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

The infrared spectrum of the cross-shaped van der Waals complex CO 2 –CS 2 is observed in the region of the CO 2 ν 3 fundamental band (≈2350 cm −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 CO 2 bend modes, and their positions and rotational structure agree extremely well (<0.1 cm −1 ) with calculations based on a recent ab initio intermolecular potential. The b -type torsional band is well-behaved, but the a -type CO 2 bending band is highly unusual, with large shifts between the subband origins for K a = 0, 2, and 4. The shifts may be rationalized as due to tunnelling effects and K a -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 CO 2 –CS 2 is not observed. [ABSTRACT FROM AUTHOR]