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:
| $\mathrm{C}_{2 \mathrm{v}}$ | $\mathrm{E}_{2}$ | $\mathrm{C}_{2}$ | $\sigma_{\mathrm{v}}$ | $\sigma_{\mathrm{v}}$ | |
|---|---|---|---|---|---|
| $\mathrm{A}_{1}$ | 1 | 1 | 1 | 1 | Z |
| $\mathrm{A}_{2}$ | 1 | 1 | -1 | -1 | $\mathrm{R}_{2}$ |
| $\mathrm{B}_{1}$ | 1 | -1 | 1 | -1 | $\mathrm{x}, \mathrm{R}_{\mathrm{y}}$ |
| $\mathrm{B}_{2}$ | 1 | -1 | -1 | 1 | y, $\mathrm{R}_{\mathrm{x}}$ |
(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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