Abstract
We report theoretical simulations of hyperthermal O(3P) collisions with CH4. C2H6 and C3H8 that are related to erosion processes in low Earth orbit (LEO). In the first part of this paper, we use electronic structure calculations to show that at energies well below those accessible in LEO, there is a whole new body of reaction channels that are open in addition to the well known lowest barrier one, H abstraction to give OH + alkyl radicals. For instance, O addition to the hydrocarbon followed by H elimination to give alkoxy radicals + H is possible through barriers of about 2 eV. Similar barrier heights are found for direct carbon-carbon bond breakage processes that can take place in hydrocarbons containing C-C bonds (ethane and propane). In the second part of this paper, we carry out reaction dynamics calculations using the quasiclassical trajectory method in conjunction with a novel semiempirical Hamiltonian termed MSINDO. Cross section calculations reveal that H elimination and C- C breakage are both competitive with H abstraction in the reactions of O(3P) with ethane and propane. In O(3P) + methane, elimination is even more important than H abstraction under LEO conditions. Energy distributions for the majority channels show different patterns according to the kinematies of the products that are formed. For H abstraction to give OH, most of the energy is released as product translation, while OH is fairly cold. For H elimination, most energy is released as internal energy of the oxyradical. Angular distributions also depend on the product channel. H abstraction shows a trend with collision energy that matches that of reactions undergoing a direct mechanism. Near threshold H elimination and C-C breakage angular distributions reveal the presence of two different saddle points that connect reagents and products for each process.
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Troya, D., Schatz, G.C. (2004). Dynamics Studies of the O(3P) + Ch4, C2H6 and C3H8 Reactions. In: Lagana, A., Lendvay, G. (eds) Theory of Chemical Reaction Dynamics. NATO Science Series II: Mathematics, Physics and Chemistry, vol 145. Springer, Dordrecht. https://doi.org/10.1007/1-4020-2165-8_15
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DOI: https://doi.org/10.1007/1-4020-2165-8_15
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