Molecular symmetry group of non-rigid molecules

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Main Question or Discussion Point

Hello everybody,

I have read some very interesting book (Molecular symmetry and Spectroscopy - Bunker and Jensen) that talk about how to find the Molecular Symmetry group (MS) of a molecule by using the concept of "feasible" operation from the Complete Nuclear Permutation Inversion (CNPI) group.
Following the explanation given by the authors, "feasible" operation is that can interconverts a numbered and equivalent equilibrium versions of the molecule (rigid or non-rigid).

Confident of this understanding, I tried and success on finding MS of some famous non-rigid molecule until I met certain hard case as of the butane molecule which have non equivalent equilibrium versions!

So, my question is :

How to define the MS group of such a non-rigid molecule that have non equivalent equilibrium versions, knowing that between theses non equivalent equilibrium versions, potential barrier is not too high and allow some interconversion between the different versions of the same molecule?

I attached here the potential energy of butane as an example of those molecule which have more than one equilibrium versions:

butane-config.png


Thank you very much everybody.

Konte.
 

Answers and Replies

DrClaude
Mentor
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3,184
What's the ultimate goal? If it is spectroscopy, then the answer would be that you will find that the spectrum shows characteristics of both stable conformers.
 
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What's the ultimate goal? If it is spectroscopy, then the answer would be that you will find that the spectrum shows characteristics of both stable conformers.
My goal is to construct the character table of the MS group and from that be able to find selection rules.
Your answer suggest that I have to consider both stable conformers as an independent (different) molecules?

Thanks .

Konte
 
DrDu
Science Advisor
6,012
749
There are no elements of the CNPI which would link energetically different minima of the potential energy surface.
 
87
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There are no elements of the CNPI which would link energetically different minima of the potential energy surface.
Ok. Is there a formal way which permit me to characterize the eigenstates (with or without solving the Schrodinger equation) of such a molecule and having some selection rules?

Thanks.

Konte.
 

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