OCS TRIMER AND TETRAMER: CALCULATED STRUCTURES AND INFRARED SPECTRA

An OCS trimer was originally observed in the 1990s by microwave spectroscopy.a New broadband chirped-pulse microwave spectra (preceding talk) reveal an OCS tetramer and a second distinct trimer isomer. In the present talk, we discuss OCS cluster structures and infrared spectra. Our structure calculations are based on a recent ab initio potential energy surfaceb and assume pairwise additivity. There are also recent direct ab initio trimer and tetramer calculations, which are (necessarily) at a lower level of theory.c We find that the observed OCS trimers indeed correspond to the two lowest energy isomers in both calculations, and that there is fairly good agreement of experimental and theoretical structures. For the tetramer the global minimum is at -2773 cm−1 relative to dissociation, and we calculate (at least) twenty different isomers within 100 cm−1 of this minimum (and seven within 20 cm−1). Remarkably, the observed microwave tetramer does correspond to our lowest calculated isomer. However this isomer is not included in the published direct ab initio calculation – it may just have been overlooked due to the large number of isomers! In the mid-infrared region of the OCS ν1 fundamental (2̃060 cm−1), we observe two bands which are clearly due to the same microwave OCS tetramer. But a third band is assigned to a different tetramer not observed in the microwave spectrum. It appears to correspond to our seventh calculated isomer, located about 20 cm−1 above the most stable one, and it is also missing from the direct ab initio calculation. Neither observed tetramer has any symmetry elements.