Toward a quantitative understanding of elementary combustion reactions

Abstract

The modelling of combustion requires data bases of reliable rate coefficients which are represented in an accurate and economic way. The combination of well defined laboratory measurements, quantum-chemical calculations of potential energy surfaces, statistical and dynamic kinetic theories, as well as optimum thermodynamic data, continuously improves such data bases. The present contribution discusses selected aspects of this active field of research. First, unimolecular reactions and recombination reactions are considered. In particular, the implementation of nonthermal determinations of collisional energy transfer properties, of selected excited state rate constants, and of reaction product identifications are described. The relation between potential energy surfaces and high pressure unimolecular bond fissions or radical recombination reactions is analyzed in terms of explicit statistical adiabatic channel calculations. It follows a detailed analysis of more complex bimolecular reactions, treated in the framework of statistical unimolecular rate theory. Particular emphasis is given to the reaction H+O2⇌HO+O which passes through an HO2* intermediate. Good agreement between simple statistical calculations and the most recent experiments is found. The contribution of electronically excited HO2 is also discussed. The accuracy of the theoretical results in this case may soon exceed that of the experiments. After the “loose entrance-loose exit” system HO2, the reaction H+HCO⇌H2+CO, passing through an H2CO* intermediate, is chosen as an example for “loose entrance-rigid exit” systems. Rotational effects, in particular “rotational channel switching,” are of great importance. The additional effects of collisions on complex-forming bimolecular reactions are modelled in detail. The relationship to the reverse multichannel unimolecular dissociation reactions for the corresponding complex molecule is illustrated. The theoretical methods described provide simple possibilities for a “sensitivity test of the rate calculations” with respect to the various molecular contributions in a rate coefficient. In this sense they complement “sensitivity tests of the modelled mechanisms” with respect to the various elementary reactions and their rate parameters.