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Spin-Forbidden Branching in the Mechanism of the Intrinsic Haber-Weiss Reaction.

ABSTRACT: The mechanism of the O2?- and H2O2 reaction (Haber-Weiss) under solvent-free conditions has been characterized at the DFT and CCSD(T) level of theory to account for the ease of this reaction in the gas phase and the formation of two different set of products (Blanksby et?al., Angew. Chem. Int. Ed. 2007, 46, 4948). The reaction is shown to proceed through an electron-transfer process from the superoxide anion to hydrogen peroxide, along two pathways. While the O3?- + H2O products are formed from a spin-allowed reaction (on the doublet surface), the preferred products, O?-(H2O)+3O2, are formed through a spin-forbidden reaction as a result of a favorable crossing point between the doublet and quartet surface. Plausible reasons for the preference toward the latter set are given in terms of the characteristics of the minimum energy crossing point (MECP) and the stability of an intermediate formed (after the MECP) in the quartet surface. These unique results show that these two pathways are associated with a bifurcation, yielding spin-dependent products.

SUBMITTER: Leitao EFV

PROVIDER: S-EPMC5474656 | biostudies-other | 2017 Jun

REPOSITORIES: biostudies-other

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Spin-Forbidden Branching in the Mechanism of the Intrinsic Haber-Weiss Reaction.

Leitão Ezequiel F V EFV Ventura Elizete E de Souza Miguel A F MAF Riveros José M JM do Monte Silmar A SA

ChemistryOpen 20170323 3

The mechanism of the O2⋅- and H2O2 reaction (Haber-Weiss) under solvent-free conditions has been characterized at the DFT and CCSD(T) level of theory to account for the ease of this reaction in the gas phase and the formation of two different set of products (Blanksby et al., Angew. Chem. Int. Ed. 2007, 46, 4948). The reaction is shown to proceed through an electron-transfer process from the superoxide anion to hydrogen peroxide, a ...[more]

PMID: 28638768

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Project description:We report REMPI spectra and velocity-mapped ion images of the O2(a1Δg) and (b1Σg+) fragments arising from the spin-forbidden photodissociation of O3 near 320 and 330 nm. The O2(a1Δg, v = 0) REMPI spectrum following a 320 nm dissociation shows enhanced peak intensity for the odd rotational states relative to the even states, which is the opposite of the trend observed by Gunthardt et al. ( J. Chem. Phys. 2019, 151, 224302) for spin-allowed dissociation at 266 nm but is consistent with the couplings between the B state and 3A' and 3A″ states calculated by Grebenshchikov and Rosenwaks ( J. Phys. Chem. A 2010, 114, 9809-9819). There are no significant differences between the ion image angular distributions of fragments in odd and even rotational states, which indicates a cold distribution of O3 and supports the explanation that the alternation in peak intensities results from a difference in the couplings. Quantitative analysis of the image angular distributions was limited due to the single laser polarization geometry accessible in one-color experiments. Radial distributions of the 320 nm images indicate a broad rotational distribution, evidenced in bimodal speed distributions with peaks corresponding to both high (j = 35-43) and low (j = 17-20) rotational states. The REMPI spectrum of O2(a1Δg) near 330 nm was collected, and while quantitative population analysis is difficult because of the perturbed resonant state, the spectrum clearly supports a broad rotational distribution as well, consistent with the images collected at 320 nm. A 2D-REMPI spectrum was collected following dissociation of O3 near 330 nm, which showed evidence of contributions from O2 fragments in both the a1Δg and b1Σg+ states. The rotational distribution for the O2(b1Σg+, v = 0) product peaks at j = 32 and is narrower than that of the O2(a1Δg) fragment, consistent with distributions reported by O'Keeffe et al. at longer dissociation wavelengths ( J. Chem. Phys. 2002, 117, 8705-8709). At smaller radii in the 2D-REMPI spectrum, there is additional signal assigned to v = 1-4 of O2(b1Σg+), with rotational distributions similar to v = 0. The vibrational distribution of the O2(b1Σg+) fragment peaks at v = 0, with populations monotonically decreasing with increasing vibrational state. Ion image angular distributions of the O2(b1Σg+) fragment and the corresponding anisotropy parameters are also reported.

Dataset Information

Spin-Forbidden Branching in the Mechanism of the Intrinsic Haber-Weiss Reaction.

Publications

Spin-Forbidden Branching in the Mechanism of the Intrinsic Haber-Weiss Reaction.

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