We are going to show the origin of th einfrared spectra of rotation, vibration and vibration-rotation of different types of molecules. At first it is necessar to recall the energy levels of anisolated molecule. The vibration is aninternal movement while the rotation and the translation are external movements. Generally Infrared Spectrometry does not permit a study of translation because the energy of this movement, as we will see later, is not quantified. In order to simplify the analysis, the problem of an isolated diatomic molecule will be processed. The most common case of polyatomic molecules will be taken up across examples.
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