ClO Reaction Research Articles

On the potential importance of the gas phase reaction CH3O2 + ClO → ClOO + CH3O and the heterogeneous reaction HOCl + HCl → H2O + Cl2 in “ozone hole” chemistry

We call attention to the great importance of the gas phase reaction ClO + CH3O2 → ClOO + CH3O and the heterogeneous reaction HCl + HOCl → Cl2 + H2O on polar stratospheric cloud (PSC) particles. These reactions may accomplish the almost complete conversion of HCl into ClOx radicals, thus leading to rapid destruction of ozone.

An Investigation of the Reactions of NO3 Radicals with Cl and ClO

AbstractThe reaction (1) Cl + NO3 → ClO + NO2 was studied at 298 K by observing the kinetic behaviour of NO3, Cl and ClO using discharge‐flow mass‐spectrometric technique. A rate constant k(1) = (2.26 ± 0.17) · 10–11 cm3 molecule−1 s−1 was obtained. The secondary processes between NO3 and ClO were also investigated and OClO was detected as a reaction product. For the rate constant of the reaction (2a) ClO + NO3 → ClOO + NO2 an upper limit of k(2a) = 1 · 10−13 cm3 molecule−1 s−1 was found. In contrast to previous findings it is shown that reaction (2b) ClO + NO3 → OClO + NO2 is the predominant channel with k(2b) = (4.3 ± 1.0) · 10−13 cm3 molecule−1 s−1.

Kinetic studies of the reactions of IO, BrO, and ClO with dimethylsulfide

AbstractThe kinetics of the reactions of IO, BrO, and ClO with DMS have been investigated at 298 K and total pressures of 0.5–6.8 mbar He using the discharge flow‐mass spectrometry technique to monitor the halogen oxide radicals. Rate coefficients of (8.8 ± 2.1) × 10−15, (2.7 − 0.5) × 10−13 and (9.5 ± 2.0) × 10−15 cm3 molecule−1 s−1 have been obtained for the reactions of IO, BrO, and ClO with DMS, respectively. The result for IO with DMS is in good agreement with two other recent studies on this reaction but is nearly three orders of magnitude slower than two earlier studies, one of which was from this laboratory. The earlier studies are now thought to be in error because of heterogeneous and secondary chemistry occurring in the systems investigated. The rate coefficient for BrO with DMS is in excellent agreement with a previously reported preliminary value from this laboratory. However, the rate coefficient for ClO with DMS is a factor of 4 lower than our previously determined value. The new data are considered more reliable in view of the much improved experimental technique in the present study. DMSO was detected as a product in all of these reactions. A semi‐quantitative determination of the DMSO yield was only possible in the case of the reaction of IO with DMS where a yield of 84 ± 40% was found.

Model simulation of chemical depletion of Arctic ozone during the winter of 1989

Ozone depletion in the Arctic vortex has become an important issue. As part of a Canadian research program designed to study the effect of polar stratospheric clouds on the chemistry of the Arctic ozone layer, project CANOZE (Canadian Arctic Northern OZone Experiment) 4, a series of ozonesondes were flown during January and February 1989 from Alert at 82.5°N. This time series of ozone profiles indicated a decrease of ozone during this period. We have applied a simple chemical model to estimate the possible ozone depletions that could have occurred during this period. The results suggest a chemical depletion of ∼ 4% per week is possible, in approximate agreement with the observed depletion rate at 20 km of 5% per week. The major features of the analysis confirm earlier suggestions that thermal dissociation of Cl2O2 could limit the amount of O3 destruction. For our simulation conditions of largely unperturbed O3 densities the ClO+O reaction is a major loss process for O3 above 19 km, while Cl2O2 photolysis is the major loss process below 19 km, with the BrO+ClO reaction contributing about 14% of the loss in both cases. The branch of the BrO+ClO reaction that results in the formation of BrCl also acts to limit the nighttime O3 destruction via BrO+ClO since BrO is rapidly converted to BrCl at twilight. The photolytic release of NO and NO2 from HNO3 at the latitudes encountered by the air parcels driving these late winter/early spring conditions is sufficiently slow that it has no major impact on O3 destruction by Clx and Brx on the time scale of a ∼ 1 month.

Preparation characterization and crystal structure of chloryl hexafluororuthenate(1-). Crystal structure of [ClF2]+[RuF6

Chloryl hexafluororuthenate(V), [ClO 2 ] + [RuF 6 ] − , is prepared either by fluorination of RuO 4 using ClF 3 in HF solution at room temperature or by the reaction of ClO 2 F with RuF 5 . This salt, which like its counterpart [ClF 2 ] + [RuF 6 ] − is a powerful oxidizer, is characterized by elemental analysis, X-ray powder data, and vibrational spectroscopy. The crystal structure of this salt shows that the ruthenium atom, located on a C 2 axis, is surrounded by a weakly distorted octahedron of fluorine atoms with six comparable bond lengths. The space group, unit cell parameters, and R factor are as follows: monoclinic, P2/n

A study of the ClO absorption cross-section between 240 and 310 nm and the kinetics of the self-reaction at 300 K

The absorption cross-section of ClO was studied between 240 and 310 nm at 300 K. Measurements of the ClO absorption bands were made at two different spectral resolutions and their oscillator strengths were determined. In addition, the complex reaction of ClO with itself was investigated at 300 K. ▪ The k 1a, k 1b and k 1c values obtained for the bimolecular channels are (7.3±1.8) × 10 −15, (7.2±1.6) × 10 −15 and (7.3±2.6) × 10 −15 cm 3 molecule −1 s −1 respectively. This investigation is compared with previous studies of the ClO absorption cross-section and the rate coefficient of reaction (1).

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.

Rate coefficient for the termolecular channel of the self-reaction of chlorine monoxide

The rate coefficient for the reaction ClO+ClO{sup M}{yields}Cl{sub 2}O{sub 2} has been measured by using a flash photolysis/UV absorption technique at temperatures from 200 to 263 K and pressures from 25 to 600 Torr in N{sub 2}, O{sub 2}, He, and SF{sub 6}. Conventional single-wavelength detection and diode array spectrometry have both been employed in monitoring the reactant and product species. The UV absorption cross section of ClO has been measured in the same temperature range, from 220 to 350 nm, by using the diode array, and at 282.65 nm (the maximum of the ClO (9,0) band) by using the single-wavelength detection. The reactions of chlorine atoms with O{sub 3} and Cl{sub 2}O have been used to produce ClO. The title reaction is believed to play a role in the loss of ozone from the Antarctic stratosphere during the austral spring. At the temperatures and pressures relevant to the Antarctic stratosphere, our measured values agree with the recent work of Sander et al. and are considerably smaller than the values obtained by Hayman et al.

ChemInform Abstract: UV Absorption Cross Sections of Cl2O2 Between 210 and 410 nm.

AbstractThe transient Cl2O2 molecule is produced by the gas‐phase reaction ClO + ClO + M → Cl2O2 + M (M: He) using three independent ClO radical source reactions.

ChemInform Abstract: Kinetics and Products of the BrO + ClO Reaction.

AbstractAn overall rate constant of k1 = 1.13·10‐11 cm3 molecule‐1 s‐1is measured for the title reaction at 298 K by using discharge flow mass spectrometry.

Effects of initial active chlorine concentrations on the Antarctic ozone spring depletion

Several one‐dimensional model experiments have been conducted in order to simulate the observed downward trend of total ozone and the shape of ozone profiles in the Antarctic during the austral spring. The results underline the fact that the O3 depletion is quadratically sensitive to the amount of active chlorine, which we have expressed as the fraction (F) of Clx that is in active form by polar sunrise. For F = 1, this result is largely independent of the occurrence of heterogeneous chemistry after polar sunrise since all of the processing has already occurred. The main O3 loss is due to formation and photolysis of Cl2O2, the mechanism first suggested by Molina et al. [1987]. By comparison, using Brx levels similar to currently measured values, it was found that the BrO + ClO reaction plays a less important role in O3 depletion. The depth of the O3 minimum in spring appears to be related to the vertical extent of the region chemically processed by polar stratospheric clouds as well as the absolute levels of active Clx; more extensive vertical regions result in larger bite outs of the O3 profile. This may account for the difference in O3 minimum between 1986 and 1987. Simulations involving downward vertical transports are unable to reproduce observed decreases in total O3. We speculate that low trace gas mixing ratios of species such as N2O may be remnants of the previous summer and fall conditions, rather than signatures of downward transport during the springtime ozone depletion.

Studies with nitryl hypochlorite: thermal dissociation rate and catalytic conversion to nitric oxide using an NO/O3 chemiluminescence detector

A NO/O 3 chemiluminescence detector equipped with a gold catalyst is adapted to provide a measure of the thermal decomposition rate of ClONO 2 in an Ni 2 /O 2 gas mixture and, in a related way, provide the absolute concentration of ClONO 2 in a flowing gas stream. The approach is to add ClONO 2 , in the parts per million by volume range, to the flow stream of the detector in the presence of excess NO. As the sample is heated, ClONO 2 is thermally dissociated and the subsequent scavenging reaction of ClO with NO produces Cl and NO 2 . Cl goes on to react with ClONO 2 to form NO 3 which, in turn, reacts with NO to produce NO 2 . The loss of NO from the flow is precisely monitored downstream in the detector by the change in the chemiluminescence produced in the reaction of NO with reagent O 3

Photoionization mass spectrometric studies of free radicals

Free radicals have been fascinating to chemists for decades, partly because of their important but emphemeral nature as intermediates in chemical reactions, e.g., combustion and atmospheric processes. For example, the reaction OH + CO {yields} H + CO{sub 2} is the dominant source of CO{sub 2} in the oxidation of hydrocarbons and is important in the chemistry of the upper atmosphere and in the formation of chemical smog. This reaction is believed to proceed through a COOH intermediate, but until very recently, this species had not been detected in the gas phase. The depletion of ozone in the stratosphere is attributed to the reaction of the free radical ClO with O{sub 3}. The chemical vapor deposition method of producing solid-state devices based on Si or Ge employs a plasma containing free-radical hydrides of Si and Ge. We have shown that the direct application of eq 1 and 2 to determine the dimerization energy of BH{sub 3} by measuring its ionization potential and its appearance potential from B{sub 2}H{sub 6} is inappropriate for this system. However, a rather lengthy analysis of the data leads to a dimerization energy of 34.3-39.1 kcal/mol, in good agreement with ab initio calculations. Preliminary results frommore » our B{sub 2}H{sub 5} and B{sub 2}H{sub 4} studies indicate that D{sub 0}(B{sub 2}H{sub 5}-H) {le} 102.7 kcal/mol, but D{sub 0}(B{sub 2}H{sub 4}-H) is much weaker, {approx} 40.1 kcal/mol.« less

Study of the reaction chlorine monoxide + methyl peroxide .fwdarw. products at 300 K

The first experimental study of the reaction ClO + CH{sub 3}O{sub 2} {yields} products (1) is reported. The rate coefficient k{sub 1} and the product distributions were investigated in the modulated photolysis of Cl{sub 2}/CH{sub 4}/Cl{sub 2}O/O{sub 2} mixtures. Reactants and products have been monitored by using UV-vis and Fourier-transform infrared gas-phase absorption spectroscopies. The value of k{sub 1} was found to be equal to (3.1 {plus minus} 1.7) {times} 10{sup {minus}12} cm{sup 3} molecule{sup {minus}1} s{sup {minus}1} at 300 K and 240 Torr total pressure. The product channel forming ClOO and CH{sub 3}O appears to be the dominant pathway at 300 K. The subsequent oxidation of CH{sub 3}O to CH{sub 2}O and its partial photolysis into H and HCO leads to enhanced production of odd hydrogen (OH, HO{sub 2}) radicals. Consequently, high concentrations of OH radicals can build up, leading to an additional source of active chlorine via the reaction HCl + OH {yields} Cl + H{sub 2}O, which destroys ozone by catalytic reactions. This strongly suggests that reaction 1 can play a substantial role in the photochemical processes that take place under stratospheric ozone hole conditions.

Rate constants, branching fractions, and energy disposal for the H+ClO and H+SF reactions

The H+ClO and SF reactions have been isolated and studied by infrared chemiluminescence in a fast flow reactor. The OH product channel is favored over the HCl channel by a factor of 4.5 and the total rate constant is (7.7±1.9)×10−11 cm3 s−1 for the H+ClO reaction. Both sets of products are accessed from a bound singlet intermediate with HCl+O(3 P) formed by a singlet–triplet surface crossing in the exit channel; the energy disposal is 〈fV (OH)〉=0.45 and 〈fV (HCl)〉=0.31. The H+SF reaction gives only HF+S(3P), but the energy disposal differs dramatically from the HCl channel of the ClO reaction. This difference arises from changes in the thermochemistry, which result in an earlier crossing to the HSF triplet surface followed by release of repulsive energy as the HF separates from the S(3P) atom.

Infrared chemiluminescence studies of H atom reactions with Cl2O, ClNO, F2O, CF3OF, ClO2, NO2, and ClO

Infrared chemiluminescence from the HF, HCl, and OH products has been used to measure the nascent vibrational distributions and the rate constants at 300 K for the title reactions in a fast flow reactor. The ClO reaction was observed as a secondary step in the ClO2 and Cl2O systems. The reactions proceeding by a direct mechanism, Cl2O, ClNO, OF2, and CF3OF release relatively small fractions, ∼0.37, of the available energy as HCl or HF vibrational energy with narrow distributions, in accord with the dynamics associated with the H atoms on a repulsive potential surface. The ClO2 and NO2 reactions, which proceed by short lived intermediates, release a larger fraction of vibrational energy to OH and with broad distributions. Although the data for the ClO reaction are not definitive, the OH formation channel is the more important by a factor of 4–5. The NO2 reaction was studied in direct comparison with Cl2 to choose the best Einstein coefficients of OH by comparing the OH and HCl formation rate constants.

Pulsed laser photolysis kinetics study of the O(3P)+ClO reaction

A pulsed laser photolysis technique has been employed to study the kinetics of the important stratospheric reaction O+ClO→k1Cl+O2 in N2 buffer gas over the temperature and pressure ranges 231–367 K and 25–500 Torr. 351 nm pulsed laser photolysis of Cl2/O3/N2 mixtures produced Cl atoms in excess over O3. After a delay sufficient for the reaction Cl+O3→ClO+O2 to go to completion, a small fraction of the ClO was photolyzed at 266 nm to produce O(3P). The decay of O(3P) in the presence of an excess, known concentration of ClO was then followed by time-resolved resonance fluorescence spectroscopy. We find that k1 is independent of pressure, but that k1(T) increases with decreasing temperature. Our results suggest that the Arrhenius expression k1(T)=(1.55±0.33)×10−11 exp{(263±60)/T} cm3 molecule−1 s−1 is appropriate for modeling stratospheric chemistry. Errors in the Arrhenius expression are 2σ and represent precison only. The absolute accuracy of k1 at any temperature within the range studied is estimated to be ±20%. Our results agree with other recent measurements of k1 at 298 K but give significantly faster rate coefficients at stratospheric temperatures. A few measurements of the rate coefficient for the reaction ClO+ClO→k7products were also carried out. These measurements were necessary to assess the time dependence of [ClO].

Kinetics and product studies of the BrO + ClO Reaction: Implications for Antarctic chemistry

The reaction of ClO with BrO has been investigated by two independent techniques, discharge flow‐mass spectrometry and flash photolysis‐UV spectrometry, over the temperature range 220‐400 K and the pressure range 1‐760 torr. Rate constants have been determined for three product channels; a) Br + ClOO, b) Br + OClO, and c) BrCl + O2. The rate constants for the overall reaction and each reaction branch were found to be inversely dependent on temperature and independent of pressure. The results for the temperature dependence of the overall rate constant from the discharge flow and flash photolysis studies are in excellent agreement, and collectively disagree substantially with the only previous temperature dependence study. Also, in contrast to previous studies, the channel forming BrCl is found to be significant (≃ 8%). These kinetic measurements have an important impact on the modeling of Antarctic chemistry; for temperatures found in the Antarctic stratosphere the rate coefficients for the channels yielding ClOO and OClO are a factor of 2‐3 larger than previously estimated. In addition, the BrCl channel, which has an impact on the nighttime partitioning of BrOX and the diurnal variability of OClO, has been omitted from previous atmospheric models.

ChemInform Abstract: Temperature Dependence of the Rate Constant and Product Channels for the BrO + ClO Reaction.

AbstractThe rate constant of the title reaction is measured over the temp. range 241‐408 K under pseudo‐first‐order conditions, ((ClO) ≫ (BrO)), using discharge‐flow mass spectrometry.

Chemistry of OClO in the Antarctic stratosphere: Implications for bromine

Observations of OClO over Antarctica in the spring of 1986 (Solomon et al., 1987a, J. geopihys. Res. 92, 8329) are used to place constraints on the abundance of stratospheric bromine. Production of OClO is thought to be due mainly to reaction of ClO with BrO. The observed diurnal variation is consistent with this source only if a fraction of the overall ClO+BrO reaction, between 3.5% and 14%, is assumed to result in production of BrCl. Otherwise the observed diurnal variation of OClO implies an additional source in twilight. The analysis suggests a concentration of stratospheric bromine of 16±4 pptv, consistent with independent measurements of gaseous bromine which, despite major uncertainty, indicate a range from about 5 to 20 pptv.