I recently came across this question, I have no idea about IR:

Which of the following vibrational modes show no IR absorption bands?

A) Symmetric CO2 stretch

B) Antisymmetric CO2 stretch

C) Symmetric S=C=O stretch

D) Antisymmetric S=C=O stretch


A) Symmetric $\ce{CO2}$ stretch.

A particular vibrational mode is considered IR active, if there is a change of transition dipole moment. $\ce{CO2}$ is linear, and the symmetric stretching mode means both oxygens move in/outward concurrently. You can see an illustration of the symmetric and antisymmetric stretching modes of $\ce{CO2}$ here: https://www.youtube.com/watch?v=TMLnUmbLwUI&t=1m50s

  • For the symmetric mode, the overall center of charge stays the same, thus no change in dipole moment, thus the mode is not IR active.
  • For the antisymmetric mode, the center of charge shifts, thus the dipole moment changes and the mode is IR active. This is also true of the $\ce{S=C=O}$ antisymmetric stretch.
  • The $\ce{S=C=O}$ symmetric stretching mode is also IR active, since $\ce{S}$ and $\ce{O}$ are two different atoms, with different partial charges. As they vibrate, the total center of charge moves.
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