The Peculiar Kinetics of the Reaction between Acetylene and the Cyclopentadienyl Radical

Abstract

The cyclopentadienyl radical (cC(5)H(5)) is a fascinating molecule characterized by several peculiar properties, such as its high internal symmetry and resonance enhanced stability. This makes cC(5)H(5) one of the most abundant radicals present in high temperature gaseous environments, such as flames. Therefore it is generally considered an interesting candidate as the starting point of reaction pathways leading to the formation of polycyclic aromatic hydrocarbons (PAH) and soot in combustion processes. However, known reaction pathways are not able to explain some recent experimental findings concerning the rapid conversion of cC(5)H(5) into C(7)H(7) and C(9)H(8) in the presence of acetylene. In this work, we used ab initio calculations and quantum Rice-Ramsperger-Kassel (QRRK) theory to investigate the cC(5)H(5) + C(2)H(2) reaction kinetics. We found that cC(5)H(5) can add acetylene to form, through a fast and not previously known reaction, the heptatrienyl radical (cC(7)H(7)), which, in many ways, can be considered the superior homologue of cC(5)H(5). The calculated reaction kinetic constant is (2.2 x 10(11))exp(-6440/T(K)) cm(3) mol(-1) s(-1) and is in good agreement with experimental data, while that of the inverse process is (4.2 x 10(16))T(-1) exp(-30 850/T(K)) s(-1). In a successive reaction, cC(7)H(7) can add a second acetylene molecule to form indene, cC(9)H(8), and H. The forward and backward kinetic constants are (6.6 x 10(11))exp(-10 080/T(K)) and (4.2 x 10(14))exp(-27 300/T(K)) cm(3) mol(-1) s(-1), respectively. These two successive reactions, leading from a single C5 cycle to a bicyclic C5-C6 species, represent a new PAH growth mechanism, characterized by a C5-C7 ring enlargement reaction.

H- and D-atom formation from the pyrolysis of C6H5CH2Br and C6H5CD2Br: Implications for high-temperature benzyl decomposition

“…These products can form from the dissociation or recombination of products/fragments of benzyl decomposition [12] or from any one of the numerous C 7 H 7 intermediates. Energetic arguments favor cycloheptatrienyl to be a likely C 7 H 7 intermediate that can form these products by isomerization to a bicyclic intermediate followed by rapid dissociation [29]. The activation energy of about 47 kcal/mol (Eq.…”

Recent contributions of flame-sampling molecular-beam mass spectrometry to a fundamental understanding of combustion chemistry

“…High-level ab initio calculations combined with flame-sampling PI-MBMS employing synchrotron-generated VUV photons were used to identify C 7 H 6 and C 7 H 8 isomers [118]. The presence of the fivemembered rings C 5 H 5 CCH or C 5 H 4 CCH 2 and of cycloheptatriene points towards the existence of C 5 -C 7 ring enlargement reactions as proposed by Fascella et al [187]. From the chemical point of view, the C 7 H 6 isomers are likely to be formed by the reactions summarized schematically in Fig.…”

“…(b) In the hydrogen-rich environment of a fuel-rich flame it is furthermore conceivable that hydrogen atom migration around the cyclopentadiene ring takes place to convert the initial isomer into the most stable -CH 2 CHCHCHC(CCH)-tautomeric species. (c) The initial C 7 H 7 radical can undergo isomerization to form the resonantly stabilized cycloheptatrienyl and benzyl radicals, subsequently forming cycloheptatriene and toluene [187,188]. The C 7 H 7 radical species are quite stable, and thus they are good precursor candidates for forming multi-ring structures, including indene and naphthalene [150,173,188].…”

Time- and Isomer-Resolved Measurements of Sequential Addition of Acetylene to the Propargyl Radical

“…It has been assumed that C 5 H 5 in R1 is the 2,4-cyclopentadienyl RSR (c-C 5 H 5 ), as it is by far the lowest-energy C 5 H 5 isomer; however, highly stable acyclic C 5 H 5 RSRs also exist. 11 It is also generally assumed that bz-C 7 H 7 is the most prominent C 7 H 7 isomer present in combustion environments, although recent calculations suggest that the most favored C 7 H 7 isomer from c-C 5 H 5 + C 2 H 2 is the cyclic tropyl RSR (2,4,6-cycloheptatrienyl, tp-C 7 H 7 ), with pathways to bz-C 7 H 7 becoming important only at T > 2000 K. 12,13 Other C 7 H 7 RSRs such as the vinylcyclopentadienyl radical (vcp-C 7 H 7 ) could also play a role in R2 and R3. 13 To resolve the isomeric form(s) of the species in this reaction sequence, we use time-resolved PI mass spectrometry similar to Knyazev and Slagle, but we incorporate tunable ionizing vacuum ultraviolet (VUV) radiation to distinguish isomers.…”