Irreducible Representation and IR stretching bands

Therefore, the number of IR active bands is 2.In summary, the number of IR active bands in methane can be determined by finding the irreducible representation, which in this case is A1 + T2. Since methane has 5 atoms, there are 9 different vibrations, but only 4 are accounted for by A1 + T2. Therefore, there are 2 IR active bands. The relationship between the irreducible representation and the number of active bands is based on the number of atoms in the molecule and the degeneracy of the representations.
  • #1
remeday86
1
0
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.
 
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  • #2
Methane is composed of N=5 atoms so there are 3N-6=9 different vibrations. A1+T2 makes up for only 4 of them.
 

1. What is an irreducible representation in the context of IR stretching bands?

An irreducible representation is a mathematical concept used to describe the behavior of molecules under different symmetry operations. In IR spectroscopy, it is used to analyze the vibrational modes of a molecule and determine the symmetry of its stretching bands.

2. How are IR stretching bands related to molecular structure?

IR stretching bands are related to the molecular structure because they represent the different types of vibrational modes that can occur in a molecule. The specific frequencies of IR stretching bands can provide information about the bond strength and connectivity of atoms within a molecule.

3. What factors affect the intensity and position of IR stretching bands?

The intensity and position of IR stretching bands can be affected by a variety of factors, including the strength of the bond, the mass of the atoms involved, and the symmetry of the molecule. Other factors such as temperature, pressure, and the surrounding medium can also influence IR stretching bands.

4. How can IR stretching bands be used to identify unknown compounds?

IR stretching bands can be used in conjunction with other spectroscopic techniques to identify unknown compounds. By comparing the IR spectra of a sample to a spectral database, the characteristic frequencies of its IR stretching bands can provide clues about the functional groups and molecular structure of the compound.

5. What is the significance of the wavenumber in IR stretching bands?

The wavenumber in IR stretching bands represents the frequency of the vibrational motion of a molecule, measured in units of reciprocal centimeters (cm^-1). It is directly related to the energy of the bond and can provide information about the strength and type of bond present in a molecule.

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