Add to Beff
Subtract from Beff
Subtract from Beff
CH2
1H
NMR of Ethyl Acetate
3
O
CH2
H3C
O
CH3
3
2
Multiplicity
Determining Hydrogen Atom
Relationships
The Substitution Test: For any pair of H’s, substitute
each separately with an X and compare the two
structures.
•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)
Determining Hydrogen Atom
Relationships
X
Cl
Cl
H
H
Cl
Cl
identical = homotopic
H
H
Cl
Cl
X
X
Me
Cl
Cl
Me
H
X
H
enantiomers = enantiotopic
H
Me
Cl
H
H
H
Me
Cl
X
Me
Cl
X
Determining Hydrogen Atom
Relationships
HO H
Me
Me
X H
HO H
diastereomers = diastereotopic
Me
Me
H H
HO H
Me
Me
H X
Me
Me
Me
H H
Me
X H
Me
diastereomers = diastereotopic
Me
H X
Coupling Constants
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
Useful Coupling Constants
H
H
H
H
H
0-3 Hz
H
H
6-12 Hz
H
H
H
12-18 Hz
H
6-8 Hz
1-3 Hz
H
0-1 Hz
(usually not
observed)
Olefin Geometry Through Coupling
Constants
A Splitting Diagram for a Doublet of
Doublets
A Quartet Vs. A Doublet Of Doublets
Coupling With “Exchangeable” Protons
Dry, ultra-pure ethanol
Ethanol with trace acid
Four Different (But Similar) Compounds
Molecular Ion = 74 [C4H10O]
Unknown #1
9
1
Unknown #2
6
2
1
1
Unknown #3
3
2
1
4
Unknown #4
1
8
1
Putting It All Together - Identifying
Unknown Compounds
86
89 (5%)
Putting It All Together - Identifying
Unknown Compounds
Putting It All Together - Identifying
Unknown Compounds
3
1
6