Vibration-rotation bands and molecular constants of methyl iodide

The rotational line structure of seven parallel and seven perpendicular vibration bands of methyl iodide has been measured with high resolution, and the molecular rotational constants have been determined. Values of a x and a x and the Coriolis coupling constants for the fundamental degenerate modes have been determined. The values for overtone and combination bands usually agree closely with those predicted from the values found for the fundamentals. There appear to be inexplicable perturbations in some of the perpendicular bands. The ratio of A values, an d frequency product rules, for the isotopic species CH3I and CD3I are in accordance with theory.