Stimulated by the recent surprising results from Wentworth et al. [Wentworth, A. D., Jones, L. H., Wentworth, P., Janda, K. D. & Lerner, R. A. (2000) Proc. Natl. Acad. Sci. USA 97, 10930–10935] that Abs efficiently catalyze the conversion of molecular singlet oxygen (1O2) 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 1O2 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 1O2 or 3O2. Pathway II differs in that step ii is replaced with the reaction between HOOOH and 1O2, leading to the formation of HOO-HOOO. This then proceeds to similar products. For a system with 18O H2O, Pathway I leads to a 2.2:1 ratio of 16O:18O in the product HOOH, whereas Pathway II leads to 3:1. These ratios are in good agreement with the 2.2:1 ratio observed in isotope experiments by Wentworth et al. These mechanisms lead to two HOOH per initial 1O2 or one, depending on whether the product of step iv is 1O2 or 3O2, in good agreement with the experimental result of 2.0. In addition to the Ab-induced reactions, the hydrogen polyoxides (H2O3 and H2O4) formed in these mechanisms and their decomposition product polyoxide radicals (HO2, HO3) may play a role in combustion, explosions, atmospheric chemistry, and the radiation chemistry in aqueous systems.

Periana et al. [Science 1998, 280, 560] previously reported two catalysts for lowtemperature methane activation to methanol: PtCl2(NH3)2 and PtCl2(bpym). It was shown that the ammine catalyst is much more active, but it decomposes rapidly in sulfuric acid to form a PtCl2 precipitate, while the bpym system is long-lived. To have a basis for developing new catalysts that would not decompose, we undertook a study of the structure, bonding, and stability of the PtCl2(NH3)2 and PtCl2(bpym) catalysts, using quantum mechanics (QM) [density functional theory (DFT) at the B3LYP/LACVP**(+) level] including solvation in sulfuric acid via the Poisson-Boltzmann continuum approximation. Critical results include the following: (1) The influence of a trans ligand Y on the Pt-X bond follows the order Cl->NH3(bpym)>OSO3H->0(empty site). Thus the Pt-N bond length is longer (up to 0.04 Å) and the Pt-N bond is weaker (up to 18 kcal/mol) when trans to a Cl-as compared to trans to OSO3H-. (2) The bpym ligand acts as both a ó-donor and a ð-acceptor. As bpym is protonated, the Pt-N bond strength decreases (by up to 51 kcal/mol). Thus, ¢H(soln, 453 K) for Pt(OSO3H)2(bpym) (69.6)> [Pt(OSO3H)2(bpymH)]+(49.4)>[Pt(OSO3H)2(bpymH2)]2+(18.7). (3) In sulfuric acid replacing the ammine ligands with bisulfate ligands is thermodynamically favorable [by△G(soln, 453 K)=-23 kcal/mol], whereas replacement of bpym with OSO3H- is unfavorable [by△G(soln, 453 K)=+16 kcal/mol]. (4) Replacement of chloride ligands with bisulfate ligands is thermodynamically unfavorable [by△G(soln, 453K)=～7kcal/mol for ammine and ～12 kcal/mol for bpym]. (5) Protonation of PtCl2(bpym) is thermodynamically favorable, leading to [PtCl2(bpymH)]+ as the stable species in sulfuric acid (by 8 kcal/mol). Thus we conclude that in hot concentrated sulfuric acid it is quite favorable for PtCl2(NH3)2 to lose its ammine ligands to form PtCl2, which in turn will dimerize and oligomerize, leading eventually to a (PtCl2)n precipitate and catalyst death. We find that PtCl2(bpym) is resistant to solvent attack, favoring retention of the bpym ligand in hot concentrated sulfuric acid. These results agree with experimental findings. The insights from these findings should help screen for stable new ligands in the design of new catalysts.

The recent observation [Wentworth, P., Jones, L. H., Wentworth, A. D., Zhu, X. Y., Larsen, N. A., Wilson, I. A., Xu, X., Goddard, W. A., Janda, K. D., Eschenmoser, A. & Lerner, R. A. (2001) Science 293, 1806–1811] that antibodies form H2O2 from 1O2 plus H2O was explained in terms of the formation of the H2O3 species that in the antibody reacts with a second H2O3 to form H2O2. There have been few reports of the chemistry for forming H2O3, but recently Engdahl and Nelander [Engdahl, A. & Nelander, B. (2002) Science 295, 482-483] reported that photolysis of the ozone-hydrogen peroxide complex in argon matrices leads to significant concentrations of H2O3. We report here the chemical mechanism for this process, determined by using first-principles quantum mechanics. We show that in an argon matrix it is favorable (3.5 kcal mol barrier) for H2O2 and O3 to form a [(HO2)(HO3)] hydrogen-bonded complex [head-to-tail sevenmembered ring (7r)]. In this complex, the barrier for forming H2O3 plus 3O2 is only 4.8 kcal mol, which should be observable by means of thermal processes (not yet reported). Irradiation of the [(HO2)(HO3)-7r] complex should break the HO–OO bond of the HO3 moiety, eliminating 3O2 and leading to [(HO2)(HO)]. This [(HO2)(HO)] confined in the matrix cage is expected to rearrange to also form H2O3 (observed experimentally). We show that these two processes can be distinguished isotopically. These results (including the predicted vibrational frequencies) suggest strategies for synthesizing H2O3 and characterizing its chemistry. We suggest that the [(HO2)(HO3)-7r] complex and H2O3 are involved in biological, atmospheric, and environmental oxidative processes.

We derive the form for an exact exchange energy density for a density decaying with Gaussian-like behavior at long range. Based on this, we develop the X3LYP (extended hybrid functional combined with Lee-Yang-Parr correlation functional) extended functional for density functional theory to significantly improve the accuracy for hydrogen-bonded and van der Waals complexes while also improving the accuracy in heats of formation, ionization potentials, electron affinities, and total atomic energies [over the most popular and accurate method, B3LYP (Becke three-parameter hybrid functional combined with Lee-Yang-Parr correlation functional)]. X3LYP also leads to a good description of dipole moments, polarizabilities, and accurate excitation energies from s to d orbitals for transition metal atoms and ions. We suggest that X3LYP will be useful for predicting ligand binding in proteins and DNA.

Based on density functional cluster model calculations, we present the first detailed mechanisms for the complete decomposition of NH3 to NHx(a) (x=0-2) on the Si(100)-(2