Rigid-rotor framework: E_J = ℏ²·J(J+1)/(2·I); ΔE_{J→J+1} = 2B(J+1), B = ℏ²/(2I)

The rigid-rotor approximation — the simplest quantum-mechanical model of molecular rotation, treating the nuclei as point masses fixed at equilibrium geometry (no vibrational motion, no centrifugal distortion). For a linear (diatomic or…