# Which vibrational modes of CO2 and CSO do not lead to IR absorption bands? [closed]

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

## closed as off-topic by J. LS, Loong♦, LDC3, Klaus-Dieter Warzecha, ronApr 19 '15 at 15:14

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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 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.