Diagram of reduced energies for the ground state (black), v₁₈ = 1 (red), v₁₃ = 1 (blue), and v₁₈ = 2 (green) excited vibrational states in trans vinyl isocyanate. The reduced energies Ered are calculated as E_red = E − (B + C)J(J + 1)/2 where E corresponds to the energy of rotational levels. Their values are obtained from the experimental spectroscopic constants and calculated vibrational energies.