On the Dissociation of Ground State trans-HOOO Radical: A Theoretical Study

The hydrotrioxyl radical (HOOO center dot) plays a crucial role in atmospheric processes involving the hydroxyl radical (HO center dot) and molecular oxygen (O-2). The equilibrium geometry of the electronic ground state (X (2)A '') of the trans conformer of HOOO center dot and its unimolecular dissociation into HO center dot (X (2)Pi) and O-2 (X (3)Sigma(-)(g)) have been studied theoretically using CASSCF and CASPT2 methodologies with the aug-cc-pVTZ basis set. On the one hand, CASSCF(19,15) calculations predict for trans-HOOO center dot (X (2)A '') an equilibrium structure showing a central O-O bond length of 1.674 angstrom and give a classical dissociation energy D-e = 1.1 kcal/mol. At this level of theory, it is found that the dissociation proceeds through a transition structure involving a low energy barrier of 1.5 kcal/mol. On the other hand, CASPT2(19,15) calculations predict for trans-HOOO center dot (X (2)A '') a central O-O bond length of 1.682 angstrom, which is in excellent agreement with the experimental value of 1.688 angstrom, and give D-e = 5.8 kcal/mol. Inclusion of the zero-point energy correction (determined from CASSCF(19,15)/aug-cc-pVTZ harmonic vibrational frequencies) in this D-e leads to a dissociation energy at 0 K of D-0 = 3.0 kcal/mol. This value of D-0 is in excellent agreement with the recent experimentally determined D-0 = 2.9 +/- 0.1 kcal/mol of Le Picard et al. (Science 2010, 328, 1258-1262). At the CASPT2 level of theory, we did not find for the dissociation of trans-HOOO center dot (X (2)A '') an energetic barrier other than that imposed by the endoergicity of the reaction. This prediction is in accordance with the experimental findings of Le Picard et al., indicating that the reaction of HO center dot with O-2 yielding HOOO center dot is a barrierless association process.