The research is directed at determination of the kinetics of radical-radical reactions important in the oxidation and pyrolysis of hydrocarbons. These reactions usually serve as chain termination and molecular-mass growth pathways. Certain members of this class of reactions have been linked to formation of aromatic rings and polyaromatic hydrocarbons, which leads, in turn, to production of soot in combustion systems. In spite of the importance of radical-radical reactions for understanding the chemistry of a wide variety of combustion and pyrolysis processes, experimental information on them is rather sparse and, in many cases, controversial. The main objective of the work is the establishment of a reliable database of accurately determined experimental data on a benchmark set of reactions. The project covers several types of radical-radical reactions, including self-reactions and cross reactions of alkyl, cycloalkyl, and delocalized and aromatic radicals. Temperature dependences of the rate constants are being determined in direct experiments under conditions where radicals are created in known concentrations and the reactions under study are isolated from side influences. Radical concentration decays and formation of products are monitored in real-time using Laser Photolysis / Photoionization Mass Spectrometry technique. Reaction-product studies are performed by online mass spectrometry as well as GC/MS of final products. Results of the study will be of fundamental importance for modeling the combustion of hydrocarbon fuels, forming an experimental basis for further advancement of high-level theories of reaction rates and semi-empirical computational tools. Benefits to society will be advanced by improving our understanding of commercial technologies such as energy generation through combustion, including the safety and environmental aspects of these processes. Other broader impacts of the proposed work will come from integration of education and research and contributions to training and education of undergraduate and graduate students. Broad dissemination of the results will be advanced via publications, presentations in multi-disciplinary conferences and workshops, web presentation of the findings, and contributing to cyberinfrastructure projects.
Free radicals are molecules with unpaired electrons. They are highly reactive and serve as the major players in the processes of combustion. Reactions between hydrocarbon radicals, including radical self reactions, are important elementary steps in the combustion and pyrolysis of hydrocarbons. Moreover, radical-radical reactions generally represent pathways of molecular mass growth. Certain members of this class of reactions have been linked to formation of aromatic rings and polyaromatic hydrocarbons (PAH), which leads, in turn, to production of soot in combustion systems. Knowledge of the rate constants of these reactions is required for modeling of the combustion of organic fuels. Information available on the rates of these reactions, however, is sparse and often controversial as radical-radical reactions are difficult to study quantitatively because of the high reactivity of radicals. Directly obtained experimental data on temperature dependences of the rate constants are limited to a handful of reactions of small radicals. Due to the lack of reliable experimental data, theoretical methods of evaluating and predicting rates of such reactions become ever more important. Validation and further development of theoretical methods requires a database of accurately determined temperature-dependent experimental data on a variety of benchmark reactions, preferably obtained in direct experiments. In the experimental part of the current project, the kinetics of three self-reactions involving relatively large (5 – 6 carbon atoms) hydrocarbon radicals representing three classes (branched alkyl, cycloalkyl, and highly delocalized cyclic radicals) have been studied using the Laser Photolysis / Photoionization Mass Spectrometry (LP/PIMS) technique. In all cases, our work was the first in which these reactions were isolated for quantitative studies at elevated temperatures. The main result of these studies was the demonstration of strong negative temperature dependences of the rate constants of these reactions. In the cases of alkyl and cycloalkyl (neopentyl and cyclohexyl) radicals, the negative temperature dependences were significantly stronger than in the cases of the only two alkyl radical self-reactions for which directly determined temperature dependent rate constants were available prior to this project: methyl and ethyl radicals: k = 3.1×10-12 exp(+506 K/T) (± 17 %) cm3 molecule-1 s-1 (neopentyl, neo-C5H11) k = 4.8×10-12 exp(+542 K/T) (± 17 %) cm3 molecule-1 s-1 (cyclohexyl, c-C6H11) A comparison between the temperature dependences obtained for the small and the large radicals is presented in Figure 1. An even more surprising, much stronger temperature dependence was observed for the rate constant of the self-reaction of the delocalized cyclopentadienyl radicals: k = 2.9×10-12 exp(+1489 K/T) cm3 molecule-1 s-1 (c-C5H5) A comparison between several strong negative temperature dependences of self-reaction rate constants is presented in Figure 2. The experimental data (symbols) include the results of the current study for the neopentyl, cyclohexyl, and cyclopentadienyl radicals. The theoretical data represented with a line are those obtained by Klippenstein et al.(Phys. Chem. Chem. Phys. 2006, 8, 1133) for the tertiary butyl radical, the radical that showed the strongest temperature dependence of its self-reaction rate constant among all alkyl radical studied by these authors. The very strong negative temperature dependence of the cyclopentadienyl radicals self-reaction rate constant is unprecedented for a radical-radical reaction and presents a challenge for theoretical methods of chemical kinetics. Formation of products was studied for the self-reactions of the cyclohexyl and the cyclopentadienyl radicals. In the cyclohexyl case, the unusually large fraction of disproportionation equal to 41 ± 7% was determined at the room temperature; analysis of earlier experimental determinations of the disproportionation-to-recombination branching ratio leads to recommending this room-temperature value for other temperatures as well. The self-reaction of the cyclopentadienyl radicals was shown to produce several C10H10 isomers at low temperatures, but at higher temperatures formation of naphthalene (C10H8) was observed. This indicates that this reaction is likely to contribute to the pathways of molecular mass growth and PAH formation in combustion systems by providing a reactive pathway leading to formation of larger aromatic species from single-ring delocalized intermediates. In the theoretical part of the current project, the recombination of the vinyl (C2H3) and the hydroxyl (OH) radicals was studied computationally. The reaction potential energy surface was studied using quantum chemistry and a master equation / RRKM model of the reaction was created. The reaction mechanism includes the initial addition, several isomerization steps, collisional stabilization, and decomposition via seven different channels. The spectrum of products predicted by the model demonstrates strong temperature dependence in the 300 – 2000 K range. One graduate student participating in the project obtained his Ph.D. degree in chemistry. The results of the project were presented at several international conferences. The results of the project contribute to improving our understanding of commercial technologies such as energy generation through combustion, including the safety and environmental aspects of these processes.
