The role of non-Boltzmann-distributed products in gas phase kinetics
One of the central aspects of the Cluster of Excellence Tailor-Made Fuels from Biomass is the study of the combustion chemistry of novel fuel compounds. In order to evaluate the potential of these novel fuel compounds, experiments, simulations and theoretical considerations of its gas phase kinetics are essential. While experiments represent reality when being properly evaluated, simulations and theory 'suffer' from assumptions made for simplifying computations.
A major assumption is that the products of a reaction are in thermal equilibrium with their environment, although these products are formed in a rovibrationally excited state by definition. This assumption is valid in the so-called high-pressure limit, i.e. if the rate of thermalization is infinite. At finite pressures, however, thermalization is sometimes slower than subsequent reactions of the rovibrationally excited products.
The Figure shows a general scheme illustrating the sequence of hydrogen abstraction by R1 from HR2 and dissociation of the rovibrationally excited R2 formed via hydrogen abstraction (taken from one of our contributions to the 36th International Symposium on Combustion). The lower dashed red line shows the rovibrational excitation of R2, which leads to direct dissociation. This is an example system for hot dissociation of rovibrationally excited unimolecular species.
A similar reaction scheme governs the hot reactions of unimolecular species formed via barrierless addition, e.g. the addition of formyl (HCO) and hydroxyl (OH) to formic acid (HC(=O)OH) . Although, the HCO + OH reaction partly forms HC(=O)OH, the rovibrational excitation of HC(=O)OH significantly exceeds the dissociation transition state of HC(=O)OH, leading to direct dissociation to CO + H2O for technically relevant pressures. The combination of low-temperature and high-pressure, however, leads to meta-stable HC(=O)OH which can undergo hot hydrogen abstraction by OH radicals.
Current research projects aim for gaining deeper understanding of non-Boltzmann effects on gas phase kinetics via combining experiments, kinetic modeling, and theory.
 Döntgen and Leonhard, J. Phys. Chem. A 120 (2016), 1819-1824
Funding: German Research Association, Grant GSC 111