Formation Mechanisms of Naphthalene and Indene: From the Interstellar Medium to Combustion Flames

The article addresses the formation mechanisms of naphthalene and indene, which represent prototype polycyclic aromatic hydrocarbons (PAH) carrying two six-membered and one five- plus a six-membered ring. Theoretical studies of the relevant chemical reactions are overviewed in terms of their potential energy surfaces, rate constants, and product branching ratios; these data are compared with experimental measurements in crossed molecular beams and the pyrolytic chemical reactor emulating the extreme conditions in the interstellar medium (ISM) and the combustion-like environment, respectively. The outcome of the reactions potentially producing naphthalene and indene is shown to critically depend on temperature and pressure or collision energy and hence the reaction mechanisms and their contributions to the PAH growth can be rather different in the ISM, planetary atmospheres, and in combustion flames at different temperatures and pressures. Specifically, this paradigm is illustrated with new theoretical results for rate constants and product branching ratios for the reaction of phenyl radical with vinylacetylene. The analysis of the formation mechanisms of naphthalene and its derivatives shows that in combustion they can be produced via hydrogen-abstraction-acetylene-addition (HACA) routes, recombination of cyclopentadienyl radical with itself and with cyclopentadiene, the reaction of benzyl radical with propargyl, methylation of indenyl radical, and the reactions of phenyl radical with vinylacetylene and 1,3-butadiene. In extreme astrochemical conditions, naphthalene and dihydronaphthalene can be formed in the C₆H₅ + vinylacetylene and C₆H₅ + 1,3-butadiene reactions, respectively. Ethynyl-substituted naphthalenes can be produced via the ethynyl addition mechanism beginning with benzene (in dehydrogenated forms) or with styrene. The formation mechanisms of indene in combustion include the reactions of the phenyl radical with C₃H₄ isomers allene and propyne, reaction of the benzyl radical with acetylene, and unimolecular decomposition of the 1-phenylallyl radical originating from 3-phenylpropene, a product of the C₆H₅ + propene reaction, or from C₆H₅ + C₃H₅

A Theoretical Study of Pyrolysis of <i>exo</i>-Tetrahydrodicyclopentadiene and Its Primary and Secondary Unimolecular Decomposition Products

Theoretical calculations of the rate constants and product branching ratios in the pyrolysis of <i>exo</i>-tetrahydrodicyclopentadiene (JP-10) and its initial decomposition products at combustion-relevant pressures and temperatures were performed and compared to the experimental results from the recently reported molecular beam photoionization mass spectrometry study of the pyrolysis of JP-10 (Zhao et al. Phys. Chem. Chem. Phys. 2017, 19, 15780−15807). The results allow us to quantitatively assess the decomposition mechanisms of JP-10 by a direct comparison with the nascent product distributionincluding radicals and thermally labile closed-shell speciesdetected in the short-residence-time molecular beam photoionization mass spectrometry experiment. In accord with the experimental data, the major products predicted by the theoretical modeling include methyl radical (CH₃), acetylene (C₂H₂), vinyl radical (C₂H₃), ethyl radical (C₂H₅), ethylene (C₂H₄), allyl radical (C₃H₅), 1,3-butadiene (C₄H₆), cyclopentadienyl radical (C₅H₅), cyclopentadiene (C₅H₆), cyclopentenyl radical (C₅H₇), cyclopentene (C₅H₈), fulvene (C₆H₆), benzene (C₆H₆), toluene (C₇H₈), and 5-methylene-1,3-cyclohexadiene (C₇H₈). We found that ethylene, allyl radical, cyclopentadiene, and cyclopentenyl radical are significant products at all combustion-relevant conditions, whereas the relative yields of the other products depend on temperature. The most significant temperature trends predicted are increasing yields of the C2 and C4 species and decreasing yields of the C1, C6, and C7 groups with increasing temperature. The calculated pressure effect on the rate constant for the decomposition of JP-10 via initial C–H bond cleavages becomes significant at temperatures above 1500 K. The initially produced radicals will react away to form closed-shell molecules, such as ethylene, propene, cyclopentadiene, cyclopentene, fulvene, and benzene, which were observed as the predominant fragments in the long-residence-time experiment. The calculated rate constants and product branching ratios should prove useful to improve the existing kinetic models for the JP-10 pyrolysis

Bottom-Up Formation of Antiaromatic Cyclobutadiene (<i>c</i>‑C₄H₄) in Interstellar Ice Analogs

Antiaromatic cyclobutadiene (c-C4H4) is the simplest prototype of [n]annulenes and a key reactive intermediate with significant ring strain, serving as the model compound for antiaromatic systems in organic chemistry. Here, we report the first bottom-up formation of cyclobutadiene in low-temperature acetylene (C2H2) ices exposed to energetic electrons. Cyclobutadiene was isolated and detected in the gas phase upon sublimation utilizing vacuum ultraviolet photoionization reflectron time-of-flight mass spectrometry along with ultraviolet photolysis studies. These findings advance our fundamental understanding of the exotic chemistry and preparation of highly strained antiaromatic cycles through non-equilibrium chemistry in interstellar environments, thus affording a possible route for the formation of highly strained molecules such as the hitherto elusive tetrahedrane (C4H4). Because acetylene is a major product of the photolysis and radiolysis of methane (CH4) ice, an abundant component of interstellar ices, our results suggest that cyclobutadiene can likely be formed in methane-rich ices of cold molecular clouds

Bottom-Up Formation of Antiaromatic Cyclobutadiene (<i>c</i>‑C₄H₄) in Interstellar Ice Analogs

Antiaromatic cyclobutadiene (c-C4H4) is the simplest prototype of [n]annulenes and a key reactive intermediate with significant ring strain, serving as the model compound for antiaromatic systems in organic chemistry. Here, we report the first bottom-up formation of cyclobutadiene in low-temperature acetylene (C2H2) ices exposed to energetic electrons. Cyclobutadiene was isolated and detected in the gas phase upon sublimation utilizing vacuum ultraviolet photoionization reflectron time-of-flight mass spectrometry along with ultraviolet photolysis studies. These findings advance our fundamental understanding of the exotic chemistry and preparation of highly strained antiaromatic cycles through non-equilibrium chemistry in interstellar environments, thus affording a possible route for the formation of highly strained molecules such as the hitherto elusive tetrahedrane (C4H4). Because acetylene is a major product of the photolysis and radiolysis of methane (CH4) ice, an abundant component of interstellar ices, our results suggest that cyclobutadiene can likely be formed in methane-rich ices of cold molecular clouds

A Combined Crossed Beam and Ab Initio Investigation of the Gas Phase Reaction of Dicarbon Molecules (C₂; X¹Σ_g⁺/a³Π_u) with Propene (C₃H₆; X¹A′): Identification of the Resonantly Stabilized Free Radicals 1- and 3‑Vinylpropargyl

The crossed molecular beam reactions of dicarbon, C₂(X¹Σ_g⁺, a³Π_u), with propene (C₃H₆; X¹A′) and with the partially deuterated D3 counterparts (CD₃CHCH₂, CH₃CDCD₂) were conducted at collision energies of about 21 kJ mol^–1 under single collision conditions. The experimental data were combined with ab initio and statistical (RRKM) calculations to reveal the underlying reaction mechanisms. Both on the singlet and triplet surfaces, the reactions involve indirect scattering dynamics and are initiated by the addition of the dicarbon reactant to the carbon–carbon double bond of propene. These initial addition complexes rearrange via multiple isomerization steps leading ultimately via atomic hydrogen elimination from the former <i>methyl</i> and <i>vinyl</i> groups to the formation of 1-vinylpropargyl and 3-vinylpropargyl. Both triplet and singlet methylbutatriene species were identified as important reaction intermediates. On the singlet surface, the unimolecular decomposition of the reaction intermediates was found to be barrier-less, whereas on the triplet surface, tight exit transition states were involved. In combustion flames, both radicals can undergo a hydrogen-atom assisted isomerization leading ultimately to the thermodynamically most stable cyclopentadienyl isomer. Alternatively, in a third body process, a subsequent reaction of 1-vinylpropargyl or 3-vinylpropargyl radicals with the propargyl radical might yield to the formation of styrene (C₆H₅C₂H₃) in <i>an entrance barrier-less</i> reaction under combustion-like conditions. This presents a strong alternative to the formation of styrene via the reaction of phenyl radicals with ethylene, which is affiliated with an entrance barrier of about 10 kJ mol^–1

A VUV Photoionization Study of the Combustion-Relevant Reaction of the Phenyl Radical (C₆H₅) with Propylene (C₃H₆) in a High Temperature Chemical Reactor

We studied the reaction of phenyl radicals (C₆H₅) with propylene (C₃H₆) exploiting a high temperature chemical reactor under combustion-like conditions (300 Torr, 1200–1500 K). The reaction products were probed in a supersonic beam by utilizing tunable vacuum ultraviolet (VUV) radiation from the Advanced Light Source and recording the photoionization efficiency (PIE) curves at mass-to-charge ratios of <i>m</i>/<i>z</i> = 118 (C₉H₁₀⁺) and <i>m</i>/<i>z</i> = 104 (C₈H₈⁺). Our results suggest that the methyl and atomic hydrogen losses are the two major reaction pathways with branching ratios of 86 ± 10% and 14 ± 10%. The isomer distributions were probed by fitting the recorded PIE curves with a linear combination of the PIE curves of the individual C₉H₁₀ and C₈H₈ isomers. Styrene (C₆H₅C₂H₃) was found to be the <i>exclusive</i> product contributing to <i>m</i>/<i>z</i> = 104 (C₈H₈⁺), whereas 3-phenylpropene, <i>cis</i>-1-phenylpropene, and 2-phenylpropene with branching ratios of 96 ± 4%, 3 ± 3%, and 1 ± 1% could account for the signal at <i>m</i>/<i>z</i> = 118 (C₉H₁₀⁺). Although searched for carefully, no evidence of the bicyclic indane molecule could be provided. The reaction mechanisms and branching ratios are explained in terms of electronic structure calculations nicely agreeing with a recent crossed molecular beam study on this system

A Crossed Beam and ab Initio Investigation on the Formation of Boronyldiacetylene (HCCCC¹¹BO; <i>X</i>¹Σ⁺) via the Reaction of the Boron Monoxide Radical (¹¹BO; <i>X</i>²Σ⁺) with Diacetylene (C₄H₂; <i>X</i>¹Σ_g⁺)

The reaction dynamics of the boron monoxide radical (¹¹BO; <i>X</i>²Σ⁺) with diacetylene (C₄H₂; <i>X</i>¹Σ_g⁺) were investigated at a nominal collision energy of 17.5 kJ mol^–1 employing the crossed molecular beam technique and supported by <i>ab initio</i> and statistical (RRKM) calculations. The reaction is governed by indirect (complex forming) scattering dynamics with the boron monoxide radical adding with its boron atom to the carbon–carbon triple bond of the diacetylene molecule at one of the terminal carbon atoms without entrance barrier. This leads to a doublet radical intermediate (C₄H₂¹¹BO), which undergoes unimolecular decomposition through hydrogen atom emission from the C1 carbon atom via a tight exit transition state located about 18 kJ mol^–1 above the separated products. This process forms the hitherto elusive boronyldiacetylene molecule (HCCCC¹¹BO; <i>X</i>¹Σ⁺) in a bimolecular gas phase reaction under single collision conditions. The overall reaction was determined to be exoergic by 62 kJ mol^–1. The reaction dynamics are compared to the isoelectronic diacetylene (C₄H₂; <i>X</i>¹Σ_g⁺)–cyano radical (CN; <i>X</i>²Σ⁺) system studied previously in our group. The characteristics of boronyl-diacetylene and the boronyldiacetylene molecule (HCCCC¹¹BO; <i>X</i>¹Σ⁺) as well as numerous intermediates are reported for the first time

Radiation-Induced Formation of Chlorine Oxides and Their Potential Role in the Origin of Martian Perchlorates

Carbon dioxide (CO₂) rich chlorine-bearing ices were exposed to energetic electrons in laboratory simulation experiments to investigate the formation of chlorine oxides (Cl_<i>x</i>O_<i>y</i>) in the condensed phase on Mars. The radiolysis-induced synthesis of chlorine oxides (Cl_<i>x</i>O_<i>y</i>) was complementarily monitored online and in situ via infrared spectroscopy (IR) and quadrupole mass spectrometry (QMS). Three discrete chlorine oxides were identified: chorine dioxide (OClO), dichlorine monoxide (ClOCl), and chloryl chloride (ClClO₂). Higher irradiation doses support the facile production of ClO₃- and ClO₂-bearing high-order chlorine oxides. We attribute manifolds of chlorine oxides, as invoked herein, to the potential origin of perchlorates as found on Mars

Formation of Hydroxylamine in Low-Temperature Interstellar Model Ices

We irradiated binary ice mixtures of ammonia (NH₃) and oxygen (O₂) ices at astrophysically relevant temperatures of 5.5 K with energetic electrons to mimic the energy transfer process that occurs in the track of galactic cosmic rays. By monitoring the newly formed molecules <i>online</i> and <i>in situ</i> utilizing Fourier transform infrared spectroscopy complemented by temperature-programmed desorption studies with single-photon photoionization reflectron time-of-flight mass spectrometry, the synthesis of hydroxylamine (NH₂OH), water (H₂O), hydrogen peroxide (H₂O₂), nitrosyl hydride (HNO), and a series of nitrogen oxides (NO, N₂O, NO₂, N₂O₂, N₂O₃) was evident. The synthetic pathway of the newly formed species, along with their rate constants, is discussed exploiting the kinetic fitting of the coupled differential equations representing the decomposition steps in the irradiated ice mixtures. Our studies suggest the hydroxylamine is likely formed through an insertion mechanism of suprathermal oxygen into the nitrogen–hydrogen bond of ammonia at such low temperatures. An isotope-labeled experiment examining the electron-irradiated D3-ammonia–oxygen (ND₃–O₂) ices was also conducted, which confirmed our findings. This study provides clear, concise evidence of the formation of hydroxylamine by irradiation of interstellar analogue ices and can help explain the question how potential precursors to complex biorelevant molecules may form in the interstellar medium