Question

Which of the following vibrational mode shows no IR absorption bands:

• A. symmetric $CO_2$ stretch
• B. antisymmetric $CO_2$ stretch
• C. CH3C≡ CHstretch
• D. symmetric O=C=S stretch

Hint:

Vibrational modes that involve a change in dipole moment are IR active, while those that do not, like symmetric CO2 stretch, are IR inactive.

Answer 1

The Correct Option is A

The symmetric $CO_2$ stretch does not result in dipole moment changes and thus does not absorb IR radiation. Other vibrational modes like the antisymmetric stretch or CC stretch do change the dipole moment and show absorption in the IR spectrum.