Irreducible Representation and IR stretching bands

  • Thread starter remeday86
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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?

Thanks

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.
 

Answers and Replies

  • #2
DrDu
Science Advisor
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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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