Computational study on the recombination reaction between benzyl and propargyl radicals

Abstract

The kinetics and mechanisms of the recombination reaction between benzyl and propargyl radicals have been computationally investigated by using the B3LYP, CBS-QB3, and CASPT2 quantum chemical methods, and the steady-state unimolecular master equation analysis based on the Rice-Ramsperger-Kassel-Marcus theory. Six distinct recombination channels arising from the radicals' nonequivalent resonance structures (three for benzyl and two for propargyl) were investigated at the CASPT2/cc-pVTZ//B3LYP/6-311G(d,p) level, and the respective rate constants were calculated with the variational transition state theory. It was found that the reaction dominantly proceeds by the addition of the propargyl radical to the alpha(CH 2 ) site of the benzyl radical and the additions to the ortho-and the para-sites are unfavorable due to the loss of the aromaticity. The isomerization reaction pathways of the initially formed adducts to methylenecyclopenta-fused compounds through diradical intermediates were identified. Methylene-indanyl radicals were also identified as products, which are produced by direct H-atom elimination reactions from the diradical and methylenated indanes. The results of the master equation analyses indicated that the 1-methylene-2-indanyl radical is dominantly produced at high temperature (>1500 K), but the overall rate constant is significantly smaller than the high-pressure limit within the pressure range studied (0.001-100 atm). The reactions of the triplet biradicals via the intersystem crossing were also suggested to be important and are discussed qualitatively. Investigations on the subsequent reactions of 1-methylene-2-indanyl radicals revealed that the radicals rapidly decompose to 1-methyleneindene or naphthalene + H. The temperature-and pressure-dependent rate expressions are proposed for kinetic modeling.

Kinetic modeling study of benzene and PAH formation in laminar methane flames

“…As mentioned, this mechanism was validated in previous investigations of laminar flames [50,53,61,64,65]. The aromatic formation submechanism was improved based on our previous investigations [53,56,66], recent theoretical calculations [43,44,51,67] and other aromatic mechanisms [54,57,[68][69][70], mainly including HACA (hydrogen-abstraction acetylene-addition) pathways [39] and the recombination reactions of resonantly stabilized radicals with small flame species [37,53,[68][69][70].…”

“…The reacting possibility was later confirmed by Matsugi and Miyoshi [44] in their quantum chemical calculation. Since it is not easy to restrict the uncertainty of the estimation of the global reaction rate, a four-steps simplified kinetic scheme (R53), (R54), (R55), (R56) provided in the work of Matsugi and Miyoshi [44] was adopted in this model. Methyleneindanyl (C 9 H 7 CH 2 ) radical is primarily formed via (R53).…”

“…The latter intermediate can isomerize to naphthalene rapidly. The rate constants of (R54), (R55), (R56) were provided in Chebyshev polynomial form in [44]. Therefore, they were fitted into Arrhenius format in this work with the maximum error of 51%.…”

“…Later, they updated their PAH mechanism based on the further validations of ethane and ethylene flames [41,42]. Their main conclusion is that three PAH formation pathways show similar importance: HACA mechanism [38,39], the addition of small molecules to aromatic molecules/radicals [43,44], and the combination reactions of aromatic molecules and radicals [45,46]. According to their extensive model analysis, these reaction pathways give main contributions in PAH formation at T > 1550 K, while part of the reaction pathways proceeds in the reverse direction or achieving equilibrium when the flame temperature is lower than 1500 K.…”

“…There are no comprehensive kinetic modeling investigations on laminar methane flames, including the validations under the premixed, counter flow and coflow diffusion flame conditions. Based on the recent progress in the combustion kinetics of methane [36], and the efforts on the combustion of benzene [49][50][51][52], toluene [43,[53][54][55] and naphthalene [44,46,[56][57][58][59], a detailed kinetic model was developed for the combustion and PAH formation in laminar methane flames. This model was validated against the experimental data of laminar flames in various previous investigations, including the premixed flames [22][23][24], and the counter flow flames [60], whose operative conditions are reported in Table 1.…”