Investigating the autoxidation accelerating reactions of peroxides and peroxyl radicals using high level quantum chemistry calculations

Increases in performance and operating temperatures of jet engines has lead to the fuel being used as a coolant to remove this excess heat. As a consequence the fuel becomes thermally stressed, and undergoes autoxidation through radical reaction pathways, causing the formation of insoluble gums. Novel fuels appearing on the market, means the importance of a fuels chemistry on its thermal stability is becoming critical. This work presents a fundamental and chemically detailed approach to this problem. The autoxidation process occurs through radical reaction mechanisms. This presents a challenge, in particular when modelling them using quantum chemistry techniques. This is particularly true for the reactions of peroxide species which link the initial oxidation process to the deposition steps. The propensity to conserve overall electronic spin angular momentum during reactions, means that high level multi-reference methods are required to properly describe them. Multi-reference self consistent field theory calculations with second-order Rayleigh-Schrödinger perturbation corrections applied (CASPT2) were carried out to investigate the self-reaction of peroxides and peroxyl radicals. These calculations demonstrate that the reactions are intrinsically multi-reference and involve open shell bi-radicals. These reactions follow non-concerted reaction pathways which are energetically more favourable than the corresponding homolytic fission of peroxides and proceed towards the formation of oxidation product. From our calculations, the self reaction of peroxyl radicals, also appears to lead to the formation highly reactive singlet oxygen as a product. Singlet oxygen will accelerate the autoxidation of fuel by reacting directly with the hydrocarbons, acting as the initiation step.