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Topic: Character tables  (Read 2010 times)

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Offline orgo814

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Character tables
« on: October 07, 2015, 09:11:45 PM »
I am struggling a little with this. How, from looking at a character table for a point group, can you determine if a molecule is IR or Raman active? I know it has to do with translational and rotational motions but I don't know how to determine that from a character table

Offline mjc123

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Re: Character tables
« Reply #1 on: October 08, 2015, 05:36:00 AM »
Here is an explanation. It looks complicated, but once you get into the habit it becomes easy.
http://www.huntresearchgroup.org.uk/teaching/teaching_symmetry_year3/L4_2014_web_Notes_VibAnalysis.pdf

Offline Corribus

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Re: Character tables
« Reply #2 on: October 08, 2015, 10:00:38 AM »
1. Assign point group for the molecule and identify symmetry elements wrt to molecular features.
2. You need to assign the vibrational modes of the molecule to specific symmetry representations. This will be the most challenging part, and involves inspecting how the atomic motions transform under specific symmetry operations. Bear in mind some possibilities are assigned to translational and rotational degrees of freedom, so should be subtracted from the overall reducible symmetry representation to arrive at only the vibrational part.
3. Check and see if any of those representations have a linear (IR) or quadratic (Raman) basis on the right side of the table. If yes, active. If no, silent.

Maybe this thread will be useful, where I walked someone through how to do it for carbon dioxide:

http://www.chemicalforums.com/index.php?topic=73681.msg267339#msg267339
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline orgo814

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Re: Character tables
« Reply #3 on: October 08, 2015, 04:57:46 PM »
so are the quadratic Raman and the linear IR?

Offline Corribus

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Re: Character tables
« Reply #4 on: October 08, 2015, 06:03:16 PM »
Basically.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

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