The gas phase reaction of singlet dioxygen with water: A water-catalyzed mechanism

Abstract

Abs efficiently catalyze the conversion of molecular singlet oxygen ( 1 O2) plus water to hydrogen peroxide (HOOH), we used quantum chemical methods (B3LYP density functional theory) to delineate the most plausible mechanisms for the observed efficient conversion of water to HOOH. We find two reasonable pathways. In Pathway I, (i) H2O catalyzes the reaction of 1 O2 with a second water to form HOOOH; (ii) two HOOOH form a dimer, which rearranges to form the HOO-HOOO ؉ H2O complex; (iii) HOO-HOOO rearranges to HOOH-OOO, which subsequently reacts with H2O to form H2O4 ؉ HOOH; and (iv) H2O4 rearranges to the cyclic dimer (HO2)2, which in turn forms HOOH plus 1 O2 or 3 O2. Pathway II differs in that step ii is replaced with the reaction between HOOOH and 1 O2, leading to the formation of HOO-HOOO. This then proceeds to similar products. For a system with 18 The results lead to plausible mechanisms in good agreement with the Scripps isotope experiments. The Discussion talks about these results, suggesting that these mechanisms account for the Scripps Ab results and they may also be significant for understanding related processes in biochemistry, combustion, explosions, atmospheric chemistry, and radiation chemistry of aqueous systems. Computational DetailsAll QM calculations use the Becke three-parameter hybrid functional with Lee-Yang-Parr correlation functional (B3LYP) flavor of density functional theory (3-7), which includes a generalized gradient approximation and some exact exchange. The 6-31G** basis set (8, 9) was used on all atoms for a full geometry optimization. Vibrational frequencies (from the analytic Hessian) were calculated to ensure that each minimum is a true local minimum (containing only positive frequencies) and that each transition state has only a single imaginary frequency (negative eigenvalue of the Hessian). All QM calculations were carried out with JAGUAR (10-12). Some of these energetics were included in ref. 2.To obtain more accurate energetics, we carried out calculations with the cc-pVTZ basis set (13) by using the optimized geometries from the 6-31G** basis. Such QM calculations lead to an accuracy of around 3 kcal͞mol for simple organic molecules (14).For molecules such as 1 O 2 and O 3 , which have significant open shell character, standard density functional theory methods often lead to much larger errors. Thus, with B3LYP, the singlet and triplet gap for O 2 is ⌬E ( 1 ⌬ g Ϫ 3 ⌺ g Ϫ ) ϭ 10.4 kcal͞mol, in poor agreement with the experimental value of 22.5 kcal͞mol (15). Consequently, we used spin projection techniques (16) to ensure a proper description of the complexes involving 1 O 2 . This leads to ⌬E ( 1 ⌬ g Ϫ 3 ⌺ g Ϫ ) ϭ 20.5 kcal͞mol, in good agreement with experiment.The calculated vibrational frequencies (with no empirical scaling) were used to calculate the zero point energy and temperature corrections so that all energetics are reported for ⌬H (298K), in kcal͞mol. QM Calculations and Plausible MechanismsFormation of H2O2 from the Reaction Between Two H2O3. First, we examined...

Peroxone chemistry: Formation of H 2 O 3 and ring-(HO 2 )(HO 3 ) from O 3 /H 2 O 2

“…The HO and HO 2 formed here can go through reaction 9, leading to the formation of H 2 O ϩ 3 O 2 . We also find that H 2 O can catalyze the decomposition of HOOOH to 1 O 2 (4,22). Thus (30) and in the pulse radiolysis of air-saturated perchloric acid solution (23).…”

“…By means of 17 O NMR spectroscopy, Plesnicar and coworkers (32) concluded that the low-temperature ozonation of isopropyl alcohol and isopropyl methyl ether yields hydrogen trioxide H 2 O 3 (in addition to the hydrotrioxides of these compounds). Recently the antibody-catalyzed oxidation of water to hydrogen peroxide by 1 O 2 has been proposed to proceed through H 2 O 3 as the critical intermediate (3)(4)(5). This situation has changed with the recent synthesis of H 2 O 3 in argon matrices by the photolysis of the ozone-hydrogen peroxide complex (2), and we have now clarified the mechanism by using first-principles quantum mechanics.…”

An Evaluation of Gas Phase Enthalpies of Formation for Hydrogen-Oxygen (HxOy) Species

“…Su et al (2002) [229] calculated the cis form to be 10.0 kJ mol −1 higher a barrier-to-isomerization of 21.8 kJ mol −1 . We note that the first quantum chemical calculations (MP2) of the rotational potential of this molecule were done by Cremer (1978) [228] and predicted the cis form to be about 15.9 kJ mol −1 less stable and that there was a very high barrier-toisomerization of 46.4 kJ mol −1 .…”

Kinetics of Nonbranched-Chain Processes of the Free-Radical Addition to Molecules of Alkenes, Formaldehyde, and Oxygen with Competing Reactions of Resulting 1:1 Adduct Radicals with Saturated and Unsaturated Components of the Binary Reaction System

“…Reactions 6 and 7 (taking into account the principle of detailed balance for the various pathways) regenerate hydrogen and oxygen [88]. 12 Ozone does not interact with molecular hydrogen.…”