THE MICROWAVE-SPECTRUM, STRUCTURE, AND HARMONIC FORCE-FIELD OF THE VANDERWAALS TRIMER AR2-OCS

The microwave rotational spectra of the van der Waals trimers Ar2-OCS, (Ar2-OCS)-S-34, and (Ar2-OCS)-18 have been observed between 7.0 and 18.0 GHz using pulsed molecular beam microwave Fourier transform (MWFT) spectroscopy. Sixty-five transitions, both a and c type, have been assigned for Ar2-OCS, as well as 37 transitions and 19 transitions for (Ar2-OCS)-S-34 and (Ar2-COS)-O-18 respectively. The rotational constants and quartic centrifugal distortion constants have been determined. The trimer has been found to be a distorted tetrahedrally shaped highly asymmetric top (K= approximately 0.377) with the OCS subunit lying in the C(s) symmetry plane. An effective ground state structure has been determined. The Ar-Ar distance and the two ’’Ar-center of mass of OCS’’ distances are slightly longer than those in the corresponding dimers Ar2 and Ar-OCS, respectively. A harmonic force field analysis has been performed on the basis of the centrifugal distortion constants, to predict the van der Waals force constants and the corresponding vibrational frequencies. The Ar-Ar stretch force constant of the trimer is 0.008 42 mdyn angstrom-1 which compares favorably with 0.0078 mdyn angstrom-1 reported for the free dimer Ar2. A comparison of the geometry and spectroscopic properties of Ar2-OCS with those of Ar2 and of the Ar2-HX (X = F, Cl, CN) trimers is given.