The angle ICI bending satellites in the millimeter-wave rotational spectra of CH2I2 and CD2I2

Rotational transitions in the first four excited states of the low-frequency angle ICI bending mode, nu(4), have been assigned in the mm-wave rotational spectra of CH2I2 and of CD2I2. Measurements of transition frequencies, made over the frequency region 167-326 GHz and for J up to 190, allowed determination of sextic level spectroscopic constants for all states. The changes in spectroscopic constants with vibrational excitation show very small anharmonicity, in spite of the very low frequency of this mode (121 cm(-1)). Vibrational excitation affects the moments of inertia in such a way that the planar moment P-b, about the plane perpendicular to both angle ICI and angle HCH, is practically invariant. Vibrational change in P-c, the moment along the principal axis in the HCH plane and perpendicular to the angle HCH bisector, has been successfully reproduced with an ab initio harmonic force field so that there is no discernible vibrational change in angle HCH on excitation of angle ICI. Finally, the change in P-a leads to estimated vibrational change of +0.12 degrees in the value of angle ICI itself. (C) 2000 Academic Press.