Infrared Absorption Frequencies

Numbers separated by dashes are ranges; when two numbers are separated by a comma, both absorptions are expected. Individual numbers not components of a range should be considered ± 5 cm-1.

Intensity of absorptions (relative height of peak) is keyed by letter: s = strong; m = medium; w = weak; v = very; br = broad.

Compound Class Structure n, cm-1 Intensity Assignment
Alkanes RCH2CH3 2850-3000

1450-1470

1370-1380

720-725

 s

s

s

m

 CH stretch

CH2 and CH3

bending modes
Alkenes RCH=CH2 3040-3140

1655

910, 990

 m

m

s

 =C-H stretch

C=C stretch

=CH out of plane
  R2C=CH2 3080-3140

1650

890

 s

m

s

 =C-H stretch

C=C stretch

=CH out of plane
  Z- RCH=CHR 3020

1660

675-725

 w

w

m

 =C-H stretch

C=C stretch

=CH out of plane
  (a) 3-ring 1641 w C=C stretch
  (b) 4-ring 1566 w "
  (c) 5-ring 1611 w "
  (d) 6-ring 1649 w "
  (e) 7-ring 1651 w "
  (f) 8-ring 1653 w "
  E- RCH=CHR 3020

1675

970

 w

w

s

 =C-H stretch

C=C stretch

=CH out of plane
  R2C=CHR 3020

1670

790-840

 w

w

s

 =C-H stretch

C=C stretch

=CH out of plane
  R2C=CR2 1670 vw to missing C=C stretch
Alkynes RCº CH 3300

2100-2140

600-700

 s

m

s

 º CH stretch

CºC stretch

ºC-H bend
  RCº CR 2190-2260 vw to missing Cº C stretch
Alkyl Halides R-F 1000-1350 s C-F stretch
  R-Cl 750-850 s C-Cl stretch
  R-Br 500-680 s C-Br stretch
  R-I 200-500 out of range C-I stretch
Alcohols RCH2OH 3400-3600

1050

 varies

s

 OH stretch

C-O stretch
  R2CHOH 3400-3600

1100

 varies

s

 OH stretch

C-O stretch
  R3COH 3400-3600 s OH stretch
Ethers R-O-R 1070-1150 s C-O stretch
Aldehydes RCHO 1725 s C=O stretch
  C=CCHO 1685 s C=O stretch
  ArCHO 1700 s C=O stretch
  All aldehydes 2720, 2820 (both) m CHO out of plane
Ketones R2CO 1715 s C=O stretch
  (a) 4-ring 1780 " "
  (b) 5-ring 1745 (doublet) " "
  (c) 6-ring 1715 " "
  C=CCOR 1675 m "
  ArCOR 1690 s "
Carboxylic Acids RCO2H 3400

1760

2800-3400

1710

 s

s

s (broad)

s

 monomer OH

monomer C=O

dimer OH

dimer C=O
  C=CCO2H 3400

1720

2800-3400

1690

 s

s

s (broad)

s

 monomer OH

monomer C=O

dimer OH

dimer C=O
  RCO2- 1550-1610

1400

 >m

"

 C-O stretch

"
Esters RCO2R' 1735 s C=O stretch
  (a) 5-ring 1770 s "
  (b) 6-ring 1735 s "
  C=CCO2R 1720 s "
  ArCO2R 1720 s "
Amides RCONH2 3500

1690

3400

1650

1600-1640

 m

s

m

s

s

 NH stretch (free)

C=O stretch (free)

NH str. (H-bond)

C=O str. (H-bond)

NH out of plane
  RCONHR 3440

1680

3330

1650

1530-1550

 m

s

m

s

m

 NH stretch (free)

C=O stretch (free)

NH str. (H-bond)

C=O str. (H-bond)

NH out of plane
  (a) 4-ring 1745 s C=O stretch
  (b) 5-ring 1700 " "
  (c) 6-ring 1640 " "
  RCONR2 1650 m C=O stretch
Anhydrides RCO2COR 1760, 1820 (both) s C=O stretch; symm., unsymm.
Acid Chloride RCOCl 1800 s C=O stretch
Amines RNH2 3400, 3500 (both)

1560-1640

1030-1230

 w

s

m

 NH stretch

NH2 in plane bend

C-N stretch
  R2NH

3310-3350

1030-1230

 w

m

 NH stretch

C-N stretch
  Ar2NH 3450

1250-1360

 w

s

 NH stretch

Ar-N stretch
Benzenes monosubst. 730-770

690-710

 m

m

 CH out of plane bending
  ortho-disub. 735-770 m "
  meta-disub. 750-810

690-710

 m

m

 "
  para-disub. 810-840 m "
  1,2,3-trisub. 760-780

705-745

 m

m

 "
  1,3,5-trisub. 810-865

675-730

 m

m

 "
  1,2,4-trisub. 805-825

870-885

 m

m

 "
  1,2,3,4-tetrasub. 800-810 m "
  1,2,4,5-tetrasub. 855-870 m "
  1,2,3,5-tetrasub. 840-850 m "
  1,2,3,4,5-pentasub. 870 w "

References

1. Bellamy, L. J., "The Infrared Spectra of Complex Molecules", 2nd edition, John Wiley & Sons, New York, 1961.

2. Nakanishi, K. "Infrared Absorption spectroscopy - Practical", Holden-Day, Inc., San Francisco, 1962.

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