CHEMISTRY @CSU Stanislaus

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.


CHARACTERISTIC INFRARED ABSORPTION FREQUENCIES
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
CC 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
CN 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


Some Practice Problems

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.

#1 #2 #3 #4 #5 #6
The Answers

comments to: j byrd jim@chem.csustan.edu

05.01.98