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Abstract
The vibration-rotation spectrum CH3I has been investigated in the 2430 to 2770 cm-1 region with a resolution of about 0.005 cm-1. Vibration-rotation constants of the parallel 2v 2 band, the perpendicular Fermi dyad v 2 + v 5/v 2 + v 3 + v 6, and the perpendicular component of the 3v 6 overtone (l = ±1) involved in anharmonic interactions affecting both v 2 + v 5 and v 2 + v 3 + v 6, have been determined. Account has been made for the x-y Coriolis interactions of v 2 + v 5 with 2v 2 and all components of 2v 5, and of v 2 + v 3 + v 6 with all components of v 3 + v 5 + v 6. The interactions involving 2v 5 and v 3 + v 5 + v 6 were handled by second order perturbation theory, since these levels occur in a very complicated band system which has not yet been clarified in detail. Both v 2 + v 5 and v 2 + v 3 + v 6 exhibit marked anomalous rotational intensity patterns, showing the enhancement of the ΔJ = 0 and ΔJ = -ΔK branches and the depletion of the ΔJ = ΔK branches at all values of K″ΔK except zero. This intensity perturbation is explained by means of the combined mechanisms of intensity stealing by v 2 + v 5 from the much stronger 2v 2 and 2v 0 5 parallel bands. With a negative value of ζ y 2, 5, the involved vibrational transition moments are found to be approximately in the ratios T 25 : T 22 : T 0 55 = 1 : 12 : -12, with a ‘negative’ intensity perturbation in the 2v 2/v 2 + v 5 pair and a ‘positive’ intensity perturbation in the 2v 0 5/v 2 + v 5 pair.