Energetics and molecular dynamics of the reaction of HOCO with HO2 radicals

The energetics of the reaction of HOCO with HO2 have been studied using the QCISD(T) method and a large basis set on the singlet and triplet PES of the system. The results show that the ground state O2+HOC(O)H products can be produced by a direct hydrogen abstraction via a TS 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 Delta O2 and HOC(O)H. On ther 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 HO2 onto the carbon atom in HOCO in a barrierless reaction: The HOC(O)OOH intermediate may dissociate intro either the CO2 + H2O2 or CO3 + H2O products through elimination reactions with four-center TSs, 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 (estimated error 1.0 kcal/mol) In addition, the dynamics of the HO2+HOCO reaction have been investigated using a scaling-all correlation coupled cluster method CCSD on the singlet PES. 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 fs (estimated error 27 fs) . At room T, the calculated product branching fractions are: 0.77 (CO2+H2O2), 0.15 (HOC(O)O+OH), 0.056 (COP3+H2O) 0.019 (first excited delta singlet O2+HoC(O)H) and 0.01 (ground state triplet O2+HOC(O)H)