The key feature of Bohr's theory of spectrum of hydrogen atom is the quantization of angular momentum when an electron is revolving around a proton. We will extend this to a general rotational motion to find quantized rotational energy of a diatomic molecule assuming it to be rigid. The rule to be applied is Bohr's quantization condition.
A diatomic molecule has moment of inertia I . By Bohr's quantization condition its rotational energy in the n th level ( n=0 is not allowed) is
(1) n21(8π2Iℏ2)
(2) n1(8π2Iℏ2)
(3) n(8π2Iℏ2)
(4) n2(8π2Iℏ2)