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Irreducible Representation and IR stretching bands

  1. Oct 3, 2011 #1
    After you find the irreducible representation of a molecule, how do you determine the number of IR active bands?

    For example, in methane, the pointgroup is Td , then you found that the irreducible representation is A1 + T2.

    How do you find the number of IR active and Raman active bands?


    Btw, a friend of mine said there are 2 IR active bands but I am not sure how to figure that out. Also, I know that T is triple degenerate and A is nondegenerate. But not sure how that relates to the number of IR active bands.
  2. jcsd
  3. Oct 4, 2011 #2


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    Science Advisor

    Methane is composed of N=5 atoms so there are 3N-6=9 different vibrations. A1+T2 makes up for only 4 of them.
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