Discussion
Consider the following six vibrational modes:
symmetric stretching of CO2, O-H symmetric stretching of H2O, stretching of HCl, stretching of H2, N-H symmetric stretching of NH3, and bending of CO2.
Among these modes, if k number of modes are IR active but Raman inactive, l number of modes are IR inactive but Raman active, and m number of modes are both IR and Raman active.
k, l, and m, respectively, are
Answer: C
k, l and m are 1, 2 and 3.
- Symmetric stretch of CO2: no dipole change, polarisability changes: Raman only.
- Stretch of H2: homonuclear, no dipole: Raman only.
- Bend of CO2: dipole changes but, by the mutual exclusion rule for a centrosymmetric molecule, Raman inactive: IR only.
- O-H symmetric stretch of H2O, the HCl stretch and the N-H symmetric stretch of NH3: molecules without a centre of symmetry, both IR and Raman active.
That gives k = 1 (IR only), l = 2 (Raman only) and m = 3 (both). The other options swap these counts.
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