Discussion
Why is the symmetric stretch of carbon dioxide infrared inactive?
Answer: A
Both oxygens move outward together, so the two bond dipoles cancel throughout the motion and the net dipole stays zero. The asymmetric stretch, where one bond lengthens as the other shortens, does change the dipole and absorbs strongly. Linearity and equal force constants are what make the cancellation exact rather than being the rule itself. A mode that is silent in the infrared is generally active in the Raman, which is why the two techniques are run together.
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