NMR & IR Tables

IR Characteristic Vibration Frequencies

Complex molecules produce a rich assortment of IR bands, but only a few of these can be assigned with confidence. The easiest bands to assign are the ones that appear between 1500-4000 cm^-1. These bands are usually due to one of three kinds of groups: XH, double bonded XY, and triple bonded XY. Characteristic frequencies (in cm^-1) for the most important categories are listed below.

OH (alcohol)

3650-3580 (“free”; often sharp)
3550-3200 (hydrogen-bonded; often “U” shaped)

OH (carboxylic acid)

3300-2500 (very broad, strong, and c entered near 3000)

NH and NH₂ (amine)

3500 + 3400 (“free” NH₂; often “V” shaped)
3400-3330 + 3330-3250 (hydrogen-bonded NH₂)
3350-3310 (NH; weak, often “V” shaped)

CH (alkyne)

3330-3260 (strong, sharp)

CH (benzene)

3100-3000 (weak)

CH_n (alkane)

3000-2840 (weak, but they add up)

CN triple bond

2300-2100 (medium, sharp)

CC triple bond

2260-2100 (terminal, weak; internal, very weak)

C=O (aldehyde, ketone)

1740-1720 (1690-1670 if conjugated)

C=O (carboxylic acid)

1720-1700 (1710-1680 if conjugated)

C=O (ester, lactone)

1750-1735 (1730-1715 formate esters)

C=O (amide)

1700-1650

C=O (acid chloride)

1815-1770

C=O (anhydride)

1818 + 1750 (1775 + 1720 if conjugated)

C=C (alkene)

1670-1640 (terminal, medium; internal, weak)