Observation of a planar isomer of the OCS-(C2H2)2 trimer

► A new planar isomer of the OCS-(C2H2)2 trimer is observed in the infrared. ► Spectra for three isotopologues are analyzed in detail, while the spectrum of a fourth is found to be perturbed. ► Observed rotational constants agree fairly well with theoretical calculations. ► It should now be possible to assign the microwave spectrum of planar OCS-(C2H2)2.