UV absorption spectrum and thermochemistry of Cl2O3 formed in the photolysis of OClO-containing mixtures at low temperatures

Mechanistic Insights into Chloric Acid Production by Hydrolysis of Chlorine Trioxide at an Air-Water Interface

Chlorine oxides play crucial roles in ozone depletion, and the final oxidation steps of chlorine oxide potentially result in the formation of chloric acid (HClO3) or perchloric acid (HClO4). Herein, the solvation and reactive uptake of three stable isomers of chlorine trioxide (Cl2O3), namely, ClOCl(O)O, ClClO3, and ClOOOCl, at the air-water interface were investigated using classical and hybrid quantum mechanics/molecular mechanics (QM/MM) molecular dynamics (MD) coupled with advanced free energy methods. Two distinct mechanisms were revealed for the hydrolysis of ClOCl(O)O and ClClO3: molecular and ionic mechanisms. A comparison of the computed free-energy profiles for the gaseous and air-water interfacial systems indicated that the air-water interface could markedly lower the free-energy barrier for ClO 3 - or HClO3 formation while stabilizing the product state. In particular, the hydrolysis of ClClO3 at the air-water interface was barrierless. In contrast, our calculations showed that the hydrolysis of ClOOOCl was very slow, indicating that ClOOOCl was inert to water at the air-water interface. This study provides theoretical evidence for the hypothesis that HClO3 is a sink for chlorine oxides and for the widespread distributions of HClO3 recently observed in the Arctic region.

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.

General-acid-catalyzed reactions of hypochlorous acid and acetyl hypochlorite with chlorite ion

The rate of oxidation of ClO2- by HOCl is first order in each reactant and is general-acid catalyzed. In the initial steps of the proposed mechanism, a steady-state intermediate, HOClOClO-, forms (k1 = 1.6 M-1 s-1) and undergoes general-acid (HA)-catalyzed reactions (k2HA) to generate a metastable intermediate, ClOClO. Values of k2HA/k-1 are 1.6 x 10(4) M-1 (H3O+), 20 M-1 (HOAc), and 8.5 M-1 (H2PO4-). Subsequent competitive reactions of ClOClO with ClO2- (k3) to give 2ClO2 and with OH- (k4OH) and other bases (k5B) to give ClO3- are very rapid. The relative yields of these products give k4OH/k3 = 1.3 x 10(5), k5HPO4/k3 = 0.20, and k5OAc/k3 = 0.06. At low pH and low buffer concentrations, the apparent yield of ClO2, based on 2ClO2 per initial HOCl, reaches 140%. This anomaly is attributed to the induced disproportionation of ClO2- by ClOClO to give ClO3- and additional HOCl. A highly reactive intermediate, ClOCl(O)OClO-, is proposed that can undergo Cl-O bond cleavage to give 2ClO2 + Cl- via one path and ClO3- + 2HOCl via another path. The additional HOCl recycles in the presence of excess ClO2- to give more ClO2. Ab initio calculations show feasible structures for the proposed reaction intermediates. Acetic acid has a second catalytic role through the formation of acetyl hypochlorite, which is much more reactive than HOCl in the transfer of Cl+ to ClO2- to form ClOClO.

Rate of formation of the ClO dimer in the polar stratosphere: implications for ozone loss

The gas-phase recombination of chlorine monoxide (ClO) has been investigated under the conditions of pressure and temperature that prevail in the Antarctic stratosphere during the period of maximum ozone (O3) disappearance. Measured rate constants are less than one-half as great as the previously accepted values. One-dimensional model calculations based on the new rate data indicate that currently accepted chemical mechanisms can quantitatively account for the observed O3 losses in late spring (17 September to 7 October). A qualitative assessment indicates that the existing mechanisms can only account for at most one-half of the measured O3 depletion in the early spring (28 August to 17 September), indicating that there may be additional catalytic cycles, besides those currently recognized, that destroy O3.