Infrared spectra of OCS–hydrogen complexes

Five distinct species of OCS–hydrogen van der Waals complexes (OCS–orthoH2,–paraH2,–paraD2,–orthoD2, and –HD) have been observed by infrared spectroscopy in the region of the OCS ν1 vibration (≈2062 cm−1) in a pulsed supersonic jet expansion using a tunable diode laser probe. The spectra are those of prolate asymmetric rotors, with observed a-type transitions (ΔK=0), but no b-type transitions (ΔK=±1). The fitted rotational parameters are consistent with structures having intermolecular separations in the range of 3.6–3.8 Å and angles of about 70° between the OCS axis and the hydrogen center of mass. The band origins are slightly red-shifted (−0.05 to −0.20 cm−1) relative to that of the free OCS molecule. The results are of interest due to recent observations of the same complexes trapped in ultracold helium droplets; there is a remarkably close correspondence of the vibrational band origins of the free and trapped species.