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Infrared Absorption Bands

Introduction

IR absorption spectroscopy uses mid-infrared light (2.5 - 50 Ám, 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.

GroupBondApprox. Energy (cm-1)
hydroxylO-H3610-3640
aminesN-H3300-3500
aromatic ringsC-H3000-3100
alkenesC-H3020-3080
alkanesC-H2850-2960
nitrilesCtriple-bondN2210-2260
carbonylC=O1650-1750
aminesC-N1180-1360

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