The dynamics of O(3P) + CO2 collisions at hyperthermal energies were investigated experimentally and theoretically. Crossed-molecular-beams experiments at «Ecoll» = 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(C2v, 3Ä) intermediate that lies 16.3 kcal mol-1 (W4 FCI excluding zero point energy) above reactants and is approached by a C2v 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(θc.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(ET), had a relatively low average of «ET» = 35 kcal mol-1, indicating that the 16O12C 18O products were formed with substantial internal energy. The QCT calculations give c.m. P(ET) and T(θc.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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry