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Published January 12, 2012 | Supplemental Material
Journal Article Open

O(^3P) +CO_2 Collisions at Hyperthermal Energies: Dynamics of Nonreactive Scattering, Oxygen Isotope Exchange, and Oxygen-Atom Abstraction


The dynamics of O(^3P) + CO_2 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 ^(12)C^(18)O_2 to distinguish products of nonreactive scattering from those of reactive scattering. The following product channels were observed: elastic and inelastic scattering (^(16)O(^3P) + ^(12)C^(18)O^2), isotope exchange (^(18)O + ^(16)O^(12)C^(18)O), and oxygen-atom abstraction (^(18)O^(16)O + ^(12)C^(18)O). Stationary points on the two lowest triplet potential energy surfaces of the O(^3P) + CO_2 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 CO_3(C_(2v),^3A″) 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 ^(16)O^(12)C^(18)O products peaked along the initial CO_2 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 ^(16)O^(12)C^(18)O 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 CO_3^* intermediate. A low yield for the abstraction reaction was seen in both the experiment and the theory. Experimentally, a fast and weak ^(16)O^(18)O 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 CO_3 intermediate, albeit only at high collision energies (149 kcal mol^(–1)). The oxygen isotope exchange mechanism for CO_2 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 O_3 and O_2.

Additional Information

© 2011 American Chemical Society. Received: August 20, 2011. Revised: October 19, 2011. Publication Date (Web): December 20, 2011. We thank Ned F. Lindholm for experimental assistance. This work was supported in part by the Missile Defense Agency under cooperative agreement HQ0006-05-2-0001. L.Y.Y. was supported by the Davidow Fund (Caltech), and G.C.S. was supported by the Air Force Office of Scientific Research grant FA955D-10-1-0205. Research at Weizmann Institute was partially supported by the Helen and Martin Kimmel Center for Molecular Design and by the Israel Science Foundation. J.M.L.M. is the incumbent of the Baroness Thatcher Professorial Chair of Chemistry.

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Supplemental Material - jp2080379_si_001.pdf

Supplemental Material - jp2080379_si_002.mpg

Supplemental Material - jp2080379_si_003.mpg

Supplemental Material - jp2080379_si_004.mpg

Supplemental Material - jp2080379_si_005.mpg


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