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Title:Ab initio exploration of the potential energy surface of the O2-SO2 open-shell complex.
Author(s):Fawzy, Wafaa M.
Contributor(s):Hougen, Jon T.
Subject(s):Theory and Computation
Abstract:The O$_2$-SO$_2$ complex is believed to be a precursor to acid rain. The previously observed FTMW spectrum suggested internal motions within the complex, but their nature was not identified. Development of an effective Hamiltonian for an open-shell molecule with tunneling requires knowledge of the potential energy surface (PES) and the intrinsic reaction coordinates (IRC) for the paths between minima. A recent ab initio study reported two different nonplanar minima in the ground electronic state of O$_2$-SO$_2$. These predictions were based on geometry optimization calculations at the MP2/aug-cc-pVnZ level of theory, with n = 2 and 3. The current work is focused on a highly correlated ab initio investigation of the global PES (a 9-D problem) in the ground triplet electronic state of O$_2$-SO$_2$. Because of the high dimensionality in the complex, the PES calculations are partitioned into several two-dimensional cuts through the PES. We have so far explored only a 3-D part of the global PES to look for stable planar configurations. These calculations included geometry optimization, frequency, and single point energy calculations. Calculations were performed using UCCSD(T)/aug-cc-pV(n+D)Z,where n = 2 and 3, level of theory. We used an axis system that defines the radial and the angular van der Waals coordinates for a planar complex as R$_{vW}$, $\theta_1$, and $\theta_2$. The bond length (R$_{vW}$) is the distance between the center of mass of the O$_2$ unit and the S atom. $\theta_1$ and $\theta_2$ are the angles between the van der Waals bond and the O$_2$ internuclear axis or one of the SO bonds in the SO$_2$ moiety, respectively. Full geometry optimization calculations predicted a minimum of C$_s$ symmetry in which both the O$_2$ and SO$_2$ units are tilted with respect to the van der Waals bond, and R$_{vW}$ = 3.63 {\AA}. 3-D PES surface calculations, which involve the R$_{vW}$, $\theta_1$, and $\theta_2$ vdW coordinates, showed that the optimized structure is the global minimum. In addition, a local minimum at R$_{vW}$ = 3.9 {\AA}, which represents a different chemical isomer, was identified. If the four oxygen atoms are labeled, each isomer is a part of four equivalent minima, and three distinguishable transition states between these various minima are identified. These results suggest that PES calculations should consider at least five dimensions. Our progress in exploring possible non-planar coordinates and IRC paths will also be presented.
Issue Date:2016-06-21
Publisher:International Symposium on Molecular Spectroscopy
Genre:Conference Paper/Presentation
Rights Information:Copyright 2016 by the authors
Date Available in IDEALS:2016-08-22

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