Question

IR active molecule(s) is/are

• A. CO$_2$
• B. CS$_2$
• C. OCS
• D. N$_2$

Hint:

Asymmetric molecules or vibrations that produce dipole change are always IR active.

Answer 1

The Correct Option is A, B, C

Step 1: IR activity condition.
A molecule is IR active if it shows a change in dipole moment during vibration. Symmetric linear molecules with no change in dipole moment are IR inactive.
Step 2: Analyzing each molecule.
(A) CO$_2$: Though linear and symmetric, its asymmetric stretching mode is IR active → correct.
(B) CS$_2$: Linear and completely symmetric; no dipole change → IR inactive → incorrect.
(C) OCS: Asymmetric linear molecule with net dipole moment → IR active → correct.
(D) N$_2$: Homonuclear diatomic molecule with no dipole → IR inactive → incorrect.
Step 3: Conclusion.
Thus, CO$_2$ and OCS are IR active molecules.