IGNOU MCH 20 2026 - The molecule has 3 fundamental vibrational frequencies. One

Question

The molecule $\text{X}_2\text{Y}$ has 3 fundamental vibrational frequencies. One is Raman active and two are IR active. Identify the structure of the molecule.

21 Jan 2026

Answer :

Word Count : 181

For a triatomic molecule X₂Y, the number of fundamental vibrational frequencies depends on whether the molecule is linear or non-linear. A non-linear triatomic molecule has 3N − 6 = 3 fundamental modes, whereas a linear triatomic molecule has 3N − 5 = 4 normal modes. However, in a linear molecule one of these modes is a doubly degenerate bending vibration, so the total number of distinct fundamental vibrational frequencies observed is three.

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