The key feature of Bohr's thoery of spectrum of hydrogen atom is 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 )

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