RESUMEN
Sufficient experimental evidence has suggested that polycyclic aromatic hydrocarbons are the building blocks of carbonaceous nanostructures in combustion and circumstellar envelops of carbon-rich stars, but their fundamental formation mechanisms remain elusive. By exploring the reaction kinetics of phenylacetylene with 1-naphthyl/4-phenanthryl radicals, we provide compelling theoretical and experimental evidence for a novel and self-consistent hydrogen-abstraction phenylacetylene-addition (HAPaA) mechanism. HAPaA operates efficiently at both low and high temperatures, leading to the formation, expansion, and nucleation of peri-condensed aromatic hydrocarbons (PCAHs), which are otherwise difficult to synthesis via traditional hydrogen-abstraction acetylene/vinylacetylene-addition pathways. The HAPaA mechanism can be generalized to other α-alkynyl PCAHs and thus provides an alternative covalent bond bridge for PCAH combination via an acetylene linker. The proposed HAPaA mechanism may contribute toward a comprehensive understanding of soot formation, carbonaceous nanomaterials synthesis, and the origin and evolution of carbon in our galaxy.
RESUMEN
Polycyclic aromatic hydrocarbons (PAHs) play a crucial role in soot inception, interstellar evolution, and nanomaterial synthesis. Although several mechanisms, such as hydrogen-abstraction acetylene/vinylacetylene addition, have previously been proposed, PAH formation and growth are not yet fully understood. We propose an alternate PAH growth mechanism wherein propargyl radical reacts with butadiyne to form larger radicals containing newly fused aromatic rings. Butadiyne is an important intermediate in hydrocarbon oxidation and carbon rich stars, while propargyl is one of the most important resonantly stabilized radicals that persists for long times. Our proposed mechanism is validated by quantum chemical calculations, elementary reaction experiments, laminar flame analysis, and kinetic modeling. Our findings challenge the conventional wisdom that radical site regeneration, being central to PAH growth, requires sequential hydrogen elimination and/or abstraction. In our proposed mechanism, PAH growth does not depend on abundant free radical consumption, and could, therefore, help explain carbonaceous nanoparticle coalescence in radical-deficient reaction environments.
RESUMEN
RATIONALE: Aromatic hydrocarbons play an important role in the formation and growth of polycyclic aromatic hydrocarbon (PAH) and soot particles. Measurements of their absolute photoionization cross-sections (PICSs), that benefit the quantitative investigation of mass spectrometry, are still lacking, however. METHODS: PICSs of some aromatic hydrocarbons were measured with tunable synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). Nitric oxide and benzene were chosen as standard references for PICS calibration, since their photoionization cross-sections are well documented in the literature. Binary liquid mixtures of the investigated molecules and their specific solvents were used in the measurements. RESULTS: The investigated aromatics include naphthalene, phenanthrene, 1-methylnaphthalene, indene, 2-/3-/4-methylphenylacetylene, 2-methylindene, diphenylacetylene, 1-/2-ethynylnaphthalene and acenaphthylene. Photo-induced fragments from the molecules were also observed with increasing photon energy. CONCLUSIONS: Based on our measurements and literature data, PICSs of most aromatic molecules have very similar values beyond their ionization energies. However, molecules that contain the phenylacetylene structure have PICSs higher than other aromatics.
RESUMEN
Pyrolytic kinetics of indene was studied in a flow reactor at 30 and 760 Torr. Indene and its decomposition products, as well as polycyclic aromatic hydrocarbons (PAHs), were measured with synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). Five literature models were selected to reproduce the experimental data and analyze the reaction kinetics of indene. The experimental and predicted results illustrate that an indenyl radical is the dominant decomposition intermediate and also the main contributor to the further growth of aromatic rings in the pyrolysis of indene. The indene consumption process needs further precise characterization, especially the subsequent dissociation reactions of indanyl and indenyl radicals. A self-recombination reaction of the indenyl radical and the combination reactions between indenyl and other radicals are found to be necessary for the efficient formation of large PAHs. The absence of these pathways leads to the underprediction of experimental measurements. In contrast, literature models adopting indenyl global reactions for PAH formation generally overestimate the system reactivity. Proper radical combination pathways proposed in a future model should consider not only the PAH formation efficiency but also its impact on system reactivity.
RESUMEN
Alkyl hydroperoxides are found to be important intermediates in the combustion and oxidation processes of hydrocarbons. However, studies of ethyl hydroperoxide (CH(3)CH(2)OOH) are limited. In this work, kinetics and mechanisms for unimolecular decomposition of CH(3)CH(2)OOH have been investigated. The potential energy surface of decomposition reactions have first been predicted at the CCSD(T)/6-311+G(3df,2p)//B3LYP/6-311G(d,p) level. The results show that the formation of CH(3)CH(2)O + OH via O-O direct bond dissociation is dominant, the branching ratio of which is over 99% in the whole temperature range from 300 to 1000 K, and its rate constant can be expressed as k1 = 9.26 × 10(52)T(-11.91)exp(-26879/T) s(-1) at 1 atm. The rate constants of the reaction CH(3)CH(2)OOH â CH(3)CH(2)O + OH at different temperatures and pressures have been calculated, which can help us to comprehend the reactions of CH(3)CH(2)OOH at experimental conditions.