New reactions of diazene and related species for modelling combustion of amine fuels

Potential energy surfaces for reactions involving N2H2 isomers of diazene (diimide) have been explored using density functional theory, with energies based on coupled-cluster theory. A focus is on processes that create or consume these species, and isomerisation between the E (trans) and Z (cis) forms of HNNH. These include isomerisation and dissociation pathways for HNNH, addition of H atoms to form N2H3, abstraction by H atoms yielding short-lived NNH, and abstraction reactions of H with N2H3. Transition state and capture theories are applied for high-pressure-limiting behaviour, while low-pressure and falloff regions are characterised via the methods of Troe and coworkers. Rate constants and thermochemistry are provided to improve models of diamine chemistry, relevant to the combustion of NH3 especially at high concentrations, high pressures or under reducing conditions. Results indicate that amine radical recombination mainly yields the E HNNH isomer, while H-abstraction from N2H3 results in E HNNH and H2NN. However, at elevated temperature E → Z isomerisation becomes competitive, and Z HNNH, being more reactive, acts to enhance the diazene consumption rate.