Ab Initio Investigation of Primary Fuel Reactions of Monoaromatic Hydrocarbons under Pyrolytic Conditions: Anisole, Phenetole, and the 2-, 3-, 4-Methylanisole Isomers.

Wooden biomass contains high amounts of lignocellulose, which is one of the main fuel components during wildfire events. Furthermore, its properties in the context of alternative energy carriers are of interest in recent research. In order to better analyze and understand these highly complex molecules and their fundamental combustion properties, a complexity reduction by using model compounds can be applied. Monoaromatic oxygenated hydrocarbons (MAHs) are an option to map these systems on a more accessible level. In the present study, the MAHs anisole, phenetole, 2-methylanisole, 3-methylanisole, and 4-methylanisole were investigated by means of quantum chemical calculations. To this end, the DLPNO-CCSD(T)/CBS(cc-pVTZ, cc-pVQZ)//B3LYP-D3BJ/def2-TZVP levels of theory were utilized to derive a range of important physical and chemical quantities. These include bond dissociation energies (BDEs), one-dimensional representations of the potential energy surface, thermodynamic properties, and reaction rate parameters. As previously demonstrated, reactions of the aromatic ring structure and the attached hydrogen atoms are energetically unfavorable. This prompted the investigation of only the reactions affecting the methyl and alkoxy side chains. The reactions examined in this study are the primary fuel reactions that are relevant to pyrolysis. This set of 47 reactions includes the H atom abstraction by Ḣ and ĊH, the unimolecular bond fissions, and the internal H atom migration reactions on the methoxy or ethoxy side chain. For all five molecules, the C-O bond on the alkoxy side chain is the weakest bond by BDEs, and the respective bond fissions are dominant. Besides the general importance of H atom abstractions, these dominant bond fissions have the highest impact on the overall reactivity among the investigated reactions. Due to the comprehensive amount of available literature for anisole, it is included as a benchmark molecule. The available literature on phenetole is limited, and the present study provides fundamental data for this species. For methylanisole, a recent publication focused on experimental and modeling efforts for these isomers. The importance of the C-O bond breaking, and the other determined reactions in this work were tested by including the calculated rate parameters in a validated chemical kinetic mechanism for methylanisole isomers from literature. The modified model was subsequently assessed in comparison to the initial version of the published model and experiments. Shock tube and rapid compression machine experiments were performed in the temperature range between 880 and 1220 K for pressures of 10 and 20 bar at stoichiometric conditions. This assessment yielded two notable findings. First, it confirmed the significant impact of the C-O bond fission. However, a comparison with recent high-level ab initio calculations revealed significant deviations in the rate constants. Second, it emphasized the importance of the subsequent phenoxy/methylphenoxy radical chemistry and the associated thermodynamic properties. Further refinement of the model descriptions of MAHs is warranted and necessary to improve the understanding of these important reference molecules.