Intermolecular vibrations of the CO2–CS2 complex: Experiment and theory agree, but understanding remains challenging

Abstract

The infrared spectrum of the cross-shaped van der Waals complex CO2–CS2 is observed in the region of the CO2 ν3 fundamental band (≈2350cm−1) using a tuneable diode laser to probe a pulsed supersonic slit jet expansion. Two combination bands are assigned, corresponding to the intermolecular torsion and CO2 bend modes, and their positions and rotational structure agree extremely well (<0.1cm−1) with calculations based on a recent ab initio intermolecular potential. The b-type torsional band is well-behaved, but the a-type CO2 bending band is highly unusual, with large shifts between the subband origins for Ka=0, 2, and 4. The shifts may be rationalized as due to tunnelling effects and Ka-dependent perturbations from other intermolecular modes. But even though they are well predicted by our calculations, there is no simple qualitative explanation. The predicted low-lying planar slipped parallel isomer of CO2–CS2 is not observed.