Spin‐Forbidden Branching in the Mechanism of the Intrinsic Haber–Weiss Reaction

The mechanism of the O(2) (⋅−) and H(2)O(2) 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 O(3) (⋅−) + H(2)O products are formed from a spin‐allowed reaction (on the doublet surface), the preferred products, O(⋅−)(H(2)O)+(3)O(2), 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.