Discussion
Among the following, the one for which the infrared active vibrational modes are Raman inactive and vice versa is
Answer: B
The molecule is the trans-planar conformer of H2O2.
The question describes the rule of mutual exclusion: in a molecule with a centre of inversion, vibrations that are IR active (odd, u) are Raman inactive, and Raman-active (even, g) ones are IR inactive. Planar trans H-O-O-H belongs to \(C_{2h}\), which contains i, so the rule applies.
None of the others has a centre of inversion. H2O is \(C_{2v}\), and all three of its modes are both IR and Raman active. Eclipsed ethane is \(D_{3h}\) (it is the staggered form, \(D_{3d}\), that is centrosymmetric). CH4 is \(T_{d}\), and its t2 modes are active in both spectra.
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