Infrared Absorption and Molecular Structure

Infrared spectroscopy is very useful for obtaining qualitative information about molecules. But molecules must possess certain properties in order to undergo absorption. 

Our discussions have been confined to molecules since nearly all absorbing species in solution are molecular in nature. In the case of single atoms (which occur in a flame or an electric arc) that do not vibrate or rotate, only electronic transitions occur. These occur as sharp lines corresponding to definite transitions and will be the subject of discussion in the next chapter. 

The molecule must undergo a change in dipole moment in order to absorb infrared radiation. 

Single atoms only undergo electronic transitions. So the spectra are sharp lines. 

Simple correlations of group vibrations to regions of infrared absorption. (From R. T. Conley, Infrared Spectroscopy, 2nd ed. Boston Allyn and Bacon, Inc., 1972. Reproduced by permission of All)m and Bacon, Inc.) 

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