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 have 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 perpendicular to a triangle formed by the center of HCCH and the carbons of OCS. The OCS dimer portion of the trimer has the two OCS monomers aligned in an almost parallel fashion such that the monomer dipole moments reinforce, rather than in the antiparallel arrangement observed in the well-known OCS dimer. This configuration has been obtained using a semiempirical model which employs a distributed multipole representation of the electrostatic interaction and analytical atom-atom terms to describe the dispersion and repulsion terms in the interaction potential. (C) 1999 American Institute of Physics. [S0021-9606(99)01147-2].