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INTERPRETATION OF INFRARED (IR) SPECTRA
 
An IR spectrum by itself does not provide an exact chemical structure of a compound, but will provide information about its functionality based on band location and intensity (see the table of functional groups). Reference spectra are required for exact structural information.

The following information identifies the requirements and limitations of FTIR analysis.
 

General Requirements

Good quality spectrum

Proper sample preparation

Relatively pure material

Correlation chart

Reference spectra for comparison
 

Limitations


 

 Closely related materials, i.e.,
Polyethylene glycols - molecular weight
Vegetable oils
Petroleum based oils
Amino/Hydroxyl functionality - amino alcohols
 


 

 Mixtures and some polymers
Urethanes
Phenolics
Silicates
 

 

 Inorganics
Difficult to identify cations
Simple anions (halogens)
 

Considerations

Relative intensities of bands are important

Any mismatch with reference spectrum negates identification
Lack of bands is positive information

 

Infrared Functional Groups

Group

Vibration

Type

Range

 

CH3

Stretch

Antisymmetric

2962± 10

 

symmetric

2872± 10

 

Bend

Antisymmetric

1460± 10

 

symmetric

1375± 10

Umbrella

CH2

Stretch

Antisymmetric

2926± 10

 

symmetric

2853± 10

 

Bend

Scissors

1455± 10

 

Rocking

Concerted

720± 10

Four or more

=CH2

Stretch

Antisymmetric

3080

 

Symmetric

2997

 

Twist

Out-of-plane 

993

Mono or Trans only

Bend

Out-of-Plane

909

Terminal alkene

1821

Overtone frequency

C=C

Stretch

Cis and Vinyl

1640+ 20

 

Trans, tri and tetra

1670+ 10

 

≡CH

Stretch

Normal

3300± 20

Always very sharp

Bend

Normal

630

 

1238

Overtone frequency

C≡C

Stretch

Normal

2220± 10

Terminal alkynes

2225± 10

Internal alkynes

C@N

Stretch

Normal

2250± 10

10 to 20 lower when conjugated

CH2

Bend

Scissoring

1426

Shifted with @

C(sp2)-H (aromatic)

Stretch

Aromatic or unsaturated

3050± 50

Not assigned to specific vibrational modes

Aromatic Ring

Ring Stretch

Symmetric

1590± 10

Non-symmetrical substitution

 

1500± 10

Variable intensity

Sideways

1450± 10

CH3 bend overlap

Bend

Hydrogen

730± 20

Out of Plane

Out of Plane Ring

690± 20

Mono, meta or 1,3,5 substitution

OH

Stretch

OH stretch

3350± 150

Broad

Bend

Broad

1400± 100

 

Wag

Band

660

Not a good frequency

C-O to C-C

Stretch

Anti-symmetrically coupled

1°: 1050± 25

2°: 1125± 25

3°: 1150± 50

 

NH2

Stretch

Antisymm

3300± 100

 

Symmetrical

3290

 

Bend

Scissoring

1615±15

1° only

Wag

Band

797

1° and 2° only

CH=O

Stretch/bend

C-H

2820

Hidden under CH stretches

2735± 15

Not hidden

Bend

C-H

1395± 10

 

C=O

Stretch

C=O

3420

1727 Overtone Frequency

C=O

1727

Aldehyde

Stretch

Conjugated

3400

1715 Overtone Frequency

Conjugated

1715

Ketone

CH2

Bend

Scissoring

1407

Shifted with carbonyl

 
COOH

Wag

Out-of-plane OH

935± 15

Acid Dimer

Stretch

Antisymmetric

3000± 500

Acid Dimer

Overtone

Bands

2500± 300

Acid group

Bend/stretch

Combination

1425± 25

Acid group

Stretch/bend

Combination

1250± 50

Acid group

O-C(O)-C

Stretch

O-C-C

1245± 15

 

1190±30

Higher esters

C=O

3484

Overtone ester

1742

ester

O=C-O-C=O

Stretch

Symmetrically

1830± 10

Anhydride

Antisymmetric

1755± 15

Anhydride

 

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