Kinetic Study of the ClOO + NO Reaction Using Cavity Ring-Down Spectroscopy

Theoretical Study of ClOO + NO Reaction: Mechanism and Kinetics

Theoretical investigations are performed on mechanism and kinetics of the reaction of halogen peroxy radical ClOO with NO radical. The electronic structure information for both of the singlet and triplet potential energy surfaces (PESs) is obtained at the MP2/6-311 + G(2df) level of theory, and the single-point energies are refined by the CCSD(T)/6-311 + G(2df) level. The rate constants for various product channels of the reaction in the pressure range of 1-7600 Torr are predicted. The main results are as follows: On the singlet surface, the addition-elimination mechanism is the most important. First, the N atom of the NO radical can attack the O atom of the ClOO radical to form an energy-riched intermediate IM1 ClOONOtp (21.3 kcal/mol) barrierlessly, then IM1 could isomerizes to IM2 ClOONOcp (22.1 kcal/mol) via a low energy barrier. Both IM1 and IM2 can dissociate to the primary product P₁ ClNO + ¹O₂ and the secondary product P₂ ClO + NO₂. On the triplet surface, the direct Cl-abstraction reaction is the most feasible pathway. The Cl-abstraction can take place via a van der Waals complex, ³IM1 ONClOO (4.1 kcal/mol), then it fragments readily to give P₁' ClNO + ³O₂ with a small barrier. The kinetic calculations show that at low temperatures, the singlet bimolecular product P₁ is the primary product, while at high temperatures, the triplet product P₁' becomes the primary one; only at high pressures and low temperatures, the unimolecular products IM1 and IM2 can be found with quite small yields. At experimentally measured temperature 213 K, ClNO is the primary product in the whole pressure range, which is consistent with the previous experiment. The present study may be useful for further experimental studies for the title reaction.

Radiation-Driven Formation of Reactive Oxygen Species in Oxychlorine-Containing Mars Surface Analogues

The present study demonstrates that γ-radiolyzed perchlorate-containing Mars soil salt analogues (in a CO₂ atmosphere) generate upon H₂O wetting the reactive oxygen species (ROS) superoxide radical (O₂^•-), hydrogen peroxide (H₂O₂), and hydroxyl radicals (^•OH). This study also validates that analogue radiolysis forms oxychlorine species that, in turn, can UV-photolyze to ^•OH upon UV photolysis. This investigation was made possible by the development of a new assay for inorganic-origin O₂^•- and H₂O₂ determination and by the modification of a previous assay for soil ^•OH. Results show that radiolyzed Mg(ClO₄)₂ generates H₂O₂ and ^•OH; and when included as part of a mixture analogous to the salt composition of samples analyzed at the Mars Phoenix site, the analogue generated O₂^•-, H₂O₂, and ^•OH, with ^•OH levels 150-fold higher than in the radiolyzed Mg(ClO₄)₂ samples. Radiolyzed Mars Phoenix site salt analogue that did not contain Mg(ClO₄)₂ generated only ^•OH also at 150-fold higher concentration than Mg(ClO₄)₂ alone. Additionally, UV photolysis of the perchlorate γ radiolysis product chlorite (ClO₂^-) generated the oxychlorine products trihalide (Cl₃^-), chlorine dioxide (ClO₂^•), and hypochlorite (ClO^-), with the formation of ^•OH by UV photolysis of ClO^-. While the generation of ROS may have contributed in part to ¹⁴CO₂ production in the Viking Labeled Release (LR) experiment and O₂ (g) release in the Viking Gas Exchange (GEx) experiment, our results indicate that they are not likely to be the major contributor to the LR and GEx results. However, due to their highly reactive nature, they are expected to play a significant role in the alteration of organics on Mars. Additionally, experiments with hypochlorite show that the thermal stability of NaClO is in the range of the thermal stability observed for thermally liable oxidant responsible for the Viking LR results. Key Words: Mars-Oxygen-Salts-Radiation-Habitability. Astrobiology 17, 319-336.

A Gas-Phase Kinetic Study of the Reaction between Bromine Monoxide and Methylperoxy Radicals at Atmospheric Temperatures

The rate constant of the reaction of BrO with CH(3)O(2) was determined to be k1 = (6.2 +/- 2.5) x 10(-12) cm3 molecule(-1) s(-1) at 298 K and 100-200 Torr of O2 diluent. Quoted uncertainty was two standard deviations. No significant pressure dependence of the rate constants was observed at 100-200 Torr total pressure of N2 or O2 diluents. Temperature dependence of the rate constants was further investigated over the range 233-333 K, and an Arrhenius type expression was obtained for k1 = 4.6 x 10(-13) exp[(798 +/- 76)/T] cm3 molecule(-1) s(-1). The product branching ratios were evaluated and the atmospheric implications were discussed.

Direct observation and reactions of Cl3 radical

The broad absorption of Cl3 radical was observed between 1150 and 1350 nm using cavity ring-down spectroscopy at 213-265 K and 50-200 Torr with He, N2, Ar, or SF6 diluents. The absorption intensity of Cl3 increased at lower temperature and higher pressure. SF6 was the most efficient diluent gas. The temperature dependent equilibrium constants for Cl3 formation from Cl+Cl2 were theoretically calculated at the MP4SDQ6-311+G(d) level. Observed decay time profiles of Cl3 and the pressure dependence of Cl3 formation are explained by the equilibrium reaction and a decay reaction of Cl+Cl3.

Kinetic Study of IO Radical with RO2 (R = CH3, C2H5, and CF3) Using Cavity Ring-Down Spectroscopy

The reactions of iodine monoxide radical, IO, with alkyl peroxide radicals, RO(2) (R = CH(3), C(2)H(5), and CF(3)), have been studied using cavity ring-down spectroscopy. The rate constant of the reaction of IO with CH(3)O(2) was determined to be (7.0 +/- 3.0) x 10(-11) cm(3) molecule(-1) s(-1) at 298 K and 100 Torr of N(2) diluent. The quoted uncertainty is two standard deviations. No significant pressure dependence of the rate constant was observed at 30-130 Torr total pressure of N(2) diluent. The temperature dependence of the rate constants was also studied at 213-298 K. The upper limit of the branching ratio of OIO radical formation from IO + CH(3)O(2) was estimated to be <0.1. The reaction rate constants of IO + C(2)H(5)O(2) and IO + CF(3)O(2) were determined to be (14 +/- 6) x 10(-11) and (6.3 +/- 2.7) x 10(-11) cm(3) molecule(-1) s(-1) at 298 K, 100 Torr of N(2) diluent, respectively. The upper limit of the reaction rate constant of IO with CH(3)I was <4 x 10(-14) cm(3) molecule(-1) s(-1).