Energetics and molecular dynamics of the reaction of HOCO with HO(2) radicals.

The energetics of the reaction of HOCO with HO(2) have been studied using the quadratic configuration interaction with single and double excitations (QCISD(T)) method and a large basis set on the singlet and triplet potential energy surfaces of the system. The results show that the ground-state O(2)+HOC(O)H products can be produced by a direct hydrogen abstraction via a transition state with a small barrier (1.66 kcal/mol) on the lowest triplet surface. A similar hydrogen abstraction can occur on the singlet electronic surface, but it leads to the singlet O(2)(a(1)Delta) and HOC(O)H. On the singlet surface, a new stable intermediate, HOC(O)OOH, hydroperoxyformic acid, has been found. This intermediate is formed by the direct addition of the terminal oxygen atom in HO(2) onto the carbon atom in HOCO in a barrierless reaction. The HOC(O)OOH intermediate may dissociate into either the CO(2)+H(2)O(2) or CO(3)+H(2)O products through elimination reactions with four-center transition states, or into HOC(O)O+OH through an O-O bond cleavage. The heat of formation of HOC(O)OOH is predicted to be -118.9+/-1.0 kcal/mol. In addition, the dynamics of the HO(2)+HOCO reaction have been investigated using a scaling-all correlation couple cluster method with single and double excitation terms (CCSD) on the singlet potential energy surface. Reaction mechanisms have been studied in detail. It was found that the direct and addition reaction mechanisms coexist. For the addition mechanism, the lifetime of the HOC(O)OOH intermediate is predicted to be 880+/-27 fs. At room temperature, the calculated thermal rate coefficient is (6.52+/-0.44)x10(-11) cm(3) molecule(-1) s(-1) with the product branching fractions: 0.77 (CO(2)+H(2)O(2)), 0.15 (HOC(O)O+OH), 0.056 (CO(3)+H(2)O), 0.019 (O(2)(a(1)Delta)+HOC(O)H), and 0.01 (O(2)(X (3)Sigma)+HOC(O)H).