Van Vleck Perturbation Theory Applied to Vibration-Rotation Interaction in Tetrahedral Molecules

Vibration‐rotation interaction in tetrahedral XY₄ molecules is discussed with particular emphasis on the resultant effective vibration‐rotation Hamiltonian in the ground and the first excited states of the F₂ vibrations in these molecules. It is shown that to second order in perturbation theory, the coefficients of effective tensor operators in ν₃ and ν₄ of the form T₀₄₄, T₁₃₄, T₂₂₄, T₂₄₄, T₂₄₆, and possibly T₂₃₄, T₁₄₄, and $T_{224}^{01}$ are expected to get contributions of comparable magnitude from a first order off‐diagonal vibration‐rotation operator in the Hamiltonian.