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.
In a CO molecule, the distance between C (mass =12 a.m.u.) and O (mass =16 a.m.u.) where 1.a.m.u. = , is close to