title: On the role of the termolecular reactions 2O2+ H2→ 2HO2and 2O2+ H2→ H + HO2+ O2in formation of the first radicals in hydrogen combustion: ab initio predictions of energy barriers.
abstract: We have investigated the role of termolecular reactions in the early chemistry of hydrogen combustion. We performed molecular chemical dynamics simulations using ReaxFF in LAMMPS to identify potential initial reactions for a 1 : 4 mixture of H2: O2in the NVT ensemble at density 276.3 kg m-3and ∼3000 K (∼4000 atm) and ∼4000 K (∼5000 atm), and then characterized the saddle points for those reactions using ab initio methods: CCSD(T) = FC/cc-pVTZ//MP2/6-31G, CCSD(T) = FULL/aug-cc-pVTZ//CCSD = FC/cc-pVTZ and CASSCF MP2/6-31G//MP2/6-31G. The main initial reaction is H2+ O2→ H + HO2, frequently occurring in the presence of a second O2as a third body; that is, 2O2+ H2→ H + HO2+ O2. The second most frequent reaction is 2O2+ H2→ 2HO2. We found three saddle points on the triplet PES of these termolecular reactions: one for 2O2+ H2→ H + HO2+ O2and two for 2O2+ H2→ 2HO2. In the latter case, one has a symmetric structure consistent with simultaneous formation of two HO2and the other corresponds to a bimolecular reaction between O2and H2that is "interrupted" by a second O2before going to completion. The classical barrier height of the symmetric saddle point for 2O2+ H2→ 2HO2is 49.8 kcal mol-1. The barrier to H2+ O2→ H + HO2is 58.9 kcal mol-1. The termolecular reaction will be competitive with H2+ O2→ H + HO2only at sufficiently high pressures.
authors.affiliation: Department of Chemistry, University of Missouri-Columbia, Columbia, MO 65211-7600, USA. mongepalaciosm@missouri.edu hryr8@mail.missouri.edu.
authors.forename: M
authors.identifier: http://orcid.org/0000-0003-1199-5026
authors.initials: M
authors.lastname: Monge-Palacios
authors.affiliation: Department of Chemistry, University of Missouri-Columbia, Columbia, MO 65211-7600, USA. mongepalaciosm@missouri.edu hryr8@mail.missouri.edu.
authors.forename: Homayoon
authors.initials: H
authors.lastname: Rafatijo