Question

100. Pure rotational spectrum of a diatomic molecule is when :
(A) It has a center of symmetry.
(B) It has a permanent dipole moment.
(C) It has no magnetic moment.
(D) None of these​

Answer 1

Explanation:

Selection rules only permit transitions between consecutive rotational levels: ΔJ=J±1, and require the molecule to contain a permanent dipole moment. Due to the dipole requirement, molecules such as HF and HCl have pure rotational spectra and molecules such as H2 and N2 are rotationally inactive.