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Abstract
The vibration-rotation spectrum of CH3I in the 2020–2440 cm-1 region consists of the absorption of the perpendicular system v 2 + v 6(E), v 5 + v 6(E), v 3 + 2v 6(E). The last two bands form a strongly coupled Fermi dyad with crossing at K′ = 4 in the (+l) stack of levels. Evidence has been found for a z-axis Coriolis-type contribution to their matrix elements, linear in K and with opposite signs in the (+l) and (-l) series. The x - y Coriolis interaction of v 2 + v 6(E) with all components of v 5 + v 6(A 1 + A 2 + E) occurs as usual in methyl halides. The spectrum does not exhibit any direct absorption to the parallel system v 5 + v 6(A 1 + A 2), v 3 + 2v 6(A 1) and v 2 + 2v 3(A 1), whose presence is revealed by extensive perturbations of the perpendicular v 5 + v 6, v 3 + 2v 6 system, mostly by l(2, -1) interactions, generating anomalous rotational patterns in a wide K″ΔK range. Accurate values of the vibration-rotation parameters of the perpendicular bands were determined by a proper use of the available data.