UV spectroscopic determination of the chlorine monoxide (ClO) / chlorine peroxide (ClOOCl) thermal equilibrium constant.

The thermal equilibrium constant between the chlorine monoxide radical (ClO) and its dimer, chlorine peroxide (ClOOCl), was determined as a function of temperature between 228 and 301K in a discharge flow apparatus using broadband UV absorption spectroscopy. A third-law fit of the equilibrium values determined from the experimental data provides the expression K eq = 2.16 × 10-27 e (8527±35 K/T) cm3 molecule-1 (1σ uncertainty). A second-law analysis of the data is in good agreement. From the slope of the van't Hoff plot in the third-law analysis, the enthalpy of formation for ClOOCl is calculated, Δ H f ° (298 K) = 130.0 ± 0.6 kJ mol-1. The equilibrium constant results from this study suggest that the uncertainties in K eq recommended in the most recent (year 2015) NASA JPL Data Evaluation can be significantly reduced.