Reinvestigation of the ν 3 - ν 6 Coriolis interaction in trifluoroiodomethane.

The lowest-frequency fundamental ν ₆ of trifluoroiodomethane (CF ₃ I) has never been directly observed and analyzed at high resolution in the gas phase. The ν ₆ (e) level interacts with ν ₃ (a ₁ ) at 286.297 cm ^-1 via a b -axis Coriolis interaction, which perturbs the rotational structure of both levels. In this work, we report low- J microwave transitions (for J ranging from 0-2) within the ν ₆ vibrational level. The l -type doubling observed in our spectrum agreed poorly with the predictions of previously published models of the interacting ν ₃ - ν ₆ levels, prompting us to refine the model. We performed ab initio anharmonic force-field calculations, which were used to constrain some of the parameters, and which served as a check on some of the floated parameters. We fit a dataset consisting of 3593 transitions, which combined our measurements with previous microwave, millimeter wave, and high-resolution infrared observations. A reasonable set of fit parameters is obtained with ν ₆ = 267.28 cm ^-1 , but we cannot rule out a lower value of ν ₆ = 261.5 cm ^-1 consistent with analyses of the vibrational level structure.