Knowledge Bank

The two mixed trimers $HCCH-(OCS)2$ and $OCS-(HCCH)2$ have been identified and assigned by FTMW spectroscopy. Rotational constants and dipole moment components for both species are consistent with triangular, barrel-shaped configurations of the monomers. The assignment of the rotational spectra of eight isotopomers for $HCCH-(OCS)2$ enabled an unambiguous structure determination and the discovery that this trimer contains a polar OCS dimmer fragment. The two OCS molecules are found to be aligned in an unusual parallel arrangement with their dipole moments reinforcing one another. Four isotopomers of the $OCS-(HCCH)2$ trimer were assigned. Due to a lack of isotopic data, a full structure determination was not possible. Semi-empirical calculations did, however, give a structure that aligned the acetylene monomers in a geometry intermediate between T-shaped and parallel that was consistent with the experimental data.