Q1(n). H₂: no IR or microwave spectrum, but Raman active

Answer in brief

H₂ has no dipole moment and its vibration creates none (IR and microwave inactive), but its polarisability changes on rotation and on vibration, so it is Raman active.

Key points

• Microwave (pure rotation) needs a permanent dipole moment; H₂ is homonuclear, μ = 0 ⇒ no rotational spectrum.
• Infrared needs a change of dipole moment during the vibration; stretching keeps μ = 0 ⇒ no vibrational spectrum.
• Raman needs a change of polarisability α; the induced dipole changes as H₂ rotates ⇒ rotational Raman, ΔJ = 0, ±2.
• Stretching enlarges the electron cloud, so α changes during the vibration ⇒ vibrational Raman, Δv = ±1, Q branch at 4161 cm⁻¹.