Pyrolysis of fulvenallene (C7H6) and fulvenallenyl (C7H5): Theoretical kinetics and experimental product detection
Graphical abstract
Highlights
► Pyrolysis of fulvenallene and fulvenallenyl were studied theoretically and experimentally. ► Fulvenallene predominantly forms fulvenallenyl + H. ► Main pyrolysis products of fulvenallenyl predicted to be C3H3 + C4H2 and C5H3 isomers + C2H2. ► Spectrometry experiment detect products consistent with theory. ► Resonance stabilized radical fulvenallenyl is a relatively long-lived combustion intermediate.
Introduction
Fulvenallene, which is well-accepted as the most stable C7H6 isomer [1], has long been known to participate in a variety of gas-phase reactions [2], and has recently been directly observed in flames [3]. Fulvenallene has assumed a more prominent role in combustion chemistry since it was shown to be the primary decomposition product of the common resonance-stabilized radical benzyl [4], and it is now included in detailed kinetic models [5]. The related fulvenallenyl radical has been suggested as the C7H5 global minima [6], due to a large resonance stabilization energy arising from propargyl- and cyclopentadienyl-like structures, yet this species was unknown prior to 2009 [7]. Soon after, a C7H5 species thought to be fulvenallenyl was detected in toluene pyrolysis [8], before being unambiguously identified as the product of fulvenallene decomposition using threshold photoionization mass spectrometry coupled to a VUV synchrotron source, where the measured photoelectron spectrum was corroborated against theoretical simulations [9]. Doubly resonance-stabilized radicals, including fulvenallenyl, have been proposed as precursors to polycyclic aromatic hydrocarbons (PAHs) in flames [7], [10], in part due to their thermal stability and resistance to oxidation.
In order to incorporate fulvenallene and fulvenallenyl chemistry in gas-phase kinetic models in a meaningful way, we require rate constants and products for all important reactions that these species undergo. For combustion models this must include pyrolysis reactions. It is apparent that fulvenallenyl is the main decomposition product of fulvenallene, and that reaction proceeds with a barrier of around 80 kcal mol−1 [8], although uncertainties remain with respect to the kinetics and mechanism. The products and kinetics of fulvenallenyl radical pyrolysis are less well-understood. Zhang et al. [5a] proposed that fulvenallenyl would decompose to propargyl (C3H3) plus diacetylene (C4H2) and to a C5H3 isomer plus C2H2, by analogy to propargyl and cyclopentadienyl radical decomposition, with both reactions assumed to have an activation energy of 62.3 kcal mol−1. A comprehensive energy surface developed to study these reactions in reverse (i.e., C3H3 + C4H2 and i/n-C5H3 + C2H2) revealed that fulvenallenyl radical pyrolysis would require a significantly larger barrier (ca. 85 kcal mol−1). Here, we adapt the C7H5 energy surface of da Silva and Trevitt [6] to theoretically determine rate constants for thermal decomposition of the fulvenallenyl radical. Similar theoretical techniques are also applied to fulvenallene decomposition. We then proceed to analyze the experimental results of Steinbauer et al. [9] who studied phthalide pyrolysis using synchrotron VUV threshold photoionization mass spectrometry and find evidence for the proposed fulvenallenyl decomposition products.
Section snippets
Experimental and theoretical methods
Theoretical kinetics (master equation/RRKM theory) calculations are similar to [6], where they are described in detail. Briefly, C7H6 and C7H5 stationary points are characterized using the composite G3SX theoretical method [11] which is expected to provide average errors in reaction energies and barrier heights below 1 kcal mol−1 [11], [12]. Product yields as a function of time in the multiple-channel multiple-well thermally activated decomposition of fulvenallene and fulvenallenyl are obtained
Results and discussion
Rate constants have been calculated for the barrierless dissociation of fulvenallene to fulvenallenyl + H, as a function of temperature and pressure. The fulvenallenyl radical (+H) is formed from fulvenallene with a thermodynamic barrier of 79.6 kcal mol−1 at the G3SX level of theory, without an adiabatic transition state. This process is modeled here using a hindered Gorin transition state [16], as has been described previously for MultiWell [17]. The reverse reaction (C7H5 + H) is assumed to
Acknowledgements
The experimental work was carried out at the VUV beamline of the Swiss Light Source of the Paul Scherrer Institute. Experimental work was funded by the German Science Foundation (Fi575/7-2). MS acknowledges a fellowship by the ‘Fonds der Chemischen Industrie’. AJT acknowledges funding through the Australian Research Council (DP1094135). Computational resources provided by the Victorian Partnership for Advanced Computing (VPAC). We are grateful to Prof. Ingo Fischer for helpful discussions.
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Present address: Paul-Scherrer-Institute, CH-5232 Villigen PSI, Switzerland.