Production Of ClO Research Articles

Dynamics of the reaction O( 1D) + HCl → ClO + H from crossed-beam experiments

The angular velocity distribution of the ClO product from the reaction O( 1D) + HCl at 12.2 kcal/mol collision energy has been obtained in a crossed-molecular-beam study. The product center-of-mass angular distribution is found to be almost backward—forward symmetric, with backward scattering being slightly favored, from which it is deduced that part of the reaction occurs via a long-lived complex and part via direct abstraction of the halogen atom. The derived large fraction (≈ 43%) of energy released into translation indicates the existence of a barrier in the exit channel. A lower limit of the branching ratio between ClO + H and OH + Cl channels is derived and is compared to recent bulk results.

Kinetic measurements of the ClO + O3 → ClOO + O2 reaction

Upper limits for the bimolecular rate constant for the reaction ClO + O3 → ClOO + O2 have been measured directly between 233 and 413 K using a discharge‐flow system at 1–2 torr total pressure. Production of ClO (which occurs quickly after thermal decomposition of ClOO in the presence of O3) was detected using Laser Magnetic Resonance spectroscopy, and distinguished from the reactant ClO through isotopic oxygen labelling. At 298 and 233 K, k1 was found to be less than 1.4 × 10−17 cm3 molecule−1 s−1. At 413 K1 k1 = (4.0 ±2.0) × 10−16 cm3 molecule−1 s−1. The experimental conditions were modelled using a kinetic simulation program to show that chemical and isotopic scrambling interferences were negligible. From these results, it is concluded that this bimolecular reaction does not contribute significantly to the observed ozone depletion within the antarctic vortex.

Production and trapping of gaseous dimeric ClO: The infrared spectrum of chlorine peroxide (ClOOCl) in solid argon

The dimeric ClO has been generated in a gaseous discharge–flow system at 220 K and 10–30 Torr by reacting atomic Cl with O3, ClOCl or OClO, and reacting atomic O with ClOCl or OClO, in a stream of Ar. The gas mixture in the flow tube was sampled through a pinhole and condensed on a CsI window maintained at 12 K. The infrared absorption spectrum of the matrix formed after 30–120 min deposition was then recorded. Absorption lines at 752.6, 649.8, and 647.6 cm−1 have been assigned to ClOOCl, and previous assignments for dimeric ClO seem to be in error. The results are in excellent agreement with recent ab initio calculations. Under our experimental conditions, no other isomer of Cl2O2 was observed.

The photolysis of concentrated perchloric acid solutions

The production of ClO 2 after the photolysis of 6 M perchloric acid (HClO 4) has been identified on the basis of its absorption spectrum. Using 3 M HClO 4, no absorption corresponding to ClO 2 was observed, but rather a weaker absorption at around 300 nm attributed to ClO − was identified.

Reactions of the chlorodifluoromethyl radical formed in the photolysis of halogenocarbon + ozone mixtures

A photolytic flow system has been used to study the reactions of CF2Cl with ozone. The radical was generated by ultraviolet photolysis of O3+ CF2Cl2 mixtures. Concentrations of reactants and products were determined by quadrupole mass spectrometry, and varying contact times obtained by varying the distance between the photolysis lamp and the sampling orifice. Quantum yields for ozone loss and for ClO production were then analysed to provide kinetic information. Comparison of the experimental data with computer models of the reaction system show that a plausible reaction sequence is [graphic omitted] with k12(298 K)=(1.4 ± 0.7)× 10–14 cm3 molecule–1 s–1, and both k10a and k11 not lower than 1 × 10–13 cm3 molecule–1 s–1. Thus, the chlorine atom in CF2Cl can be converted into ClO, which has implications for the ClOx catalytic O3 destruction cycle in stratospheric chemistry. CF2O is produced in reactions of CF2Cl and not in the reaction of O(1D) with CF2Cl2.

Photochemistry of the ozone—halocarbon system

A photolytic flow system was used to study the primary and secondary steps in the reaction of O( 1D) with chlorofluorocarbons. Concentrations of reactants and products were determined by quadrupole mass spectrometry; varying contact times were obtained by altering the distance between the photolysis lamp and the sampling orifice. “Intermediate” quantum yields for ozone loss and for ClO production were then analysed to provide kinetic information. Preliminary results with CF 2Cl 2 as the halocarbon suggest that O( 1D), produced by ozone (O 3) photolysis, reacts to yield mainly CF 2Cl and ClO fragments. The CF 2Cl radicals then react with O 3, with a rate constant of the order of 10 −14 molecule −1 cm 3s −1 to produce a further ClO radical, probably together with CF 2. CF 2O appears only at relatively long contact times, suggesting that is not formed directly in a branch of the O( 1D) + CF 2Cl 2 reaction but rather in a secondary reaction between CF 2 and O 3.

Some aspects of the stratospheric Cl-ClO-Cl cycle: Possible roles of ClO *, ClNO 3 and HOCl

Depending on such factors as (a) the probabilities of exciting the various vibrational states in ClO formed in the reaction of Cl with O 3, (b) the radiative lifetime of ClO *, (c) Δ H ƒ(ClO 3), and (d) the rate coeffic`ient of the relevant three-body reaction, the production of ClO 3 via the reaction ClO * +O 2 +M→ClO 3 +M may be quite substantial in the stratosphere. The significance of this result lies in the subsequent elimination (from the stratosphere) of ClO 3 and its associated chlorine atom as HClO 4, in the manner recently suggested by Samonaitis and Heicklen. In the stratosphere, ClO 3 most probably photodissociates primarily into OClO and O. Upon photodissociation, OClO may also yield atomic oxygen. Thus the formation of ClO 3 from ClO * and O 2, and the above-mentioned photodissociation steps constitute an interesting, indirect mechanism of O 2 dissociation into two odd oxygen species. Other aspects of ClO * chemistry, applicable in stratospheric conditions, also deserve attention in view of Nicholl's recent interpretation of the Umkehr measurements by Brewer et al. The reactions of ClO with HO 2, and NO 2, possess the potential of significantly obstructing the completion of the C1-ClO-Cl cycle, at least in the region below 35 km. An accurate and critical study of the chemistry of oxyacids, higher oxides, and nitrates of chlorine in the stratospheric environment is needed. Obviously, this is only a partial list of the difficult problems associated with a proper understanding of stratospheric chlorine chemistry which appears to be far more complex than what is implied in the literature. (See also notes added in proof stage.)