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