103 Infrared spectroscopy.
Absorbing infrared radiation makes covalent bonds vibrate. Different types of bond absorb
different wavelengths of infrared:
Instead of wavelength, infrared spectroscopists record the wavenumber; the number of waves
that fit into 1 cm. (This is easily converted to the energy of the wave.)
For some reason the spectra are recorded backwards (from 4000 to 500 cm-1is typical),
often with a different scale below 1000 cm-1(to see the fingerprint region more clearly) and
upside-down (% radiation transmitted is recorded instead of the absorbance of radiation).
The wavenumbers of the absorbed IR radiation are characteristic of many bonds, so IR spec-
troscopy can determine which functional groups are contained in the sample. For example,
the carbonyl (C=O) bond will absorb at 1650-1760cm-1.
103.1 Summary of absorptions of bonds in organic molecules.
w:Infrared Spectroscopy Correlation Table^1
Bond Minimum wavenum-
ber(cm-1)Maximum
wavenumber(cm-1)Functional group
(and other notes)
C-O 1000 1300 Alcohols and esters
N-H 1580 1650 Amine or amide
1610 1680 Alkenes
1650 1760 Aldehydes, ketones,
acids, esters, amides
O-H 2500 3300 Carboxylic acids (very
broad band)
C-H 2850 3000 Alkane
C-H 3050 3150 Alkene (Compare in-
tensity to alkane for
rough idea of relative
number of H atoms in-
volved.)
O-H 3230 3550 H-bonded in alcohols
N-H 3300 3500 Amine or amide1 https://en.wikipedia.org/wiki/Infrared%20Spectroscopy%20Correlation%20Table