Fangtong Zhang

Low temperature formation of naphthalene and its role in the synthesis of PAHs (Polycyclic Aromatic Hydrocarbons) in the interstellar medium

Polycyclic aromatic hydrocarbons (PAHs) are regarded as key molecules in the astrochemical evolution of the interstellar medium, but the formation mechanism of even their simplest prototype—naphthalene (C10H8)—has remained an open question. Here, we show in a combined crossed beam and theoretical study that naphthalene can be formed in the gas phase via a barrierless and exoergic reaction between the phenyl radical (C6H5) and vinylacetylene (CH2 = CH-C ≡ CH) involving a van-der-Waals complex and submerged barrier in the entrance channel. Our finding challenges conventional wisdom that PAH-formation only occurs at high temperatures such as in combustion systems and implies that low temperature chemistry can initiate the synthesis of the very first PAH in the interstellar medium. In cold molecular clouds, barrierless phenyl-type radical reactions could propagate the vinylacetylene-mediated formation of PAHs leading to more complex structures like phenanthrene and anthracene at temperatures down to 10 K.

Formation of benzene in the interstellar medium

Polycyclic aromatic hydrocarbons and related species have been suggested to play a key role in the astrochemical evolution of the interstellar medium, but the formation mechanism of even their simplest building block—the aromatic benzene molecule—has remained elusive for decades. Here we demonstrate in crossed molecular beam experiments combined with electronic structure and statistical calculations that benzene (C6H6) can be synthesized via the barrierless, exoergic reaction of the ethynyl radical and 1,3-butadiene, C2H + H2CCHCHCH2 → C6H6 + H, under single collision conditions. This reaction portrays the simplest representative of a reaction class in which aromatic molecules with a benzene core can be formed from acyclic precursors via barrierless reactions of ethynyl radicals with substituted 1,3-butadiene molecules. Unique gas-grain astrochemical models imply that this low-temperature route controls the synthesis of the very first aromatic ring from acyclic precursors in cold molecular clouds, such as in the Taurus Molecular Cloud. Rapid, subsequent barrierless reactions of benzene with ethynyl radicals can lead to naphthalene-like structures thus effectively propagating the ethynyl-radical mediated formation of aromatic molecules in the interstellar medium.

Untangling the chemical evolution of Titan's atmosphere and surface–from homogeneous to heterogeneous chemistry

In this article, we first explored the chemical dynamics of simple diatomic radicals (dicarbon, methylidyne) utilizing the crossed molecular beams method. This versatile experimental technique can be applied to study reactions relevant to the atmospheres of planets and their moons as long as intense and stable supersonic beam sources of the reactant species exist. By focusing on reactions of dicarbon with hydrogen cyanide, we untangled the contribution of dicarbon in its singlet ground and first excited triplet states. These results were applied to understand and re-analyze the data of crossed beam reactions of the isoelectronic dicarbon plus acetylene reaction. Further, we investigated the interaction of ionizing radiation in form of energetic electrons with organic molecules ethane and propane sequestered on Titans surface. These experiments presented compelling evidence that even at irradiation exposures equivalent to about 44 years on Titans surface, aliphatic like organic residues can be produced on Titans surface with thicknesses up to 1.5 m. Finally, we investigated how Titans nascent chemical inventory can be altered by an external influx of matter as supplied by (micro)meteorites and possibly comets. For this, we simulated the ablation process in Titans atmosphere, which can lead to ground and electronically excited atoms of, for instance, the principal constituents of silicates like iron, silicon, and magnesium, in laboratory experiments. By ablating silicon species and seeding the ablated species in acetylene carrier gas, which also acts as a reactant, we produced organo silicon species, which were then photoionized utilizing tunable VUV radiation from the Advanced Light Source. In combination with electronic structure calculations, the structures and ionization energies of distinct organo-silicon species were elucidated.

PAH formation under single collision conditions: reaction of phenyl radical and 1,3-butadiene to form 1,4-dihydronaphthalene.

The crossed beam reactions of the phenyl radical (C(6)H(5), X(2)A(1)) with 1,3-butadiene (C(4)H(6), X(1)A(g)) and D6-1,3-butadiene (C(4)D(6), X(1)A(g)) as well as of the D5-phenyl radical (C(6)D(5), X(2)A(1)) with 2,3-D2-1,3-butadiene and 1,1,4,4-D4-1,3-butadiene were carried out under single collision conditions at collision energies of about 55 kJ mol(-1). Experimentally, the bicyclic 1,4-dihydronaphthalene molecule was identified as a major product of this reaction (58 ± 15%) with the 1-phenyl-1,3-butadiene contributing 34 ± 10%. The reaction is initiated by a barrierless addition of the phenyl radical to the terminal carbon atom of the 1,3-butadiene (C1/C4) to form a bound intermediate; the latter underwent hydrogen elimination from the terminal CH(2) group of the 1,3-butadiene molecule leading to 1-phenyl-trans-1,3-butadiene through a submerged barrier. The dominant product, 1,4-dihydronaphthalene, is formed via an isomerization of the adduct by ring closure and emission of the hydrogen atom from the phenyl moiety at the bridging carbon atom through a tight exit transition state located about 31 kJ mol(-1) above the separated products. The hydrogen atom was found to leave the decomposing complex almost parallel to the total angular momentum vector and perpendicularly to the rotation plane of the decomposing intermediate. The defacto barrierless formation of the 1,4-dihydronaphthalene molecule involving a single collision between a phenyl radical and 1,3-butadiene represents an important step in the formation of polycyclic aromatic hydrocarbons (PAHs) and their partially hydrogenated counterparts in combustion and interstellar chemistry.

Indene formation under single-collision conditions from the reaction of phenyl radicals with allene and methylacetylene--a crossed molecular beam and ab initio study.

Polycyclic aromatic hydrocarbons (PAHs) are regarded as key intermediates in the molecular growth process that forms soot from incomplete fossil fuel combustion. Although heavily researched, the reaction mechanisms for PAH formation have only been investigated through bulk experiments; therefore, current models remain conjectural. We report the first observation of a directed synthesis of a PAH under single-collision conditions. By using a crossed-molecular-beam apparatus, phenyl radicals react with C(3)H(4) isomers, methylacetylene and allene, to form indene at collision energies of 45 kJ mol(-1). The reaction dynamics supported by theoretical calculations show that both isomers decay through the same collision complex, are indirect, have long lifetimes, and form indene in high yields. Through the use of deuterium-substituted reactants, we were able to identify the reaction pathway to indene.

Formation of the Phenyl Radical [C6H5(X2A1)] under Single Collision Conditions: A Crossed Molecular Beam and ab Initio Study

Reactions of dicarbon molecules (C(2)) with C(4)H(6) isomers such as 1,3-butadiene represent a potential, but hitherto unnoticed, route to synthesize the first aromatic C(6) ring in hydrocarbon flames and in the interstellar medium where concentrations of dicarbon transient species are significant. Here, crossed molecular beams experiments of dicarbon molecules in their X(1)Sigma(g)(+) electronic ground state and in the first electronically excited a(3)Pi(u) state have been conducted with 1,3-butadiene and two partially deuterated counterparts (1,1,4,4-D4-1,3-butadiene and 2,3-D2-1,3-butadiene) at two collision energies of 12.7 and 33.7 kJ mol(-1). Combining these scattering experiments with electronic structure and RRKM calculations on the singlet and triplet C(6)H(6) surfaces, our investigation reveals that the aromatic phenyl radical is formed predominantly on the triplet surface via indirect scattering dynamics through a long-lived reaction intermediate. Initiated by a barrierless addition of triplet dicarbon to one of the terminal carbon atoms of 1,3-butadiene, the collision complex undergoes trans-cis isomerization followed by ring closure and hydrogen migration prior to hydrogen atom elimination, ultimately forming the phenyl radical. The latter step emits the hydrogen atom almost perpendicularly to the rotational plane of the decomposing intermediate and almost parallel to the total angular momentum vector. On the singlet surface, smaller contributions of phenyl radical could not be excluded; experiments with partially deuterated 1,3-butadiene indicate the formation of the thermodynamically less stable acyclic H(2)CCHCCCCH(2) isomer. This study presents the very first experimental evidence, contemplated by theoretical studies, that under single collision conditions an aromatic hydrocarbon molecule can be formed in a bimolecular gas-phase reaction via reaction of two acyclic molecules involving cyclization processes at collision energies highly relevant to combustion flames.

Elementary Reactions of Boron Atoms with Hydrocarbons—Toward the Formation of Organo-Boron Compounds

The chemical reactivity of atomic boron, B(Pj), with inorganic and organic molecules is a fascinating subject of research from the experimental1-35 and theoretical viewpoints.29,30,36-46 Atomic boron resides in the same row as carbon, nitrogen, and oxygen.47 Although the reaction dynamics and kinetics of the latter elements (C, N, O) have been studied in depth,48-53 an investigation of elementary reactions of boron atoms has remained sketchy so far. These studies are of significant interest due to the position of boron between metals and nonmetals. Apart from the fundamental importance of the elementary boron reactions,54,55 bimolecular collisions involving atomic boron are relevant to material sciences56 such as boron assisted nanotube growth57 and the production of boron-doped diamond thin films,58,59 chemical vapor deposition (CVD), high temperature combustion processes,60,61 interstellar chemistry,62 and the synthesis of novel organo-boron molecules.39-41,45,46,63-69

A CROSSED MOLECULAR BEAM, LOW-TEMPERATURE KINETICS, AND THEORETICAL INVESTIGATION OF THE REACTION OF THE CYANO RADICAL (CN) WITH 1,3-BUTADIENE (C4H6). A ROUTE TO COMPLEX NITROGEN-BEARING MOLECULES IN LOW-TEMPERATURE EXTRATERRESTRIAL ENVIRONMENTS

We present a joint crossed molecular beam and kinetics investigation combined with electronic structure and statistical calculations on the reaction of the ground-state cyano radical, CN(X 2Σ+), with the 1,3-butadiene molecule, H2CCHCHCH2(X 1 A g), and its partially deuterated counterparts, H2CCDCDCH2(X 1 A g) and D2CCHCHCD2(X 1 A g). The crossed beam studies indicate that the reaction proceeds via a long-lived C5H6N complex, yielding C5H5N isomer(s) plus atomic hydrogen under single collision conditions as the nascent product(s). Experiments with the partially deuterated 1,3-butadienes indicate that the atomic hydrogen loss originates from one of the terminal carbon atoms of 1,3-butadiene. A combination of the experimental data with electronic structure calculations suggests that the thermodynamically less favorable 1-cyano-1,3-butadiene isomer represents the dominant reaction product; possible minor contributions of less than a few percent from the aromatic pyridine molecule might be feasible. Low-temperature kinetics studies demonstrate that the overall reaction is very fast from room temperature down to 23 K with rate coefficients close to the gas kinetic limit. This finding, combined with theoretical calculations, indicates that the reaction proceeds on an entrance barrier-less potential energy surface (PES). This combined experimental and theoretical approach represents an important step toward a systematic understanding of the formation of complex, nitrogen-bearing molecules--here on the C5H6N PES--in low-temperature extraterrestrial environments. These results are compared to the reaction dynamics of D1-ethynyl radicals (C2D; X 2Σ+) with 1,3-butadiene accessing the isoelectronic C6H7 surface as tackled earlier in our laboratories.

A crossed molecular beams study of the reaction of the ethynyl radical (C2H(X2Σ+)) with allene (H2CCCH2(X1A1))

The crossed beams reaction of ground state ethynyl radicals, C(2)H(X(2)Sigma(+)), with allene, H(2)CCCH(2)(X(1)A(1)), was conducted under single collision conditions at a collision energy of 22.0 +/- 0.4 kJ mol(-1). The center-of-mass functions were combined with earlier ab initio calculations and revealed that the reaction was barrier-less, proceeded via indirect reaction dynamics through an addition of the ethynyl radical to the terminal carbon atom of the allene molecule, and was terminated by atomic hydrogen emission via a tight exit transition state to form the ethynylallene product. The overall reaction was found to be exoergic by 93 +/- 15 kJ mol(-1). Since the reaction is barrier-less, exoergic, and all transition states involved are located below the energy level of the separated reactants, the formation of ethynylallene is predicted to take place in low temperature atmospheres of planets and their satellites such as Titan and also in cold molecular clouds via the neutral-neutral reaction of ethynyl radicals with allene. Implications to interstellar chemistry and a comparison with the chemistry of the isoelectronic cyano radical, CN(X(2)Sigma(+)), are also presented.

ON THE FORMATION OF ORTHO-BENZYNE (o-C6H4) UNDER SINGLE COLLISION CONDITIONS AND ITS ROLE IN INTERSTELLAR CHEMISTRY

The elementary reaction of the D1-ethynyl radical (C2D) with vinylacetylene (C4H4) was studied under single collision conditions via crossed molecular beam experiments and electronic structure calculations. The results suggested that besides two acyclic isomers as predicted computationally, the ortho-benzyne (o-C6H4)—in its singly deuterated form—which is considered as an important intermediate in the formation of polycyclic aromatic hydrocarbons, can be formed in this process. This reaction was carried out at a collision energy of 40.9 kJ mol −1 , which is roughly equal to about 4000 K as present in photospheres of carbon stars and protoplanetary nebulae close to the central star. As the reaction is exoergic and involves no barriers higher than the separated reactants, our findings suggest that ortho-benzyne can also be formed in molecular clouds like TMC-1.