Interpreting IR/Raman Spectra Using Character Tables

In summary, the conversation is about interpreting data for IR/Raman and assigning them into stretch, bend, etc. The speaker suggests finding the symmetry of the system and looking at a character table to determine which motions are Raman active. They ask if this table can be found online or if a book must be consulted.
  • #1
greisen
76
0
hi,

I am trying to interpret some data which have been calculated for IR/Raman but I don't know how to interpret them exactly. The schema says
mode \nu
1 b_u 82
1 a_g 216
2 b_g 239

I presume that the u/g are gerade/ungerade from the bonding/antibonding wave functions but how to assign these informations into stretch, bend etc. Is there a table where one can look these values up?

How to proceed any help appreciated. Thanks in advance.

 
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  • #2
greisen said:
hi,

I am trying to interpret some data which have been calculated for IR/Raman but I don't know how to interpret them exactly. The schema says
mode \nu
1 b_u 82
1 a_g 216
2 b_g 239

I presume that the u/g are gerade/ungerade from the bonding/antibonding wave functions but how to assign these informations into stretch, bend etc. Is there a table where one can look these values up?

How to proceed any help appreciated. Thanks in advance.
You have to find out the symmetry of the system and then look at the character table for that point group. The meanings of a and b etc are all in the character table and will tell you which motions (bend, stretch) are raman active and which are not.
 
  • #3
hi,

A character table - is it possible to find that online or does one have to consult a certain book?
 

1. What is the difference between IR and Raman spectra?

IR (infrared) and Raman spectra are both types of spectroscopy techniques used to analyze the vibrational modes of molecules. The main difference between the two is the type of radiation used. IR spectroscopy measures the absorption of infrared light, while Raman spectroscopy measures the scattering of light. Additionally, IR spectra primarily give information about the dipole moments of molecules, while Raman spectra are more sensitive to changes in polarizability.

2. How is symmetry related to IR and Raman spectra?

Symmetry refers to the spatial arrangement of atoms within a molecule. IR and Raman spectra are affected by the symmetry of a molecule because they are both based on the vibrational modes of molecules. Symmetry can determine which vibrational modes are active in IR or Raman spectroscopy, and it can also affect the intensity and selection rules for certain vibrational bands.

3. What is the importance of analyzing IR/Raman spectra in chemistry?

IR and Raman spectroscopy are important tools in analytical chemistry because they provide valuable information about the structure and chemical properties of molecules. They can be used to identify unknown compounds, determine the functional groups present in a molecule, and study the effects of molecular symmetry on chemical properties. IR and Raman spectra are also used in various fields such as pharmaceuticals, forensics, and environmental analysis.

4. How is the information from IR/Raman spectra interpreted?

The information obtained from IR/Raman spectra is typically displayed as a graph with the intensity of absorption or scattering on the y-axis and the wavenumber (or frequency) on the x-axis. By comparing the peaks in the spectrum to known reference spectra, the functional groups present in a molecule can be identified. The positions and intensities of the peaks can also provide information about the molecular structure, such as bond lengths and strengths.

5. Can IR/Raman spectra be used to determine the absolute configuration of chiral molecules?

Yes, IR and Raman spectra can provide information about the absolute configuration of chiral molecules. This is because the vibrational modes of chiral molecules are affected by their stereochemistry. However, the spectra alone may not be enough to determine the absolute configuration, and other techniques such as X-ray crystallography or NMR spectroscopy may be needed for confirmation.

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