Absorption Bands Stretching and Bending

From the basic theory it is easy to understand that an absorption band appears only when the frequency of the radiation incident upon a compound is the same as the vibrational frequency of that compound. This phenomenon is related to the change of the dipole moment or to the change in polarizability. If it is related to 

and are given under each mode. The arrows show the direction of the vibration, while the plus and minus signs denote the vibrations that are in a direction perpendicular to the page. 

The factor that determines the position of the absorption bands is the potential energy of the molecule. For SO2, the potential energy V can be expressed as 

The stretching frequency v of a diatomic molecule is, in general, based on the classical mechanics of a harmonic oscillator  

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