Infrared spectra of OCS-CO2 complexes are studied in a pulsed supersonic slit-jet expansion using a tunable diode laser probe in the 2060cm1 region of the C–O 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 Å 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 Å. 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.

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