Infrared Vibrational Modes

By Gregory Brust
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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