Ab initio study on the decomposition of first excited state HOOO radicals

Abstract

The HOOO radical plays a crucial role in atmospheric processes involving the OH radical and O₂ molecule. We present an ab initio molecular orbital theory study on the decomposition reaction of the first excited state HOOO(²A′) with respect to OH and O₂. The geometries and harmonic vibrational frequencies of all stationary points are calculated at the CASSCF and MRCI levels of theory in conjunction with the 6-31+G(d,p) basis set. The potential energy profile of the decomposition reaction is studied at the CASSCF/6-31+G(d,p) level of theory, in which the complete valence orbitals and electrons are included in the active space. The energies of the potential energy profile are further refined at the CASPT2 and MRCI levels of the theory. Additionally, we have determined the interesting reaction process: the HOOO(²A′) radical with C_s symmetry does not dissociate to OH(²Π) and O₂(³Σ⁻_g) directly as this is forbidden by orbital symmetry, but dissociates to OH(²Π) and O₂(³Σ⁻_g) via the change in symmetry from C_s to C_∞v symmetry with a low barrier.