Heat capacity of a diatomic gas ($C_V⟶7R/2$)

For an ideal diatomic gas with intact molecules and all translational, rotational and harmonic vibrational degrees of freedom active, the classical molar heat capacity is $7R/2$. Quantum freeze-out of vibrations and then rotations produces approximate $5R/2$ and $3R/2$ regimes. The model excludes dissociation, condensation and electronic excitation.