An ab initio quantum chemical investigation of the structure and stability of ozone-water complexes

Can Ozone Dissociate at the Surface of Water (Water Droplet and Ice) without Light?

Ozone is a major source of OH radicals in the troposphere. It is well-known that photodissociation of ozone is key for the conversion of ozone into OH radicals. In the present study, using Born-Oppenheimer molecular dynamics simulation, we have shown that on the surface of the droplet and ice, ozone can dissociate without light. In addition, the dissociation time of ozone is found to be much less on the ice surface than the same time on the water droplet. As the dissociation of ozone on the water surface can happen during the day as well as in the night time, we believe this route of forming OH radicals can be even more important than the photodissociation. The present study suggests that the cloud and ice surface can enhance the oxidizing power of the troposphere.

Effect of (H2O)n (n = 1 and 2) on HOCl + Cl reaction

In the present work, we investigate the effect of water molecules (H₂O and (H₂O)₂) on HOCl + Cl˙ → ClO˙ + HCl (R1), and HOCl + Cl˙ → OH˙ + Cl₂ (R2) reactions using quantum chemical and kinetics calculations. The present investigation suggests that a water molecule decreases the energy barrier of both reactions significantly, compared to uncatalyzed reaction. However, the effective rate constants for the water catalyzed path for both channels (R1 and R2) were found to be lower than the bimolecular rate constant of the uncatalyzed path. Further, it was found that the R2 reaction will dominate over the R1 reaction, with or without catalyst. Interestingly, the uncatalyzed title reaction was found to be two times faster than the HOCl + OH˙ reaction, but in the presence of water, HOCl + OH˙ becomes the dominant reaction compared to the HOCl + Cl˙ reaction in the atmosphere. In addition, the concentration of bimolecular complexes formed in the presence of a catalyst are found to be higher than the precursor molecule of the uncatalyzed reaction, which suggests that in the presence of catalyst, the HOCl + Cl˙ reaction would favor the catalyzed path rather than the uncatalyzed path.

Effect of water on the oxidation of CO by a Criegee intermediate

The present work employs the CCSD(T)/CBS//M06-2X/aug-cc-pVTZ level of theory to investigate the effect of a water monomer and dimer on the oxidation of carbon-monoxide by a Criegee intermediate (CH2OO). The present work suggests that in the presence of a water monomer the energy barrier of the title reaction reduced to ∼3.4 kcal mol-1 from the corresponding uncatalyzed barrier (∼12.4 kcal mol-1), whereas, in the presence of a water dimer it became as low as ∼-3.2 kcal mol-1. It has also been found that, in the presence of catalysts, additional channels become available from which the title reaction can proceed. The estimated values of rate constants suggest that within the temperature range of 210-320 K, the effective bimolecular rate constant for the water monomer catalyzed channel is 10 to 100 times lower than the bimolecular rate constant of the uncatalyzed channel, whereas in the case of the water dimer it is ∼5-10 times higher than that of the uncatalyzed channel.

Photochemistry of the ozone-water complex in cryogenic neon, argon, and krypton matrixes

The photochemistry of ozone-water complexes and the wavelength dependence of the reactions were studied by matrix isolation FTIR spectrometry in neon, argon, and krypton matrixes. Hydrogen peroxide was formed upon the irradiation of UV light below 355 nm. Quantitative analyses of the reactant and product were performed to evaluate the matrix cage effect of the photoreaction. In argon and krypton matrixes, a bimolecular O((1)D) + H2O → H2O2 reaction was found to occur to form hydrogen peroxide, where the O((1)D) atom generated by the photolysis of ozone diffused in the cryogenic solids to encounter water. In a neon matrix, hydrogen peroxide was generated through intracage photoreaction of the ozone-water complex, indicating that a neon matrix medium is most appropriate to study the photochemistry of the ozone-water complex.