On the Kirkwood theory of natural optical activity

Vibration-rotation intensities have been calculated for transitions between states of arbitrary vibrational symmetries, for tetrahedral molecules. For this purpose eigenfunctions of first order Coriolis interactions, which are assumed to be much smaller than anharmonic splittings, have been used. While some bands, among which fundamentals and overtones, follow the ΔR = 0 selection rule, for others the most intense vibration-rotation lines are those with ΔR maximum, in agreement with our double-resonance investigations of stretching levels of methane. One such investigation is presented here, in which the lower (3v₃, F ₂) level of CH₄ has been found at 8906·78 cm^-1, in close agreement with local-mode predictions. Several I.R. lines in this region have been assigned, the effective rotational constants being 5·214 and 5·24 cm^-1 for R = J and R ≠ J Coriolis sublevels respectively.