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 H-2 : O-2 in the NVT ensemble at density 276.3 kg m(-3) and similar to 3000 K (similar to 4000 atm) and similar to 4000 K (similar to 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-2 + O-2 -> H + HO2, frequently occurring in the presence of a second O-2 as a third body; that is, 2O(2) + H-2 -> H + HO2 + O-2. The second most frequent reaction is 2O(2) + H-2 -> 2HO(2). We found three saddle points on the triplet PES of these termolecular reactions: one for 2O(2) + H-2 -> H + HO2 + O-2 and two for 2O(2) + H-2 -> 2HO(2). In the latter case, one has a symmetric structure consistent with simultaneous formation of two HO2 and the other corresponds to a bimolecular reaction between O-2 and H-2 that is interrupted by a second O-2 before going to completion. The classical barrier height of the symmetric saddle point for 2O(2) + H-2 -> 2HO(2) is 49.8 kcal mol(-1). The barrier to H-2 + O-2 -> H + HO2 is 58.9 kcal mol(-1). The termolecular reaction will be competitive with H-2 + O-2 -> H + HO2 only at sufficiently high pressures.