High-resolution infrared spectroscopy of the interacting ν9, ν5 + ν6 and 3ν6 levels of CH279BrF

► High resolution FTIR spectrum of CH279BrF in the region of the ν9 and ν5 + ν6 bands. ► Coriolis resonances between v9 = 1, v5 = v6 = 1 and v6 = 3 dark state. ► Determination of upper-state parameters up to quartic distortion terms of ν9 and ν5 + ν6. ► Spectroscopic information about the dark state 3ν6.