Not only does each group of atoms in a molecule vibrate at its own frequency, there are six different ways the molecules can vibrate. Each of these vibrations can occur in any molecule, but they all occur at different frequencies. These 6 modes can be seen in the Shockwave animation below.

Notice that different groups of atoms vibrate at different frequencies, and that one group of atoms can vibrate six different ways. That's a lot of different vibrations! How could someone possibly ever get any information from an infrared spectrum? Well, it's all Edisonian learning by trial and error. We've learned where the absorbances of these different groups show up by getting an IR spectrum of lots of known substances and seeing where each group absorbs. They then make tables of these values so that you and I can learn from their findings and interpret our own spectra. Sounds great, doesn't it? Well, as a special treat to you, below you will find a table which shows some of the most typical infrared absorptions and where they show up in a spectrum. Learn and enjoy!

Typical Infrared Absorptions

vibration	frequency range (cm-1)
carbonyl (C=O) stretch	1870 - 1650
Alcohols
O-H stretch	3640 - 3250
C-OH stretch	1160 - 1030
C-OH in-plane bend	1440 - 1260
C-OH wag	700 - 600
Alkanes
C-H stretch	2980 - 2850
CH2 wag	1470 - 1450
CH2 rock	740 - 720
CH3 wag	1390 - 1370
CH3 twist	1470 - 1440
Alkenes
=CH2 stretch	3040 - 3010
=CH2 wag	950 - 900
C=C stretch (cis isomer)	1665 - 1635
C=C stretch (trans isomer)	1675 - 1665
Amines
N-H stretch	3460 - 3280
NH2 wag	1650 - 1590
C-N stretch	1190 - 1130
C-N-C scissor	510 - 480
Esters
C-O-C asymmetrical stretch	1290 - 1180
O-C-O scissors	645 - 575
Nitro compounds
NO2 symmetrical stretch	1570 - 1550
NO2 asymmetrical stretch	1380 - 1360
NO2 scissors	650 - 600
NO2 rock	530 - 470
Sulfones
SO2 symmetrical stretch	1170 - 1120
SO2 asymmetrical stretch	1360 - 1290
SO2 scissors	610 - 545

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