Isomers on the [H, S 2 , Cl] potential energy surface: A high level investigation Yuri Alexandre Aoto, Antonio Gustavo S. de Oliveira-Filho, Fernando R. Ornellas * Departamento de Química Fundamental, Instituto de Química, Universidade de São Paulo, Av. Lineu Prestes, 748, Caixa Postal 26077, São Paulo 05513-970, Brazil article info Article history: Received 13 November 2008 Received in revised form 16 February 2009 Accepted 16 February 2009 Available online 26 February 2009 Keywords: Sulfur–chlorine chemistry Chlorine reservoir Structure and bonding Vibrational spectra CCSD(T) abstract This study reports a systematic state-of-the-art characterization of new sulfur–chlorine species on the [H, S 2 , Cl] potential energy surface. Coupled cluster theory singles and doubles with perturbative contribu- tions of connected triples, using the series of correlation consistent basis sets with extrapolations to the complete basis set limit (CBS), were employed to quantify the energetic quantities involved in the isomerization processes on this surface. The structures and vibrational frequencies are unique for some species and represent the most accurate investigation to date. These molecules are potentially a new route of coupling the sulfur and chlorine chemistries in the atmosphere, and conditions of high concen- tration of H 2 S (HS) like in volcanic eruptions might contribute to their formation. Also an assessment of the MP2/CBS approach relative to CCSD(T)/CBS provides insights on the expected performance of MP2/ CBS on the characterization of polysulfides, and also of more complex systems containing disulfide bridges. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction The destruction of ozone in the stratosphere has been a subject of major concern both for the public and the scientific community. In this context, the role of halogen-containing species like ClO, BrO, IO, HOCl, HOBr, and HOI in reactions cycles leading to the catalytic depletion of ozone is quite well documented in the literature [1–9]. The question of how radical carriers react with each other leading to another carrier or to a reservoir molecule is a central one to the understanding of the mechanisms underlying these catalytic cy- cles. In the case of the reaction between the radicals ClO and OH, it can lead to two other carriers HO 2 and Cl, and a further reaction of HO 2 with Cl can either generate HCl and O 2 in one case, or HO and ClO in the other. To account for the large magnitude of the ki- netic results of the reaction between HO 2 and Cl, a mechanism has been proposed which involves the formation of an intermediate of the type HOOCl [10,11]. This possibility motivated theoretical studies concerned with structural features and relative stabilities of the various isomers on the [H, O 2 , Cl] potential energy surface, and also on possible reactions between different carriers having these isomers as intermediates [12,13]. Guided initially by an analogy of these oxygenated molecules with their isovalent counterpart containing sulfur, our group has recently characterized the thermodynamic properties, the vibra- tion spectra, and the structural parameters of a series of new spe- cies, namely SCl, SBr, SI, HSCl, HClS, HSBr, HBrS, HSI, and HIS, of potential relevance to atmospheric chemistry [14–19]. In principle, these species are a possibility of coupling the sulfur and halogen chemistries in the atmosphere. In the case of HSCl, we have shown that after strong volcanic eruptions, a condition in which the con- centration of HS becomes significant, it is very likely that the spe- cies HSCl can be formed, as well as under high pressures [20,21]. Experimentally, to the best of our knowledge, only data on the photoionization spectrum and the ionization energy of the chlorine species HSSCl, detected in the reaction system C/Cl 2 /H 2 S in a dis- charge-flow reactor with a photoionization mass spectrometer, have been reported in the literature [22]; this study also presented geometric parameters optimized at the MP2(full)/6-31G * level of theory, and ionization energies computed with the G2 model. The only other study in the literature is the Atoms-in-Molecules (AIM) analysis of the isomerization reaction SSHCl ? HSSCl carried out by Zeng et al. [23], with geometrical parameters optimized with the DFT/B3LYP/6-311++G(2df, pd) approach, followed by en- ergy calculations at the MP2/6-311++G(2df, pd) level on top of the DFT/B3LYP/6-311++G(2df,p d) optimized geometries for four stationary points. Systems with a disulfide bond (XSSX, X = F, Cl, Br) were investigated by Prascher and Wilson [24], and more re- cently a high level (CCSD(T)/CBS) characterization of the isomeri- zation reaction SSX 2 ? XSSX (X = F, Br), was also systematic studied in our group [25,26]. In this work, as another step in the long-term investigation of halogen–sulfur species, our efforts were directed to a systematic characterization of the [H, S 2 , Cl] potential energy surface (PES) at a high level of correlation treatment, using extended correlation consistent basis sets. Here also we are guided by the same type of analogy we have carried out so far for the halogen–sulfur com- pounds mentioned above. It is our hope that the reliability and 0166-1280/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2009.02.016 * Corresponding author. Tel.: +55 11 30913895. E-mail address: frornell@usp.br (F.R. Ornellas). Journal of Molecular Structure: THEOCHEM 902 (2009) 90–95 Contents lists available at ScienceDirect Journal of Molecular Structure: THEOCHEM journal homepage: www.elsevier.com/locate/theochem