Second-order coriolis resonance in the ν2, ν5 Raman bands of CH 3F

The vibration-rotation Raman spectrum of the ν₂ and ν₅ fundamentals of CH ₃F is reported, from 1320 to 1640 cm ^-1, with a resolution of about 0.3 cm ^-1. The Coriolis resonance between the two bands leads to many perturbation-allowed transitions. Where the resonance is still sufficiently weak that the quantum number K' retains its meaning, perturbation-allowed transitions are observed for all values of ∆ K from +4 to -4; in regions of strong resonance, however, we can only say that the observed transitions obey the selection rule ∆( k- l) = 0 or ±3. The spectrum has been analyzed by band contour simulation using a computer program based on exact diagonalization of the Hamiltonian within the ν₂, ν₅ vibrational levels, and improved vibration-rotation constants for these bands are reported. The relative magnitudes and relative sings of polarizability derivatives involved in these vibrations are also reported.