Different vibrational modes:

The Raman activity of different vibrational modes can be predicted on the basis of whether or not the polarisability changes during the vibration. However, it is not very simple and in many cases requires a detailed analysis. We would not get into these details. It would be suffice to mention here that in addition to the CCl₄ example where only some modes have IR as well as Raman activities, we may have situations where all the vibration modes are active in IR as well as in Raman and in some cases there is no mode which is active in both. Water is an example of the former although carbon dioxide belongs to the later category. An IR and Raman activities of the vibration modes of H₂O and CO₂ are compiled in Table.

Table: The IR and Raman activities of the vibration modes of water and carbon dioxide molecules