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

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.