Chlorine peroxide (Cl₂O₂) and its isomers: structures, spectroscopy, formation and thermochemistry

ABSTRACT

We report ab initio calculations of the molecular structures of the various Cl₂O₂ isomers, transition states, vibrational frequencies and vertical excitation energies, as well as the relative energies of the Cl₂O₂ isomers with respect to 2ClO, ClOO + Cl and OClO + Cl dissociation channels employing up to the CCSD(T)/aug-cc-pVQZ level of theory. Our best theoretical estimate for the dissociation wave number D₀ of chlorine-peroxide, dichloride-dioxide ClOOCl relative to 2ClO is 6825 cm⁻¹ (including harmonic zero-point energy correction), compared to recent experimental estimates in the range 5700–7000 cm⁻¹, thus favouring the higher values. The chlorine chlorite structure ClOClO is found to be weakly bound by ∼3400 cm⁻¹ with respect to 2ClO. The chloryl chloride, chlorine peroxide ClClO₂ is observed to be stabilised with respect to the chlorine peroxide ClOOCl when large basis sets with diffuse functions are used, and ClClO₂ is predicted to be about hc 700 cm⁻¹ lower in energy than ClOOCl (including harmonic zero-point energy correction). However, ClClO₂ is not assumed to be significant for the ClO self-reaction due to the high barrier to association. The isomerisations appear also unlikely under stratospheric conditions, as the transition states optimised at CCSD/aug-cc-pVTZ level of theory are found to lie high above the reactants. We also discuss the relation to recent research on parity violation and stereomutation tunnelling in this molecule.

GRAPHICAL ABSTRACT

KEYWORDS:

• ClO dimer
• ClOOCl
• ClClO₂
• ClOClO
• coupled cluster
• ab initio
• dichlorine dioxide atmospheric chemistry
• ozone depletion

Acknowledgements

Our work is supported by ETH Zürich, the Swiss National Science Foundation and an ERC Advanced Grant. In particular, the research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7 2007-2013) ERC grant agreement No. 290 925. H. F. Schaefer is supported by the U.S. National Science Foundation and by visiting professorship of ETH Zürich. We enjoyed discussions with Mitchio Okumura, Fabio Mariotti, Robert Prentner and Jürgen Stohner, as well as Csaba Fabri, in particular in the final stages of this work.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

ETH Zürich; the Swiss National Science Foundation; an ERC Advanced Grant; European Research Council: European Union’s Seventh Framework Programme [grant number FP7 2007-2013].