On the role of the termolecular reactions 2O + H → 2HO and 2O + H → H + HO + O in formation of the first radicals in hydrogen combustion: ab initio predictions of energy barriers.

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 H : O in the NVT ensemble at density 276.3 kg m and ∼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 H + O → H + HO, frequently occurring in the presence of a second O as a third body; that is, 2O + H → H + HO + O. The second most frequent reaction is 2O + H → 2HO. We found three saddle points on the triplet PES of these termolecular reactions: one for 2O + H → H + HO + O and two for 2O + H → 2HO. In the latter case, one has a symmetric structure consistent with simultaneous formation of two HO and the other corresponds to a bimolecular reaction between O and H that is "interrupted" by a second O before going to completion. The classical barrier height of the symmetric saddle point for 2O + H → 2HO is 49.8 kcal mol. The barrier to H + O → H + HO is 58.9 kcal mol. The termolecular reaction will be competitive with H + O → H + HO only at sufficiently high pressures.