Interpreting IR/Raman Spectra Using Character Tables

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SUMMARY

This discussion focuses on interpreting IR/Raman spectra using character tables, specifically addressing the assignment of vibrational modes to molecular motions. The provided data includes modes and their corresponding frequencies: 1 b_u at 82 cm-1, 1 a_g at 216 cm-1, and 2 b_g at 239 cm-1. Participants emphasize the importance of determining the symmetry of the molecular system and consulting the character table for the relevant point group to identify which vibrational modes are Raman active. Access to character tables is also discussed, with inquiries about online resources versus traditional textbooks.

PREREQUISITES
  • Understanding of IR/Raman spectroscopy principles
  • Familiarity with molecular symmetry and point groups
  • Knowledge of character tables and their applications
  • Basic concepts of vibrational modes (stretching, bending)
NEXT STEPS
  • Research how to determine molecular symmetry and identify point groups
  • Explore online resources for character tables specific to various point groups
  • Study the relationship between vibrational modes and their Raman activity
  • Learn about the significance of bonding and antibonding wave functions in molecular vibrations
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Chemists, spectroscopists, and students studying molecular vibrations and spectroscopy, particularly those involved in interpreting IR/Raman spectra and understanding molecular symmetry.

greisen
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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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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.
 
hi,

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