Interpretation of Infrared Spectra
The interpretation of infrared spectra involves the correlation of absorption bands in the spectrum of an unknown compound with the known absorption frequencies for types of bonds. This table will help users become more familiar with the process. Significant for the identification of the source of an absorption band are intensity (weak, medium or strong), shape (broad or sharp), and position (cm-1) in the spectrum. Characteristic examples are provided in the table below to assist the user in becoming familiar with the intensity and shape absorption bands for representative absorptions.
You can make use of this Table by doing the set of practice problems given at the end of this page.
| Bond | Compound Type | Frequency range, cm-1 |
|---|---|---|
| C-H | Alkanes | 2960-2850(s) stretch |
| 1470-1350(v) scissoring and bending | ||
| CH3 Umbrella Deformation | 1380(m-w) - Doublet - isopropyl, t-butyl | |
| C-H | Alkenes | 3080-3020(m) stretch |
| 1000-675(s) bend | ||
| C-H | Aromatic Rings | 3100-3000(m) stretch |
| Phenyl Ring Substitution Bands | 870-675(s) bend | |
| Phenyl Ring Substitution Overtones | 2000-1600(w) - fingerprint region | |
| C-H | Alkynes | 3333-3267(s) stretch |
| 700-610(b) bend | ||
| C=C | Alkenes | 1680-1640(m,w)) stretch |
| CºC | Alkynes | 2260-2100(w,sh) stretch |
| C=C | Aromatic Rings | 1600, 1500(w) stretch |
| C-O | Alcohols, Ethers, Carboxylic acids, Esters | 1260-1000(s) stretch |
| C=O | Aldehydes, Ketones, Carboxylic acids, Esters | 1760-1670(s) stretch |
| O-H | Monomeric -- Alcohols, Phenols | 3640-3160(s,br) stretch |
| Hydrogen-bonded -- Alcohols, Phenols | 3600-3200(b) stretch | |
| Carboxylic acids | 3000-2500(b) stretch | |
| N-H | Amines | 3500-3300(m) stretch |
| 1650-1580 (m) bend | ||
| C-N | Amines | 1340-1020(m) stretch |
| CºN | Nitriles | 2260-2220(v) stretch |
| NO2 | Nitro Compounds | 1660-1500(s) asymmetrical stretch |
| 1390-1260(s) symmetrical stretch |
v - variable, m - medium, s - strong, br - broad, w - weak
For steps to follow in the analysis of an infrared spectrum select this link ----> Quick Infrared Analysis
Select an unknown and determine as much as you can about its structure from the infrared spectrum. A helpful table of characteristic IR frequencies can be found below.
comments to: j byrd jim@chem.csustan.edu
02.22.00
