O(3P) + CO2 Collisions at Hyperthermal Energies: Dynamics of Nonreactive Scattering, Oxygen Isotope Exchange, and Oxygen-Atom Abstraction

The dynamics of O(P-3) + 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 (CO2)-C-12-O-18 to distinguish products of nonreactive scattering from those of reactive scattering. The following product channels were observed: elastic and inelastic scattering (O-16(P-3) + (CO2)-C-12-O-18), isotope exchange (O-18 + (OCO)-O-16-C-12-O-18) and oxygen atom abstraction ((OO)-O-18-O-16 + (CO)-C-12-O-18). Stationary, points on the two lowest triplet potential energy surfaces of the O(P-3) + 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, (3)A '') 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-a-mass (c.m.) angular distribution, T(theta(c.m.)), of the (OCO)-O-16-C-12-O-18 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 (OCO)-O-16-C-12-O-18 products were formed with substantial internal energy. The QCT calculations give c.m. P(E-T) and T(theta(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 (OO)-O-16-O-18 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(P-3) atoms with hyperthermal translational energies can be generated by photodissociation of O-3 and O-2.