Assigning peaks in a Raman spectrum/ group theory

[baldywaldy]

Homework Statement

I need help assigning the peaks in a Raman spectrum of acetylene(ethyne).
The peaks are :
Wavenumber Contours
3372 OQS
1973 OQS
613 OPRS


2. The attempt at a solution
Ethyne has 7 vibrational modes ( with two degenerative modes)
It has a point group of d infinity h

I don't see how I calculate the reducibile representitive because its an inifinity group, the order of the group will be infinity for a start.

All help is greatly appreciated , thanks!

 

[nate808]

Hello! I would be happy to assist you with assigning the peaks in the Raman spectrum of acetylene. First, let's review the basics of Raman spectroscopy. Raman spectroscopy is a technique that measures the scattering of light by molecules. When a molecule is exposed to a laser, some of the photons are scattered at a different frequency than the incident light. This difference in frequency is known as the Raman shift and is related to the vibrational modes of the molecule.

In your case, you are looking at the Raman spectrum of acetylene, which has 7 vibrational modes. These modes can be classified based on their symmetry under the d infinity h point group. The d infinity h point group is a special case where the molecule has an infinite number of symmetry elements, making it difficult to calculate the reducible representation. However, we can still assign the peaks based on the symmetry of the vibrational modes.

The OQS and OPRS contours correspond to the symmetric and asymmetric stretching modes, respectively. These modes are degenerate and will appear as a single peak in the Raman spectrum. The wavenumber for the symmetric stretching mode is 3372 cm^-1 and the asymmetric stretching mode is 1973 cm^-1.

The remaining peak at 613 cm^-1 corresponds to the bending mode of acetylene. This mode is non-degenerate and should be assigned to the OPRS contour.

In summary, the peaks in the Raman spectrum of acetylene can be assigned as follows:

3372 cm^-1 - Symmetric stretching mode (OQS)
1973 cm^-1 - Asymmetric stretching mode (OQS)
613 cm^-1 - Bending mode (OPRS)

I hope this helps with assigning the peaks in the Raman spectrum of acetylene. If you have any further questions, please don't hesitate to ask. Good luck with your research!