Chlorine-catalyzed ozone destruction: Cl atom production from ClOOCl photolysis

New Potential Energy Surface for the H + Cl2 Reaction and Quantum Dynamics Studies

The reaction of H + Cl2 → HCl + Cl plays a crucial role in various fields. However, no previous study has investigated this reaction using accurate quantum mechanical methods. In this paper, we construct a global potential energy surface (PES) using the neural network method with more than 20,000 ab initio energies obtained by the MRCI-F12+Q method with the aug-cc-pV5Z basis and extrapolated to the complete basis set limit. The spin-orbit coupling of the Cl atom has been considered in the PES. With this new PES, product state-resolved quantum dynamics calculations for the H + Cl2 (v0 = 0, j0 = 0-2) → HCl + Cl reaction was carried out. Numerical results show that the initial rotational excitation of the Cl2 has negligible effects on the reactivity. Product state-resolved integral cross sections (ICS) and rate constants reveal that the HCl is most favorably produced in its v' = 2 vibrational state. The calculated product vibrational state-resolved and total reaction rate constants suggest that the new global PES is accurate enough, as compared with the available experimental measurements.

Method for the production of a compact source of atomic line spectra in the vacuum ultraviolet

Atomic emission spectra provide a means to identify and to gain insight into the electronic structure of emitting or absorbing matter. Detailed procedures are provided for the construction of low-pressure electrodeless discharge lamps that yield targeted emission in the vacuum ultraviolet for the spectroscopic study of water vapor and halogen species aboard an array of airborne observation platforms in the upper atmosphere, as well as in laboratory environments. While specific to the production of Lyman-alpha, atomic chlorine, and atomic bromine emissions in this study, the configuration of the lamps and their interchangeability with respect to operation lend these procedures to constructing sources engaging a wide selection of atomic and molecular spectra with straightforward modifications. The features and limitations of each type of lamp are discussed, as well as methods to improve spectral purity and factors affecting operational lifetime.

Product State-Resolved Reactive Scattering Studies of the H + Cl2 (v0 = 1-3, j0 = 0) → HCl + Cl Reaction by the Time-Dependent Wave Packet Method

The typical hydrogen atom plus halogen molecule reaction H + Cl2 → HCl + Cl has implications across many fields. In this paper, product state-resolved quantum dynamics calculations for the vibrationally excited reaction H + Cl2 (v0 = 1-3, j0 = 0) → HCl + Cl were conducted using the time-dependent wave packet method on a newly developed accurate potential energy surface. Numerical results indicate that the initial vibrational excitation of Cl2 does enhance the reactivity for this early barrier reaction, although less than the enhancement of the translational energy. The calculated product vibrational state-resolved integral cross sections and rate constants reveal that the product vibrational state distribution and the initial vibrational state of Cl2 are highly correlated. The thermal rate constant in the temperature range from 100 to 1000 K was given and is found to be in reasonable agreement with the experimental measurements.

Dichlorine peroxide (ClOOCl), chloryl chloride (ClCl(O)O) and chlorine chlorite (ClOClO): very accurateab initiostructures and actinic degradation

The structural parameters of the three most stable isomers with formula Cl₂O₂, dichlorine peroxide, chloryl chloride and chlorine chlorite, were determined by high-levelab initiotheory. The photodissociation pathways were investigated.

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.

Kinetic and thermochemical studies of the ClO + ClO + M <=> Cl(2)O(2) + M reaction

Recent work by von Hobe et al. [Atmos. Chem. Phys., 2007, 7, 3055] has highlighted significant inconsistencies between laboratory results, theoretical calculations and field observations concerning the ClO dimer ozone destruction cycle. This work investigates the temperature dependence of the equilibrium constant of one of the key reactions in this cycle, ClO + ClO + M <=> Cl(2)O(2) + M (1, -1), by means of laser flash photolysis and time-resolved UV absorption spectroscopy. ClO radicals were generated via laser flash photolysis of Cl(2)/Cl(2)O mixtures in synthetic air. Radicals were monitored via UV absorption spectroscopy: the use of a charge coupled device (CCD) detector allowed time resolution over a broad range of wavelengths giving unequivocal concentrations of radicals. The equilibrium constant K(eq) was determined as the ratio of the rate constants of the forward and reverse over the temperature range T = 256.55-312.65 K. Second Law and Third Law thermodynamic methods were employed to determine the standard enthalpy and entropy changes of , Δ(r)H° and Δ(r)S°, from the measured equilibrium constants. The values obtained from Second Law analysis were Δ(r)H° = - 80.7 ± 2.2 kJ mol(-1) and Δ(r)S° = -168.1 ± 7.8 J K(-1) mol(-1). Third Law analysis gave Δ(r)H° = -74.65 ± 0.4 kJ mol(-1) and Δ(r)S° = -148.0 ± 0.4 J K(-1) mol(-1). These values are in good agreement with previous work by Nickolaisen et al. [J. Phys. Chem., 1994, 98, 155] but greater in (negative) magnitude than current JPL-NASA recommendations [Sander et al., Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies, JPL Publication 06-2, NASA Jet Propulsion Laboratory, Pasadena, 2006 (interim update to this reference, 2009)]. The discrepancy between the Second and Third Law analyses also agrees with Nickolaisen et al., possibly indicating an aspect of the ClO recombination reaction not yet fully elucidated. The atmospheric implications of the results and their impact on the current understanding on polar ozone depletion are briefly discussed.