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Infrared Spectroscopy

Absorption of IR light in different

regions is characteristic of the

various functional groups present

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Electromagnetic Radiation Spectrum

E =hn

ln = c

Molecular Excitation

electronic | vibrational

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Infrared Spectrometer(double-beam)

I0

I

Detector Types

diode arrayFourier Transfer

Ours is FT single beam

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IR Sample Types and Holders

Solid (KBr pellet)

or

powder w/ ATR*

Liquid (neat / pure) Solution Gas

*ATR = attenuated total

reflectance apparatus

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Typical IR spectrum

% Transmittance = (I/I0)(100)

Absorbance (A) = log(I0/I)

n = 104/l (microns)_

wavenumbers(Kaysers) cm-1

2.5m 15m

m = 10-6m

cm= 10-2m

Basic frequency (n

)

and wavelength (l

)

relationships

E = hn

n = c / l

wavelength

~ frequency (n)_

(l)

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Typical IR spectrum

2.5m 15m

Diagnostic Region Fingerprint Region

1500

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Isomer IR Spectra

similar, but different

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Uses of IR Spectra

1. Identity - by exact spectral match

2. Purity - by band resolution &additional peaks

3. Concentration Absorbance + Std. curves

4. Structure Analysis by characteristic bands

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"Back of the Envelop" Regions

3600 3200 OH, NH stretch nOH, NH

3300 3000 CH unsat'd stretch nCH unsat

3000

2700 CH sat'd stretch nCH sat2300 2100 triple bonds nCN, CC

1800 1650 C=O stretch nC=O

1700 1600 C=C stretch nC=C

1600 1500 aromatic C-H bend dCH arom

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Carbonyl (C=O) Details

1740 cm-1 1720 cm-1 1680 cm-1

ester aldehyde amideketone

carboxylic acid

conjugation effects

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Colthup Chart 1

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Key Spectral Features of

Major Functional Groups

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Alkane

|nCH satd (below 3000 cm-1)

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Alkene

n

CH unsatd| (above 3000 cm

-1

)

| nC=C (1600-1700 cm-1)

***

**

*

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Aromatic

4 regions (cm-1) *> 3000

1600-2000

1500-1600700-850

* * **

out of planein plane

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Aromatic Substitution Patterns

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Ethers, Alcohols

1000-1200 nC-O-C

3200-3600 nO-Hstrong, broad, symmetrical

hard to identify

easy to identify

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Phenols

Aromatic +

OH strong, broad, symmetrical

Note: the nOHregion

appears broad, butsimply

due to scale of X-axis

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Ketones

ca 1720 nC=O

note C=O shift ->

due to conj'n

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Aldehydes & Carboxlic Acids

ca 172

nC=O

nC=O

ca172

&

nO-H

2700 3600

v. strong & broad

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Esters

ca.174 nC=O

ca 1780

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Carbonyl (C=O) Details

1740 cm-1 1720 cm-1 1680 cm-1

ester aldehyde amideketone

carboxylic acid

conjugation effects

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Amine3200-3600(weaker than OH)

n

N-H

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Amine classes

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Amide

ca168

nC=O

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Problem Assignment

Spectrum Interpretation label peaks

3600 3200 OH, NH stretch nOH, NH

3300 3000 CH unsat'd stretch nCH unsat

3000 2700 CH sat'd stretch nCH sat

2300 2100 triple bonds nCN,CC

1800 1650 C=O stretch nC=O

1700 1600 C=C stretch nC=C

1600 1500 aromatic C-H bend dCH arom

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Lab Assignment

Spectrum Interpretation label peaks

3600 3200 OH, NH stretch nOH, NH

3300 3000 CH unsat'd stretch nCH unsat

3000 2700 CH sat'd stretch nCH sat

2300 2100 triple bonds nCN,CC

1800 1650 C=O stretch nC=O

1700 1600 C=C stretch nC=C

1600 1500 aromatic C-H bend dCH arom

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Spectra for Problems

Spectrum Interpretation label peaks

3600 3200 OH, NH stretch nOH, NH

3300 3000 CH unsat'd stretch nCH unsat

3000 2700 CH sat'd stretch nCH sat

2300 2100 triple bonds nCN,CC

1800 1650 C=O stretch nC=O

1700 1600 C=C stretch nC=C

1600 1500 aromatic C-H bend dCH arom

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Anhydride, Acid chloride

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Nitrile & Nitro

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Amino Acid

C-Cl bond