Question

The IR spectrum of water shows three bands. How many bands do you predict for CO2 (a linear triatomic molecule)?

• A. 1
• B. 2
• C. 3
• D. 4

Hint:

Count only the IR-active fundamental vibrational modes, keeping degeneracy in mind.

Answer 1

The Correct Option is B

Mutual exclusion rule (CO2 has center of symmetry): gerade modes (symmetric stretch) IR inactive; ungerade modes (asym stretch, bend) IR active. Degenerate bend counts as 1 band. Total 2 bands, vs water's 3 (no symmetry center).

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