10/23/2019 IR-frequencies

Characteristic IR Absorption Frequencies of Organic Functional Groups

Functional Type of Characteristic
Intensity
Group Vibration Absorptions (cm-1)
Alcohol
(stretch, H-
O-H 3200-3600 strong, broad
bonded)
(stretch,
O-H 3500-3700 strong, sharp
free)
C-O (stretch) 1050-1150 strong
Alkane
C-H stretch 2850-3000 strong
-C-H bending 1350-1480 variable
Alkene
=C-H stretch 3010-3100 medium
=C-H bending 675-1000 strong
C=C stretch 1620-1680 variable
Alkyl
Halide
C-F stretch 1000-1400 strong
C-Cl stretch 600-800 strong
C-Br stretch 500-600 strong
C-I stretch 500 strong
Alkyne
C-H stretch 3300 strong,sharp
stretch 2100-2260 variable, not present in symmetrical alkynes
Amine
medium (primary amines have two bands; secondary
N-H stretch 3300-3500
have one band, often very weak)
C-N stretch 1080-1360 medium-weak
N-H bending 1600 medium
Aromatic
C-H stretch 3000-3100 medium
C=C stretch 1400-1600 medium-weak, multiple bands
Analysis of C-H out-of-plane bending can often distinguish substitution patterns
Carbonyl Detailed Information on Carbonyl IR
C=O stretch 1670-1820 strong
(conjugation moves absorptions to lower wave numbers)
Ether
1000-1300 (1070-
C-O stretch strong
1150)
Nitrile
CN stretch 2210-2260 medium
Nitro
N-O stretch 1515-1560 & 1345- strong, two bands
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IR Absorption Frequencies of Functional Groups Containing a Carbonyl (C=O)

Functional Group Type of Vibration Characteristic Absorptions (cm-1) Intensity
Carbonyl
C=O stretch 1670-1820 strong
(conjugation moves absorptions to lower wave numbers)
Acid
C=O stretch 1700-1725 strong
O-H stretch 2500-3300 strong, very broad
C-O stretch 1210-1320 strong
Aldehyde
C=O stretch 1740-1720 strong
=C-H stretch 2820-2850 & 2720-2750 medium, two peaks
Amide
C=O stretch 1640-1690 strong
N-H stretch 3100-3500 unsubstituted have two bands
N-H bending 1550-1640
Anhydride
C=O stretch 1800-1830 & 1740-1775 two bands
Ester
C=O stretch 1735-1750 strong
C-O stretch 1000-1300 two bands or more
Ketone
acyclic stretch 1705-1725 strong
3-membered - 1850
4-membered - 1780
cyclic stretch 5-membered - 1745 strong
6-membered - 1715
7-membered - 1705
a,b-unsaturated stretch 1665-1685 strong
aryl ketone stretch 1680-1700 strong

A good general reference for more detailed information on interpretation of infrared spectra (as well as other
spectroscopic techniques) is Silverstein, R.M.; Bassler, G.C.; and Morrill, T.C. Spectrometric Identification
of Organic Compounds. 4th ed. New York: John Wiley and Sons, 1981. QD272.S6 S55

A more complete listing of functional group absorption frequencies ma be found in: Nakanishi, Koji Infrared
Absorption Spectroscopy. QD95.N383

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