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 Title: Ocs Trimer And Tetramer: Calculated Structures And Infrared Spectra Author(s): McKellar, Bob Contributor(s): Moazzen-Ahmadi, Nasser; Dehghany, Mehdi Subject(s): Clusters/Complexes Abstract: An OCS trimer was originally observed in the 1990s by microwave spectroscopy.\footnote{J.P. Connelly, A. Bauder, A. Chisholm, and B.J. Howard, Mol. Phys. \textbf{88}, 915 (1996); R.A. Peebles and R.L. Kuczkowski, J. Phys. Chem. A \textbf{103}, 6344 (1999).} 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 \textit{ab initio} potential energy surface\footnote{J. Brown, X.-G. Wang, R. Dawes, and T. Carrington, Jr., J. Chem. Phys. \textbf{136}, 134306 (2012).} and assume pairwise additivity. There are also recent direct \textit{ab initio} trimer and tetramer calculations, which are (necessarily) at a lower level of theory.\footnote{N. Sahu, G. Singh, and S.R. Gadre, J. Phys. Chem. A \textbf{117}, 10964 (2013).} 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 $\wn$ relative to dissociation, and we calculate (at least) twenty different isomers within 100 $\wn$ of this minimum (and seven within 20 \wn). Remarkably, the observed microwave tetramer does correspond to our lowest calculated isomer. However this isomer is not included in the published direct \textit{ab initio} calculation -- it may just have been overlooked due to the large number of isomers! In the mid-infrared region of the OCS \nub{1} fundamental (\~2060 \wn), 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 $\wn$ above the most stable one, and it is also missing from the direct \textit{ab initio} calculation. Neither observed tetramer has any symmetry elements. Issue Date: 2014-06-20 Publisher: International Symposium on Molecular Spectroscopy Citation Info: McKellar, B.; Moazzen-Ahmadi, N.; Dehghany, M. OCS TRIMER AND TETRAMER: CALCULATED STRUCTURES AND INFRARED SPECTRA. Proceedings of the International Symposium on Molecular Spectroscopy, Urbana, IL, June 16-21, 2014. DOI: 10.15278/isms.2014.FB11 Genre: Conference Paper / Presentation Type: Text Language: English URI: http://hdl.handle.net/2142/51233 DOI: https://doi.org/10.15278/isms.2014.FB11 Rights Information: Copyright 2014 by the authors. Licensed under a Creative Commons Attribution 4.0 International License. http://creativecommons.org/licenses/by/4.0/ Date Available in IDEALS: 2014-09-172015-04-14
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