Rotational spectrum, structure, and modeling of the HCCH-(OCS)(2) trimer: Observation of a polar OCS dimer fragment

The rotational spectra of eight isotopomers of the HCCH-(OCS)(2) trimer hav e been assigned by pulsed nozzle, Fourier-transform microwave spectroscopy. The rotational constants and dipole moment components are consistent with a nonplanar, triangular-twisted structure of C-1 symmetry which aligns the three monomer axes approximately 3 degrees-27 degrees away from perpendicul ar to a triangle formed by the center of HCCH and the carbons of OCS. The O CS dimer portion of the trimer has the two OCS monomers aligned in an almos t parallel fashion such that the monomer dipole moments reinforce, rather t han in the antiparallel arrangement observed in the well-known OCS dimer. T his configuration has been obtained using a semiempirical model which emplo ys a distributed multipole representation of the electrostatic interaction and analytical atom-atom terms to describe the dispersion and repulsion ter ms in the interaction potential. (C) 1999 American Institute of Physics. [S 0021-9606(99)01147-2].