Reaction of Aniline with Singlet Oxygen (O₂¹Δ_g)

Dissolved organic matter (DOM) acts as an effective photochemical sensitizer that produces the singlet delta state of molecular oxygen (O₂¹Δ_g), a powerful oxidizer that removes aniline from aqueous solutions. However, the exact mode of this reaction, the p- to o-iminobenzoquinone ratio, and the selectivity of one over the other remain largely speculative. This contribution resolves these uncertainties. We report, for the first time, a comprehensive mechanistic and kinetic account of the oxidation of aniline with the singlet delta oxygen using B3LYP and M06 functionals in both gas and aqueous phases. Reaction mechanisms have been mapped out at E, H, and G scales. The 1,4-cycloaddition of O₂¹Δ_g to aniline forms a 1,4-peroxide intermediate (M1), which isomerizes via a closed-shell mechanism to generate a p-iminobenzoquinone molecule. On the other hand, the O₂¹Δ_g ene-type reaction forms an o-iminobenzoquinone product when the hydroperoxyl bond breaks, splitting hydroxyl from the 1,2-hydroperoxide (M3) moiety. The gas-phase model predicts the formation of both p- and o-iminobenzoquinones. In the latter model, the M1 adduct displays the selectivity of up to 96%. A water-solvation model predicts that M1 decomposes further, forming only p-iminobenzoquinone with a rate constant of k = 1.85 × 10⁹ (L/(mol s)) at T = 313 K. These results corroborate the recent experimental findings of product concentration profile in which p-iminobenzoquinonine represents the only detected product.