How Does Group Theory Aid in Predicting Vibrational Modes in Spectroscopy?

[excalibur313]

Hi Everyone,
I am giving a talk on group theory as related to spectroscopy (IR & Raman) and I was curious about how to explain the jump from knowing which modes in the charcter table are Raman active to predicting what kind of vibration they will have. For example the Td point group has raman modes: A1, T3, E and then consequentially you know that the A1 is x^2+y^2+z^2, the T3 has xy,zy,zx, etc. How can I then take that information to infer what kind of vibrational mode it is? So I know the x^2+y^2+z^2 will be this breathing mode because it is simple to visualize and I know that A1 is totally symmetric, but the others are more difficult to see what kind of motion it is. I want to know this because I want to correlate it back to the raman spectrum I have and I have to know the types of modes so I can then figure out the energy shift they should have.

I was told there is a chart that will tell me what a xy vibration is, for example, but I haven't found one. Additionally, I'd also want to find a chart that could correlate a particular vibration to an energy shift, so the students can predict what each of the peaks are. Thanks a lot for your help!
Take care,
Stephen

PS- I basically want to do what they do on this website in general: http://fy.chalmers.se/~brodin/MolecularMotions/CCl4modes.html

 

[Dr Transport]

It is a tedious process to calculate how all of the atoms in a 3-d lattice move according to the different modes. The classic example is a triangle which takes about 5 pages and a square taking about 10-12 to do.

try here as a start...

http://symmetry.jacobs-university.de/cgi-bin/group.cgi?group=902&option=2&a1=12&a2=0&a3=0&a4=0&a5=2

 

[charng]

Hello Stephen,

Thank you for reaching out to me about your talk on group theory and spectroscopy. It sounds like you are interested in understanding the relationship between the modes in a character table and the corresponding vibrational motions in a molecule. This is an important topic in spectroscopy and can be explained using the concepts of symmetry and selection rules.

First, let's discuss the connection between the modes in a character table and the vibrational motions they represent. The character table for a particular point group provides information about the symmetry of the molecule, including the symmetry of its vibrational modes. The modes listed in the character table are classified as either symmetric or asymmetric based on their behavior under the symmetry operations of the molecule. For example, the A1 mode in the Td point group is totally symmetric, meaning that it does not change under any of the symmetry operations. This mode is often associated with a stretching motion along the x, y, and z axes, as you mentioned.

On the other hand, the T3 mode in the Td point group is asymmetric and transforms under the symmetry operations. This mode is often associated with a bending motion, such as a xy, yz, or zx bending motion. The exact type of bending motion depends on the specific molecule and its symmetry.

To determine the type of vibrational mode, you can use the concept of selection rules. Selection rules are rules that govern which transitions are allowed in spectroscopy. In Raman spectroscopy, the selection rule is that the transition must involve a change in polarizability, which is related to the symmetry of the molecule. In the Td point group, only modes that are asymmetric can be Raman active, meaning they can be observed in the Raman spectrum. This is why the A1 mode, which is totally symmetric, is not Raman active.

To correlate a particular vibrational mode to an energy shift, you can use the concept of vibrational coupling. Vibrational coupling refers to the interaction between different vibrational modes in a molecule. In general, the more coupled the modes are, the larger the energy shift will be. Therefore, modes that are strongly coupled will result in larger energy shifts in the Raman spectrum.

In terms of finding a chart that correlates a particular vibration to an energy shift, I would recommend looking into the concept of normal modes. Normal modes are the fundamental vibrational motions of a molecule, and they can be calculated using quantum mechanical methods. There are various software