Temperature Dependence of the Product Branching Ratios of the ClO Self-Reaction in Oxygen

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.

Thermal decomposition of ClOOCl

ClOOCl was prepared in situ in a temperature controlled photoreactor (v = 420 L) by photolyzing OClO/N2 mixtures in the wavelength range 300-500 nm at temperatures between 242 and 261 K and total pressures between 2 and 480 mbar. After switching off the lights, excess NO2 was added, and IR and UV spectra were monitored simultaneously as a function of time. By spectral stripping of all other known UV absorbers (in particular, other chlorine oxides and chlorine nitrate), we determined rate constants k-1 of the reaction ClOOCl (+M) --> ClO + ClO (+M) from the first-order decay of the residual UV absorption of ClOOCl at 246 and 255 nm. k-1,0 = [N2] x 7.6 x 10(-9) exp[(-53.6 +/- 6.0) kJ mol(-1)/RT] cm3 molecule(-1) s(-1) (2sigma) was derived for the low-pressure limiting rate constant. Application of Troe's expression for the limiting low-pressure rate constants of unimolecular decomposition reactions leads to E0 = Delta(r)H0(0)(ClOOCl-->ClO+ClO) = 66.4 +/- 3.0 kJ mol(-1). k-1,0 started to fall off from the pressure proportional low pressure behavior at p approximately 30 mbar; however, reliable extrapolation to the high pressure limit was not possible. The decomposition rate constants of ClOOCl were directly measured for the first time, and they are higher, depending on temperature and pressure, by factors between 1.5 and 4.2 as compared to experimental data on k-1 by Nickolaisen et al. [J. Phys. Chem. 1994, 98, 155] which were derived from the approach of ClO to thermal equilibrium with its dimer ClOOCl. Combination of the present dissociation rate constants with recommended temperature and pressure dependent data on the reverse reaction (k1) demonstrate inconsistencies between the dissociation and recombination rate constants. Summarizing laboratory data on k1 and k-1 above 250 K and field measurements on the ClO + ClO <= => ClOOCl equilibrium in the nighttime polar stratosphere close to 200 K, the expression Kc = k1/k-1 = 3.0 x 10(-27) exp(8433 K/T) cm3 molecule(-1) is derived for the temperature range 200-300 K.

Theoretical Investigation of Product Channels in the CH3O2 + Br Reaction

Several reaction pathways on the potential energy surface (PES) for the reaction of CH3O2 radicals with Br atoms are examined using both ab initio and density functional methods. Analysis of the PES suggests the presence of the stable intermediates CH3OOBr and CH3OBrO. CH3OOBr is calculated to be more stable than CH3OBrO by 9.7 kcal mol(-1) with a significant barrier preventing formation of CH3OBrO via isomerization of CH3OOBr. The relative importance of bi- and termolecular product channels resulting from the initially formed CH3OOBr adduct are assessed based on calculated barriers to the formation of CH2OO + HBr, CH3O + BrO, CH3Br + O2, and CH2O + HOBr.

Quasiclassical trajectory studies on the gas-phase BrO self-reaction

The effect of initial vibrational energy and the mode of disposal of available energy into products for the disproportionation reaction between BrO radicals are studied using the quasiclassical trajectory (QCT) technique. The calculations are performed on an analytical potential enegy surface based on ab initio results reported for the three isomeric forms of (BrO)2 system. The results indicate a mild dependence on reactant vibrational energy, which is discussed in terms of the mechanistic aspects of the reaction. The study of the distribution of total accessible energy among products indicates a different mode of energy disposal in each path and hence important differences on the mechanism by which each channel takes place.