A DFT‐Based Investigation of Hydrogen Abstraction Reactions from Methylated Polycyclic Aromatic Hydrocarbons

Abstract

The growth of polycyclic aromatic hydrocarbons (PAHs) is in many areas of combustion and pyrolysis of hydrocarbons an inconvenient side effect that warrants an extensive investigation of the underlying reaction mechanism, which is known to be a cascade of radical reactions. Herein, the focus lies on one of the key reaction classes within the coke formation process: hydrogen abstraction reactions induced by a methyl radical from methylated benzenoid species. It has been shown previously that hydrogen abstractions determine the global PAH formation rate. In particular, the influence of the polyaromatic environment on the thermodynamic and kinetic properties is the subject of a thorough exploration. Reaction enthalpies at 298 K, reaction barriers at 0 K, rate constants, and kinetic parameters (within the temperature interval 700-1100 K) are calculated by using B3LYP/6-31+G(d,p) geometries and BMK/6-311+G(3df,2p) single-point energies. This level of theory has been validated with available experimental data for the abstraction at toluene. The enhanced stability of the product benzylic radicals and its influence on the reaction enthalpies is highlighted. Corrections for tunneling effects and hindered (or free) rotations of the methyl group are taken into account. The largest spreading in thermochemical and kinetic data is observed in the series of linear acenes, and a normal reactivity-enthalpy relationship is obtained. The abstraction of a methyl hydrogen atom at one of the center rings of large methylated acenes is largely preferred. Geometrical and electronic aspects lie at the basis of this striking feature. Comparison with hydrogen abstractions leading to arylic radicals is also made.

Competitive Reactions of Organophosphorus Radicals on Coke Surfaces

“…[65] Tunneling corrections were not included, since it was previously shown that Eckart tunneling refinements lead to a decrease of approximately 5 kJ mol À1 in activation energies of hydrogen abstraction reactions from hydrocarbons. [19,25,26] This decrease is negligible compared to the difference in activation energies between hydrogen abstraction and addition reactions, as will be shown below. Reaction rate coefficients were calculated in the temperature range 700-1100 K, which is characteristic for thermal cracking (see Supporting Information), and kinetic parameters were estimated by fitting the Arrhenius equation to these data.…”

“…The intrinsic reaction coordinate (IRC) [55,56] paths were traced to find the two associated minima directly connected to each transition state (TS) on the potential-energy surfaces. The BMK [57] functional, particularly developed for kinetics and well-known for its good performance in both radical and nucleophilic substitution reactions, [19,25,27,39,58] was employed in conjunction with the 6-311 + GA C H T U N G T R E N N U N G (3df,2p) basis set to further refine energies of B3LYP optimized structures. The BMK/6-311 + G-A C H T U N G T R E N N U N G (3df,2p)//B3LYP/6-31 + GA C H T U N G T R E N N U N G (d,p) level of theory was recently shown to effectively reproduce experimental heats of formation and thermochemical data in reactions of organophosphorus compounds and derived radicals.…”

“…[16,22] An extended theoretical assessment of a reference reaction, namely, methyl radical induced hydrogen abstraction from benzene, [25] was followed by studies on larger polyaromatic surfaces, for both unsubstituted [26] and methyl-substituted aromatic compounds. [19] Peripheral hydrogen abstractions leading to arylic radicals were favored in unsubstituted aromatics, while resonance-stabilized benzylic radicals formed in methylated compounds have rate constants systematically larger than those predicted for the formation of arylic radicals. The size of the aromatic species and the abstraction site were shown to affect the overall rate constant by more than a factor of ten and led to approximately 10 kJ mol À1 difference in activation energies.…”

“…The size of the aromatic species and the abstraction site were shown to affect the overall rate constant by more than a factor of ten and led to approximately 10 kJ mol À1 difference in activation energies. [19] In addition to intrinsic reaction rate coefficients at actual working temperatures, thermochemical data, such as bond dissociation energies and enthalpies, which can be used to improve the cokegrowth model through single-event kinetic modeling, were calculated for a large set of hydrocarbons involved in thermal cracking. [23,27,28] Abstract: The efficacy of organophosphorus radicals as anticoking agents was subjected to a computational study in which a representative set of radicals derived from industrially relevant organophosphorus additives was used to explore competitive reaction pathways on the graphene-like coke surface formed during thermal cracking.…”

Validation of DFT-Based Methods for Predicting Qualitative Thermochemistry of Large Polyaromatics

“…[12][13][14][15][16] Our interest stems from their importance as key intermediates in the coke formation process during steam cracking. [17][18][19][20][21][22][23][24] In particular, we have used various polyaromatic structures to model the coke surface and to study carbon-hydrogen bond dissociation properties and corresponding hydrogen abstraction reactions from both pure and methylated PAHs.…”