Origin of infrared absorptions

The frequency at which two masses, joined together by a spring, vibrate, is described by the equations of simple harmonic motion  

As equation (10.1) indicates, the frequency at which the spring vibrates will increase with the strength of the spring, but will decrease with increasing mass of either particle. 

The analogy holds well for simple gas-phase molecules. Of course, we cannot directly observe the vibration of a molecule itself, only the transition between one state and another. However, the frequency of the transition between vibrational states is comparable to the fundamental frequency of the vibration itself. 

For a transition between vibrational states to be observed it must be infrared active—i.e. it must lead to the absorption or emission of a photon of infrared light. The rules governing whether or not this can occur are rather complicated—especially for larger molecules—and to be applied require a knowledge of the symmetry properties of the species 

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