The microwave spectrum, structure, and harmonic force field of the van der Waals trimer Ar2–OCS

Abstract

The microwave rotational spectra of the van der Waals trimers Ar2–OCS, Ar2–OC34S, and Ar2–18OCS 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–OC34S and Ar2–18OCS, 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 (κ=∼0.377) with the OCS subunit lying in the Cs 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 Å−1, which compares favorably with 0.0078 mdyn Å−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.