O(³ P) + CO₂ collisions at hyperthermal energies: Dynamics of nonreactive scattering, oxygen isotope exchange, and oxygen-atom abstraction

The dynamics of O(³P) + CO₂ collisions at hyperthermal energies were investigated experimentally and theoretically. Crossed-molecular-beams experiments at «E_coll» = 98.8 kcal mol^-1 were performed with isotopically labeled ¹²C¹⁸O₂ to distinguish products of nonreactive scattering from those of reactive scattering. The following product channels were observed: elastic and inelastic scattering (¹⁶O(³P) + ¹²C¹⁸O₂), isotope exchange (¹⁸O + ¹⁶O¹²C¹⁸O), and oxygen-atom abstraction ( ¹⁸O¹⁶O + ¹²C¹⁸O). Stationary points on the two lowest triplet potential energy surfaces of the O(³P) + CO₂ system were characterized at the CCSD(T)/aug-cc-pVTZ level of theory and by means of W4 theory, which represents an approximation to the relativistic basis set limit, full-configuration-interaction (FCI) energy. The calculations predict a planar CO₃(C_2v, ³Ä) intermediate that lies 16.3 kcal mol^-1 (W4 FCI excluding zero point energy) above reactants and is approached by a C_2v transition state with energy 24.08 kcal mol^-1. Quasi-classical trajectory (QCT) calculations with collision energies in the range 23-150 kcal mol^-1 were performed at the B3LYP/6-311G(d) and BMK/6-311G(d) levels. Both reactive channels observed in the experiment were predicted by these calculations. In the isotope exchange reaction, the experimental center-of-mass (c.m.) angular distribution, T(θ_c.m.), of the ¹⁶O ¹²C¹⁸O products peaked along the initial CO₂ direction (backward relative to the direction of the reagent O atoms), with a smaller isotropic component. The product translational energy distribution, P(E_T), had a relatively low average of «E_T» = 35 kcal mol^-1, indicating that the ¹⁶O¹²C ¹⁸O products were formed with substantial internal energy. The QCT calculations give c.m. P(E_T) and T(θ_c.m.) distributions and a relative product yield that agree qualitatively with the experimental results, and the trajectories indicate that exchange occurs through a short-lived CO₃* intermediate. A low yield for the abstraction reaction was seen in both the experiment and the theory. Experimentally, a fast and weak ¹⁶O¹⁸O product signal from an abstraction reaction was observed, which could only be detected in the forward direction. A small number of QCT trajectories leading to abstraction were observed to occur primarily via a transient CO₃ intermediate, albeit only at high collision energies (149 kcal mol^-1). The oxygen isotope exchange mechanism for CO₂ in collisions with ground state O atoms is a newly discovered pathway through which oxygen isotopes may be cycled in the upper atmosphere, where O(³P) atoms with hyperthermal translational energies can be generated by photodissociation of O₃ and O₂.