The rotational spectrum of the acetylene–carbonyl sulfide trimer: HCCH–(OCS)2

The rotational spectrum of the HCCH–OCS–OCS trimer has been assigned. The rotational constants and dipole moment components are consistent with a non-planar, barrel-like structure which aligns the three monomer axes in an almost parallel fashion. In this structure, the OCS dimer portion of the trimer has the two OCS molecules aligned in a parallel-like arrangement rather than the antiparallel geometry found in the OCS dimer. This conformation gives good agreement with rotational constants and dipole moment components predicted from a semi-empirical model and with isotopic shifts observed for two other isotopomers.