Infrared Absorption Bands
Introduction
IR absorption spectroscopy uses mid-infrared light
(2.5 - 50 µm, 4000 - 200 cm-1) to detect specific types
of chemical bonds in a sample for identification of organic and organometallic
molecules. There are two regions in IR absorption spectra: the "functional
group region" and the "fingerprint region". The functional group region
spans from 4000 to 1300 cm-1. In this region the bands are characteristic
of specific functional groups in a molecule. The fingerprint region spans
from 1300 to 900 cm-1. In this portion of the spectrum the energy
of the absorption bands varies depending on the structure of a molecule.
Table of characteristic IR bands
The following table lists some of the more common functional groups and
their characteristic IR absorption energy. More extensive tables can be
found in organic chemistry texts and other reference sources.
| Group |
Bond |
Approx. Energy (cm-1) |
| hydroxyl |
O-H |
3610-3640 |
| amines |
N-H |
3300-3500 |
| aromatic rings |
C-H |
3000-3100 |
| alkenes |
C-H |
3020-3080 |
| alkanes |
C-H |
2850-2960 |
| nitriles |
C º N |
2210-2260 |
| carbonyl |
C=O |
1650-1750 |
| amines |
C-N |
1180-1360 |
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