Discussion
Which of the following molecules give no infrared or microwave spectra but Rotational Raman spectra ?
Answer: C
The molecules are \(O_{2}\) and \(H_{2}\).
A pure rotational (microwave) spectrum needs a permanent dipole moment, and an infrared vibrational band needs a vibration that changes the dipole moment. Homonuclear diatomics such as \(O_{2}\) and \(H_{2}\) have no dipole and their stretching cannot create one, so they are inactive in both. Their polarizability is anisotropic, however, so they show rotational Raman spectra.
HCl, HCN, CO and \(H_{2}O\) are polar and show microwave and IR spectra. \(CO_{2}\) has no microwave spectrum but its bending and asymmetric stretch are IR active, and \(CH_{4}\), a spherical top, is IR active and shows no rotational Raman spectrum.
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