Kinetics of the CH₃ + C₅H₅ Reaction: A Theoretical Study

Abstract

Formation of fulvene and benzene through the reaction of cyclopentadienyl (CH) with methyl radical (CH) and consequent dissociation of its primary CH products has been studied using ab initio and theoretical kinetics calculations. The potential energies and geometries of all involved species have been computed at the CCSD(T)-F12/cc-pVTZ-f12//B2PLYPD3/aug-cc-pVDZ level theory. Multichannel/multiwell RRKM-Master Equation calculations have been utilized to produce phenomenological pressure- and temperature-dependent absolute and individual-channel rate constants for various reactions at the CH and CH potential energy surfaces. The kinetic scheme combining the primary and secondary reactions has been used to generate the overall rate constants for the production of fulvene and benzene and their branching ratios. Analyses of the kinetic data revealed that at low pressures (0.01 atm) benzene formation prevails, with branching ratios exceeding 60%, whereas at the highest pressure (100 atm) fulvene formation is prevalent, with the branching ratio of benzene being lower than 40%. At intermediate pressures (1 and 10 atm) the two product channels compete and fulvene formation is preferable at temperatures above 1600 K. The results demonstrate that a five-member ring can be efficiently transformed into an aromatic six-member ring by methylation and corroborate the potentially important role of the methyl radical in the mechanism of PAH growth where CH additions alternate with H abstractions and acetylene additions.

Transformation of an Embedded Five-Membered Ring in Polycyclic Aromatic Hydrocarbons via the Hydrogen-Abstraction–Acetylene-Addition Mechanism: A Theoretical Study

“…Based on their simulations using reaction rate constants computed at the high pressure limit employing transition state (TS) theory, the authors concluded that such a mechanism could contribute to the formation of pyrene from cyclopenta­[ d , e , f ]­phenanthrene, but its role in the presence of other competing PAH mechanisms still needs to be determined. A similar mechanism is also well known from theoretical studies on the formation of benzene in the cyclopentadienyl C 5 H 5 + CH 3 reaction − and of naphthalene in the indenyl C 9 H 7 + CH 3 reaction; the latter has been recently confirmed experimentally in a pyrolytic chemical micro reactor with the product detection by means of photoionization mass spectrometry …”

“…Alternatively, the five-membered ring in R1 can be converted to a six-membered ring via the MAC mechanism involving methyl radical addition to the radical site, followed by a H loss, CH 2 insertion into a C–C in the five-membered ring, elimination of another H-atom, and aromatization, R1 + CH 3 → pyrene + 2H. Because of the structural and mechanistic similarity, the rate constant for the overall process is anticipated to be similar to that for the C 5 H 5 + CH 3 → C 6 H 6 + 2H reaction, 2.5 × 10 12 cm 3 mol –1 s –1 at 1500 K and 1 atm, that is, ∼300 times higher than that for R2 + C 2 H 2 → 1-pyrenylmethyl + H. The R1 + CH 3 reaction is likely to be somewhat slower than C 5 H 5 + CH 3 due to a different degree of electron delocalization and steric constraints caused by the additional presence of the six-member rings in R1. Therefore, a more quantitative conclusion about the competition of R1 + CH 3 and R2 + C 2 H 2 can be made only after the kinetics of R1 + CH 3 is characterized and a fuller kinetic modeling involving all significant competing reactions is carried out.…”

Molecular mass growth through ring expansion in polycyclic aromatic hydrocarbons via radical–radical reactions

“…1). Although the formation of naphthalene via recombination of two cyclopentadienyl radicals (C 5 H 5 • ) 20,21 and methylation of indenyl has been predicted theoretically 22,23 , similar to the formation of benzene via methylation of the cyclopentadienyl radical 24–26 , the validity of elementary reactions between free hydrocarbon radicals leading to PAHs at high temperatures have not been realized experimentally. Indeed there is no experiment to date under high temperature conditions that has followed the kinetics and mechanism of a hydrocarbon free radical with a second hydrocarbon radical, and hence the reaction route through methylation of the indenyl radical provides a benchmark for the conversion of a five-membered ring to a six-membered ring in PAHs.…”

An Aromatic Universe–A Physical Chemistry Perspective

“…Reactions between two hydrocarbon radicals such as propargyl (C 3 H 3 )-propargyl (C 3 H 3 ), methyl (CH 3 )-cyclopentadienyl (C 5 H 5 ), − and cyclopentadienyl (C 5 H 5 )-cyclopentadienyl (C 5 H 5 ) , have been suggested to lead to aromatic systems such as benzene (C 6 H 6 ) and naphthalene (C 10 H 8 ), respectively. However, the validity of elementary reactions between free hydrocarbon radicals leading to aromatic hydrocarbons at high temperatures have been scarce under well-defined experimental conditions in molecular beams.…”