High-resolution FTIR spectroscopy of HNSO-Analysis of the highly perturbed ν4, ν6 and 2ν5 bands

We report a rovibrational analysis of the ν4 and ν6 fundamentals and the 2ν5 overtone of HNSO from high-resolution Fourier transform infrared spectra. The ν6 band (out-of-plane bend) centred at 757.5 cm−1 is c-type. The ν4 band (HNS bend) centred at 905.9 cm−1 is predominantly a-type with a very weak b-type component (μa2/μb2=12.4). Numerous global perturbations and localized avoided crossings affecting the v4 = 1 rotational levels were successfully treated by inclusion of Fermi and c-axis Coriolis resonance terms between v4 = 1 and v5 = 2, and a b-axis Coriolis resonance term between v4 = 1 and v6 = 1. The latter term gives rise to an avoided crossing with an extraordinary ∣ΔKa∣ = 5 selection rule. The Fermi resonance between v4 = 1 and v5 = 2 gives rise to strong mixing of their rotational wavefunctions in the vicinity of Ka = 18. The resultant borrowing of intensity made it possible for 2ν5 transitions in the range Ka = 16-19 to be assigned and included in a global rovibrational treatment of all three band systems.