The six fundamental bands of CH3I and CD3I, both as pure liquids and in solution with CS2, have been studied in order to obtain information about the molecular rotational and vibrational dynamics. Emphasis was placed on the features of the spectra that can be interpreted independent of a particular model. An examination for Boltzmann asymmetry in theA1andEbands indicates that the former, being symmetric, are amenable to classical description, while the latter, being asymmetric, definitely require quantum mechanical interpretations. With respect to the validity of the assumption that rotationhyphen;vibration coupling can be ignored, we give evidence of substantial coupling effects in a nondegenerateA1mode (v1) as well as in the doubly degenerateEmodes. We emphasize that, in attempting to obtain dynamical information, several bands of any given liquid must be studied and compared in order to decide which can be used with confidence in a detailed analysis. The consistency of the rotational diffusion model for describing the tumbling motion of the methyl iodide molecules has been confirmed.
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