Infrared spectra of the OCS-CO2 complex: observation of two distinct slipped near-parallel isomers

Infrared spectra of OCS-CO2 complexes are studied in a pulsed supersonic slit-jet expansion using a tunable diode laser probe in the 2060\u2002cm 121 region of the C\u2013O stretching fundamental of OCS. Two bands are observed and analyzed, corresponding to two distinct isomers of the complex. Isomer a is the known form which has been previously studied in the microwave region. Isomer b is a new form, expected theoretically but first observed here. Structures are determined with the help of isotopic substitution. Both isomers are planar, with slipped near-parallel geometries. In isomer a, the intermolecular (center of mass) separation is 3.55 \uc5 and the C atom of the CO2 is closer to the S atom of the OCS. In isomer b, the C atom of CO2 slides closer to the O atom of OCS and the center of mass separation increases to 3.99 \uc5. Isomer a is the lowest energy form, but paradoxically isomer b appears to be stronger in our infrared spectra. Predicted pure rotational transition frequencies are given to help in a search for the microwave spectrum of isomer b.Peer reviewed: YesNRC publication: Ye