Density functional theory calculations of the energetics and kinetics of set fuel autoxidation reaction

Density functional theory calculations of the energetics and kinetics of important reactions for jet fuel oxidation are reported. The B3LYP functional along with 6-31G(d) and larger basis sets are used for calculation of peroxy radical abstraction reactions from hydrocarbons and heteroatomic species, the reaction of sulfides, disulfides, and phosphines with hydroperoxides to produce nonradical products, and the metal catalysis of hydroperoxide decomposition. Reaction enthalpies and activation energies are determined via DFT calculations of the structures and energies of stable species and transition states. The peroxy radical abstraction study shows the high reactivity (E-a's of 6-11 kcal/mol) of the H atoms which are weakly bonded to heteroatoms, including nitrogen, oxygen, and sulfur. These species, at part-per-million levels, are able to compete for peroxy radicals with the bulk fuel hydrocarbon species. Benzylic hydrogens on aromatic hydrocarbons are shown to be significantly more reactive (by 4 to 5 kcal/mol) than paraffinic hydrogens with the result that the aromatic portion of fuel sustains the bulk of the autoxidation process. Sulfides and disulfides are found to react readily with fuel hydroperoxides (E-a's of 26-29 kcal/mol) to produce alcohols and the oxidized sulfur species. Triphenylphosphine reacts with hydroperoxides with a very low activation energy (12.9 kcal/mol). The metal catalysis of hydroperoxide decomposition is calculated to occur through the formation of a complex with subsequent decomposition to form radical species without regeneration of the metal ion. The reaction pathways found and activation energies calculated can be used to improve chemical kinetic models of fuel autoxidation and deposition.