Hi!
I would love it if someone can explain how to read the vibrational frequency data listed in NIST website. For example, there are four vibrational modes (in cm-1) listed for Methane (https://webbook.nist.gov/cgi/cbook.cgi?ID=C74828&Mask=800), 2917 (symmetric stretching), 1534 and 1306 (both degenerate deformation), and 3019 (degenerate stretching) . Methane should have 3N-6 = 9 vibrational frequencies, so how do I know which of the above mentioned degenerate modes are doubly or triply degenerate? This website (https://lweb.cfa.harvard.edu/hitran/vibrational.html) mentions 1534 is doubly degenerate and 3019 and 1306 modes are the triply degenerate modes which makes up the 9 frequencies for methane. But it also mentions the following 12 modes for Ethane (2954, 1388, 995, 289, 2896, 1379, 2969, 1468, 823, 2985, 1469, 822) where the last 3 are stated as doubly degenerate. But that would still give us 15 frequencies instead of 18. NIST mentions almost similar frequencies for ethane (https://webbook.nist.gov/cgi/cbook.cgi?ID=C74840&Mask=800) but lists 2969, 1468, 2985, & 1469 as degenerate modes. Hence, my question is, how do I know which of the NIST listed vibrational modes are doubly or triply degenerate to make up the 3N-5 or 3N-6 frequencies for linear and non-linear molecules?
Thank you so much!
Yes the NIST data is often a bit confusing.
You are correct there are 9 NORMAL modes of methane there's a nice animation here:
https://www.chem.purdue.edu/jmol/vibs/ch4.html
You could predict the number and degeneracy (and the atom movements) using group theory (Td point group).
Problem is when you come to experimentally measure the spectrum:
1. Many of the modes "couple"
2. Many are only Raman active. Raman and IR often show at (slightly) different frequencies.
3. Gas phase and liquid phase will have different frequencies
4. There will also be rotational excitation.
So it is often difficult to find two frequency tabulations that agree (exactly).
David Stephenson Thank you Dr. Stephenson. That clears up a lot about the differences in tabulated frequencies between different sources. Does the point group helps us identify which modes are doubly/triply degenerate?
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