Combustion Chemistry via Metadynamics: Benzyl Decomposition Revisited

Abstract

Large polycyclic aromatic hydrocarbons (PAHs) are thought to be responsible for the formation of soot particles in combustion processes. However, there are still uncertainties on the course that leads small molecules to form PAHs. This is largely due to the high number of reactions and intermediates involved. Metadynamics combined with ab initio molecular dynamics can provide a very precious contribution because offers the possibility to explore new possible pathways and suggest new mechanisms. Here, we adopt this method to investigate the chemical evolution of the benzyl radical, whose role is very important in PAHs growth. This species has been intensely studied, and though most of its chemistry is known, there are still open questions regarding its decomposition. The simulation reproduces the most commonly accepted decomposition pathway and it suggests also a new one which can explain recent experimental data that are in contradiction with the old mechanism. In addition, quantitative free energy evaluation of some key reaction steps sheds light on the role of entropy.

Benzyl Radical Photodissociation Dynamics at 248 nm

“…35,36,39,40,62,63 Although the other fragment measured in this experiment at m/z = 90 has not been unambiguously identified, theoretical work has suggested that it is most likely the fulvenallene molecule. 7,8,41,42 The identification of an H loss channel is corroborated here by the agreement between the Newton diagram in Figure 7 with the angular distribution of signal and by the excellent agreement between the P(E T ) distribution from Song et al, based on measuring the H atom fragment, with our P(E T ) in Figure 8, determined from TOF spectra for the C 7 H 6 counterfragment. We cannot differentiate between the production of fulvenallene and 2-ethynylcyclopentadiene or distinguish which pathway in Figure 1 produces fulvenallene because all of these pathways are energetically very similar.…”

Gas-Phase Optical Detection of 3-Ethynylcyclopentenyl: A Resonance-Stabilized C7H7 Radical with an Embedded 1-Vinylpropargyl Chromophore

“…Three channels have been identified from flash pyrolysis VUV-PIMS experiments, , of which the most important proceeds via the (unobserved) intermediate 4 (Figure ) to produce H atoms and fulvenallene (C 5 H 4 =C=CH 2 ). However, computational studies that are broadly consistent with these findings also predict a plethora of C 7 H 7 RSRs (isomers 5–10 in Figure ) that remain undiscovered. ,,− …”

The thermal decomposition of the benzyl radical in a heated micro-reactor. II. Pyrolysis of the tropyl radical

“…The PIE(m/z 65) resulting from the thermal cracking of the C 6 H 5 CH 2 radical indicates that there is little (or no) (CH 2 ) 2 C-C≡≡CH present in the pyrolysis of benzyl. It was also predicted by the metadynamics calculations 18 that the initial C 6 H 5 CH 2 radicals could isomerize to a pair of isomeric, dimethylene-cyclopentenyl radicals. The spectroscopic probes of Ref.…”

“…A recent paper 18 applied metadynamics to the pyrolysis of benzyl. These calculations suggested that both C 5 H 5 and its isomer, (CH 2 ) 2 C-C≡≡CH, are intermediates in the high temperature pyrolysis of benzyl.…”

“…In the early literature, the interconversion of benzyl to tropyl (C 6 H 5 CH 2 C 7 H 7 ) was considered by several groups. [9][10][11][12][13][14][15] However, all recent theoretical studies [16][17][18] find no pathways below 2000 K for the isomerization of benzyl to tropyl. A photoionization search for the isomerization of the C 6 H 5 CD 2 benzyl radical to the C 7 H 5 D 2 tropyl radical with tunable VUV radiation found no evidence for tropyl radical formation.…”