Q90 · CSIR-NET Chemistry, December 2012

Paper: CSIR-NET December 2012 · Subject: Physical Chemistry · Chapter: Group Theory · Topic: Applications To Spectroscopy · Marks: 2 · Difficulty: Easy

The number of IR active vibrational modes of pyridine is:
$\mathrm{C}_{2 \mathrm{v}}$$\mathrm{E}_{2}$$\mathrm{C}_{2}$$\sigma_{\mathrm{v}}$$\sigma_{\mathrm{v}}$
$\mathrm{A}_{1}$1111Z
$\mathrm{A}_{2}$11-1-1$\mathrm{R}_{2}$
$\mathrm{B}_{1}$1-11-1$\mathrm{x}, \mathrm{R}_{\mathrm{y}}$
$\mathrm{B}_{2}$1-1-11y, $\mathrm{R}_{\mathrm{x}}$
(a)12
(b)20
(c)24
(d)33
Answer
Answer: C ✓ checked by 4AB · confidence high
Explanation
Pyridine (C₂v, 11 atoms): 27 normal modes = 10A₁ + 3A₂ + 5B₁ + 9B₂; only the 3 A₂ modes are IR-inactive, so 24 are IR-active.

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