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, 3A″) 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 16O12C18O 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 16O12C18O 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.
|Journal||Journal of Physical Chemistry A|
|Early online date||20 Dec 2011|
|Publication status||Published - 12 Jan 2012|