Water-carbon disulfide dimers: observation of a new isomer and ab initio structure theory

The weakly bound dimer water-carbon disulfide is studied by ab initio structure theory and high-resolution infrared spectroscopy. The calculations yield three stable minima in the potential energy surface, all planar. The most stable, isomer 1, was observed previously by microwave spectroscopy. It has C_2v symmetry, an S-O bond, and a linear O-S-C-S backbone. Isomer 2, the next most stable, has not been considered previously by theory or experiment. It also has C_2v symmetry, but with a side-by-side structure. Isomer 3, which is slightly less stable, is side-by-side with one O-H bond pointing toward an S atom. The first gas phase water-CS₂ infrared spectra are reported. For isomer 1, H₂O-, HDO-, and D₂O-CS₂ are observed in the CS₂ν₁ + ν₃ region, H₂O-CS₂ in the H₂O ν₂ bend region, and D₂O-CS₂ in the D₂O ν₁ and ν₃ stretch regions. The latter ν₃ spectrum enables an experimental determination of the A rotational parameter, which turns out to be larger than expected. The new isomer 2 is observed by means of a band in the H₂O ν₂ region, confirming its C_2v symmetry and calculated structure.