An experimental and theoretical investigation of the formation of C7H7 isomers in the bimolecular reaction of dicarbon molecules with 1,3-pentadiene
Graphical abstract
Introduction
Resonantly stabilized free radicals (RSFRs) and aromatic radicals (ARs) are considered key reaction intermediates in hydrocarbon flames and in extraterrestrial environments classifying them as important reaction intermediates involved in the mass growth processes and in the formations of polycyclic aromatic hydrocarbons (PAHs) [1], [2], [3], [4]. Due to this importance, the role of various C7H7 radicals – benzyl (C6H5CH2), o-, m-, p-tolyl (or 2-, 3-, and 4-tolyl) (C6H4CH3), and cycloheptatrienyl (C7H7) – have been explored computationally and experimentally [5], [6], [7]. Due to the potential key role of the benzyl radical, which is both aromatic and resonance-stabilized, reaction pathways to distinct C7H7 isomers have been explored theoretically [6], [8], [9]. The reaction of methylene (CH2) with the phenyl radical (C6H5), of acetylene (C2H2) with the cyclopentadienyl radical (c-C5H5) [10], of atomic hydrogen with fulvenallene (C7H6) and/or 1-ethynyl-cyclopentadiene (C7H6) [5], and of the propargyl radical (C3H3) with vinylacetylene (C4H4) have been proposed to access various points of the C7H7 potential energy surfaces (PESs). Alternatively, bimolecular reactions via C7H8 complex formation followed by hydrogen atom elimination might involve reactions of methyl (CH3) with the phenyl radical (C6H5) [8] and of methylene (CH2) with benzene (C6H6) [8]. Similarly, acetylene (C2H2) was predicted to react with cyclopentadiene (C5H6) via photochemically [2+2] or thermally induced [4+2] cycloaddition [11]. However, the formation of C7H7 isomers – among them the thermodynamically most stable benzyl (C6H5CH2) radical – via the bimolecular reaction of ubiquitous dicarbon molecules (C2) in their electronic ground (X1Σg+) and/or first excited (a3Πu) states with C5H8 isomers such as 1-methyl-1,3-butadiene (1,3-pentadiene, C5H8; X1A’) has never been explored. The dicarbon molecule is abundant in hydrocarbon flames and in the interstellar medium [12], [13] while the 1-methyl-1,3-butadiene can be formally derived from 1,3-butadiene (C4H6) by replacing the hydrogen atom at the C1 carbon atom by a methyl group. 1,3-Butadiene together with its C4H6 isomers 1,2-butadiene, 1-butyne, and 2-butyne is omnipresent in combustion flames such as of ethylene [14] and cyclohexane [15]. Distinct C5H8 isomers, including 1,3-pentadiene, have been probed in hydrocarbon flames such as of premixed methane/oxygen/cyclopentene [16] and ethylene/oxygen/argon systems [17]. The C7H7 species have been identified explicitly via mass spectrometric detection coupled with photoionization in premixed combustion flames of hydrogen/argon/benzene [18], hydrogen/argon/toluene [18], hydrogen/argon/cyclohexane [18], benzene/oxygen/argon [19] and toluene/oxygen/argon [20]. Photoionization efficiency curves suggest the benzyl radical to be the major C7H7 species. The benzyl radical is also suggested to be the major intermediate detected in the decomposition of benzylallene [21] and phenylacetic acid [22]. In combustion processes, the benzyl radicals may also form in the high temperature thermal decomposition of mono-substituted aromatics such as toluene, ethylbenzene, propylbenzene, and butylbenzene, which represent primary aromatic surrogates for gasoline, diesel, and jet fuel [23].
Since the C7H7 radicals can reach significant concentrations in combustion flames due to their inherent thermodynamical stability, understanding of their chemistry, in particular their formation and decomposition processes as well as bimolecular reactions, is essential for the development of accurate and predictive combustion engine models. Note that the dicarbon reactions are also relevant for carbon-rich circumstellar environments. For example, Dhanoa and Rawlings implicated dicarbon as a crucial building block in the synthesis of AR and RSFR; therefore, the reaction of dicarbon with 1-methyl-1,3-butadiene may provide a convenient pathway to synthesize C7H7 radicals in those environments [24]. However, the formation of these C7H7 radicals including the benzyl radical (C6H5CH2) via the bimolecular reaction of dicarbon with 1-methyl-1,3-butadiene has to be verified experimentally and computationally. The chemical evolution of macroscopic environments such as combustion flames and the interstellar medium can be best understood in terms of successive bimolecular reactions [10], [25], [26], [27]. This understanding must be achieved on the molecular level exploiting experiments conducted under single collision conditions, in which the nascent reaction products fly undisturbed toward the detector [28], [29]. Very recently, we have shown that the benzyl radical can be synthesized via reaction of dicarbon with 2-methyl-1,3-butadiene (isoprene) [30]. Herein, we report on the results of the crossed molecular beams reaction of dicarbon molecules with the 1-methyl-1,3-butadiene isomer accessing various collision complexes and chemically activated reactive intermediates on the singlet and triplet C7H8 surfaces, which then decompose to products including distinct C7H7 isomers.
Section snippets
Experimental methods
The experiments were conducted under single collision conditions utilizing a universal crossed molecular beam machine [28], [31]. Briefly, a pulsed supersonic dicarbon beam, C2 (X1Σg+, a3Πu), was generated via laser ablation of graphite. A graphite rod was ablated by focusing about 10 mJ per pulse of the output of a Spectra-Physics Quanta-Ray Pro 270 Nd:YAG laser operating at 30 Hz and 266 nm. The ablated species were then seeded in helium carrier gas (99.9999%, Airgas) introduced via a
Theoretical methods
Stationary points on the singlet and triplet C7H8 PES accessed by the reaction of dicarbon, C2(X1Σg+/a3Πu), with 1-methyl-1,3-butadiene, including intermediates, transition states, and possible products, were optimized at the hybrid density functional B3LYP level of theory [34] with the 6-311G∗∗ basis set. Vibrational frequencies were computed using the same B3LYP/6-311G∗∗ method and were used to obtain zero-point vibrational energy (ZPE) corrections. Relative energies of various species were
Laboratory data
Reactive scattering signal from the reactions of dicarbon (C2; 24 amu) with 1-methyl-1,3-butadiene (C5H8; 68 amu) was observed at m/z = 91 (C7H7+), m/z = 90 (C7H6+), and m/z = 89 (C7H5+) with data at m/z = 89 depicting the best signal-to-noise. The TOF spectra at these mass-to-charge rations were – after scaling – superimposable suggesting that signal at m/z = 90 and 89 originated from dissociative ionization of the C7H7 product in the electron impact ionizer of the detector. Therefore, our data suggest
Discussion
In case of complex, polyatomic reactants it is often beneficial to combine the crossed molecular beams data with results from electronic structure calculations (Figure 3, Figure 4). Let us attempt to identify the reaction product(s) first. Recall that based on the center-of-mass translational energy distribution, the reaction to form C7H7 isomers plus atomic hydrogen was found to be exoergic by 412 ± 52 kJ mol−1. A comparison of this data with the results from the electronic structure calculations
Conclusion
We performed the crossed molecular beam reaction of dicarbon, C2(X1Σg+, a3Πu), with 1,3-pentadiene (C5H8; X1A′) at a collision energy of 43 kJ mol−1, which accessed the triplet and singlet C7H8 PESs under single collision conditions. The experimental data were combined with ab initio and statistical calculations to reveal the underlying reaction mechanism and chemical dynamics. On both the singlet and triplet surfaces, the reactions involve indirect scattering dynamics and are initiated by the
Acknowledgments
This work was supported by the US Department of Energy, Basic Energy Sciences (Grants no. DE-FG02-03ER15411 to RIK and the University of Hawaii and DE-FG02-04ER15570 to A.M.M. at FIU).
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