Ab initio study of the elusive HO3(X2A″) radical and the reaction

Electronic structures, vibrational properties, and dissociation energies of the elusive HO3(X2A″) radical have been calculated using various benchmark ab initio methods, including multireference RS2, MRCI+Q, MR-AQCC, NEVPT2, and the explicitly correlated RCCSD(T)-F12, RS2-F12, and MRCI-F12. The RS2 results strongly depend on the semiempirical level shifts applied to modify the zeroth-order Hamiltonian and the basis sets. The sizes of active spaces are crucial to the MRCI+Q data. The calculated dissociation energy (De) of HO3 into OH + O2 ranges from 4.6 to 6.2 kcal/mol. Theoretical intermolecular vibrational frequencies are in excellent agreement with the experimental measurements. The zero-point energy correction to De is estimated to be 2.6 kcal/mol. The formation of HO3 from OH(X2Π) and O 2 ( X 3 Σ g - ) is determined to be a barrierless process. Temperature and pressure dependent kinetics for the association reaction are calculated variationally using the Master equations. It is revealed that the rate constants exhibit steep negative temperature dependence and typical non-Arrhenius behavior. The experimental low-temperature rate constants have been well reproduced theoretically.