The self-reaction of hydroperoxyl radicals: ab initio characterization of dimer structures and reaction mechanisms

Abstract

The global potential energy surfaces of singlet and triplet H ₂ O ₄ systems have been searched at the B3LYP/6-311G(d, p) level of theory; their relative energies have been calculated at the G2M(CC5)// B3LYP/6-311G(d, p) level. The results show that the most stable intermediate out of the 11 open-chain and cyclic dimers of HO ₂ is the singlet HO ₄ H chain-structure with C ₁ symmetry which lies 19.1 kcal mol ^–1 below the reactants. The transition states for the production of H ₂ O ₂  + O ₂ (singlet and triplet), H ₂ O + O ₃ and H ₂  + 2O ₂ have been calculated at the same level of theory. The results show that the most favored product channel, producing H ₂ O ₂  +  ³ O ₂ , occurs by the formation of a triplet six-member-ring intermediate through head-to-tail association with a dual hydrogen-bonding energy of 9.5 kcal mol ^–1 . The intermediate fragments to give H ₂ O ₂  +  ³ O ₂ via a transition state, which lies below the reactants by about 0.5 kcal mol ^–1 . There are four channels over the singlet surface which can produce ¹ O ₂ ; all the transition states associated with these channels lie above the reactants by 2.8–5.6 kcal mol ^–1 at the G2M level. Similarly, the O ₃ and H ₂ formation channels also occur over the singlet surface with high energy barriers, 5.2 and 74.2 kcal mol ^–1 , respectively; their formation is kinetically unimportant.