Exploring the chemical dynamics of phenanthrene (C14H10) formation via the bimolecular gas-phase reaction of the phenylethynyl radical (C6H5CC) with benzene (C6H6).

The exploration of the fundamental formation mechanisms of polycyclic aromatic hydrocarbons (PAHs) is crucial for the understanding of molecular mass growth processes leading to two- and three-dimensional carbonaceous nanostructures (nanosheets, graphenes, nanotubes, buckyballs) in extraterrestrial environments (circumstellar envelopes, planetary nebulae, molecular clouds) and combustion systems. While key studies have been conducted exploiting traditional, high-temperature mechanisms such as the hydrogen abstraction-acetylene addition (HACA) and phenyl addition-dehydrocyclization (PAC) pathways, the complexity of extreme environments highlights the necessity of investigating chemically diverse mass growth reaction mechanisms leading to PAHs. Employing the crossed molecular beams technique coupled with electronic structure calculations, we report on the gas-phase synthesis of phenanthrene (C14H10)-a three-ring, 14π benzenoid PAH-via a phenylethynyl addition-cyclization-aromatization mechanism, featuring bimolecular reactions of the phenylethynyl radical (C6H5CC, X2A1) with benzene (C6H6) under single collision conditions. The dynamics involve a phenylethynyl radical addition to benzene without entrance barrier leading eventually to phenanthrene via indirect scattering dynamics through C14H11 intermediates. The barrierless nature of reaction allows rapid access to phenanthrene in low-temperature environments such as cold molecular clouds which can reach temperatures as low as 10 K. This mechanism constitutes a unique, low-temperature framework for the formation of PAHs as building blocks in molecular mass growth processes to carbonaceous nanostructures in extraterrestrial environments thus affording critical insight into the low-temperature hydrocarbon chemistry in our universe.

Elucidating the chemical dynamics of the elementary reactions of the 1-propynyl radical (CH3CC; X2A1) with 2-methylpropene ((CH3)2CCH2; X1A1).

Exploiting the crossed molecular beam technique, we studied the reaction of the 1-propynyl radical (CH3CC; X2A1) with 2-methylpropene (isobutylene; (CH3)2CCH2; X1A1) at a collision energy of 38 ± 3 kJ mol-1. The experimental results along with ab initio and statistical calculations revealed that the reaction has no entrance barrier and proceeds via indirect scattering dynamics involving C7H11 intermediates with lifetimes longer than their rotation period(s). The reaction is initiated by the addition of the 1-propynyl radical with its radical center to the π-electron density at the C1 and/or C2 position in 2-methylpropene. Further, the C7H11 intermediate formed from the C1 addition either emits atomic hydrogen or undergoes isomerization via [1,2-H] shift from the CH3 or CH2 group prior to atomic hydrogen loss preferentially leading to 1,2,4-trimethylvinylacetylene (2-methylhex-2-en-4-yne) as the dominant product. The molecular structures of the collisional complexes promote hydrogen atom loss channels. RRKM results show that hydrogen elimination channels dominate in this reaction, with a branching ratio exceeding 70%. Since the reaction of the 1-propynyl radical with 2-methylpropene has no entrance barrier, is exoergic, and all transition states involved are located below the energy of the separated reactants, bimolecular collisions are feasible to form trimethylsubstituted 1,3-enyne (p1) via a single collision event even at temperatures as low as 10 K prevailing in cold molecular clouds such as G+0.693. The formation of trimethylsubstituted vinylacetylene could serve as the starting point of fundamental molecular mass growth processes leading to di- and trimethylsubstituted naphthalenes via the HAVA mechanism.

Unconventional gas-phase synthesis of biphenyl and its atropisomeric methyl-substituted derivatives.

The biphenyl molecule (C12H10) acts as a fundamental molecular backbone in the stereoselective synthesis of organic materials due to its inherent twist angle causing atropisomerism in substituted derivatives and in molecular mass growth processes in circumstellar environments and combustion systems. Here, we reveal an unconventional low-temperature phenylethynyl addition-cyclization-aromatization mechanism for the gas-phase preparation of biphenyl (C12H10) along with ortho-, meta-, and para-substituted methylbiphenyl (C13H12) derivatives through crossed molecular beams and computational studies providing compelling evidence on their formation via bimolecular gas-phase reactions of phenylethynyl radicals (C6H5CC, X2A1) with 1,3-butadiene-d6 (C4D6), isoprene (CH2C(CH3)CHCH2), and 1,3-pentadiene (CH2CHCHCHCH3). The dynamics involve de-facto barrierless phenylethynyl radical additions via submerged barriers followed by facile cyclization and hydrogen shift prior to hydrogen atom emission and aromatization to racemic mixtures (ortho, meta) of biphenyls in overall exoergic reactions. These findings not only challenge our current perception of biphenyls as high temperature markers in combustion systems and astrophysical environments, but also identify biphenyls as fundamental building blocks of complex polycyclic aromatic hydrocarbons (PAHs) such as coronene (C24H12) eventually leading to carbonaceous nanoparticles (soot, grains) in combustion systems and in deep space thus affording critical insight into the low-temperature hydrocarbon chemistry in our universe.

Gas-Phase Synthesis of Coronene through Stepwise Directed Ring Annulation.

Molecular beam experiments together with electronic structure calculations provide the first evidence of a complex network of elementary gas-phase reactions culminating in the bottom-up preparation of the 24π aromatic coronene (C24H12) molecule─a representative peri-fused polycyclic aromatic hydrocarbon (PAH) central to the complex chemistry of combustion systems and circumstellar envelopes of carbon stars. The gas-phase synthesis of coronene proceeds via aryl radical-mediated ring annulations through benzo[e]pyrene (C20H12) and benzo[ghi]perylene (C22H12) involving armchair-, zigzag-, and arm-zig-edged aromatic intermediates, highlighting the chemical diversity of molecular mass growth processes to polycyclic aromatic hydrocarbons. The isomer-selective identification of five- to six-ringed aromatics culminating with the detection of coronene is accomplished through photoionization and is based upon photoionization efficiency curves along with photoion mass-selected threshold photoelectron spectra, providing a versatile concept of molecular mass growth processes via aromatic and resonantly stabilized free radical intermediates to two-dimensional carbonaceous nanostructures.

Competing Si2CH4-H2 and SiCH2-SiH4 Channels in the Bimolecular Reaction of Ground-State Atomic Carbon (C(3Pj)) with Disilane (Si2H6, X1A1g) under Single Collision Conditions.

The bimolecular gas-phase reaction of ground-state atomic carbon (C(3Pj)) with disilane (Si2H6, X1A1g) was explored under single-collision conditions in a crossed molecular beam machine at a collision energy of 36.6 ± 4.5 kJ mol-1. Two channels were observed: a molecular hydrogen elimination plus Si2CH4 (reaction 1) pathway and a silane loss channel along with the formation of SiCH2 (reaction 2), with branching ratios of 20 ± 3 and 80 ± 4%, respectively. Both channels involved indirect scattering dynamics via long-lived Si2CH6 reaction intermediate(s); the latter eject molecular hydrogen and silane in "molecular" elimination channels within the rotational plane of the fragmenting intermediate nearly perpendicularly to the total angular momentum vector. These molecular elimination channels are associated with tight exit transition states as reflected in a significant electron rearrangement as visible from the chemical bonding in the light reaction products molecular hydrogen and silane. Once these hydrogenated silicon-carbide clusters are formed within the inner envelope of carbon stars such as of IRC + 10216, the stellar wind can drive both Si2CH4 and SiCH2 to the outside sections of the envelope, where they can be photolyzed. This is of particular importance to unravel potential formation pathways to disilicon monocarbide (Si2C) observed recently in the circumstellar shell of IRC + 10216.

Exploring the Chemical Dynamics of Phenylethynyl Radical (C6H5CC; X2A1) Reactions with Allene (H2CCCH2; X1A1) and Methylacetylene (CH3CCH; X1A1).

The bimolecular gas-phase reactions of the phenylethynyl radical (C6H5CC, X2A1) with allene (H2CCCH2), allene-d4 (D2CCCD2), and methylacetylene (CH3CCH) were studied under single-collision conditions utilizing the crossed molecular beams technique and merged with electronic structure and statistical calculations. The phenylethynyl radical was found to add without an entrance barrier to the C1 carbon of the allene and methylacetylene reactants, resulting in doublet C11H9 collision complexes with lifetimes longer than their rotational periods. These intermediates underwent unimolecular decomposition via atomic hydrogen loss through tight exit transition states in facile radical addition─hydrogen atom elimination mechanisms forming predominantly 3,4-pentadien-1-yn-1-ylbenzene (C6H5CCCHCCH2) and 1-phenyl-1,3-pentadiyne (C6H5CCCCCH3) in overall exoergic reactions (-110 kJ mol-1 and -130 kJ mol-1) for the phenylethynyl-allene and phenylethynyl-methylacetylene systems, respectively. These barrierless reaction mechanisms mirror those of the ethynyl radical (C2H, X2Σ+) with allene and methylacetylene forming predominantly ethynylallene (HCCCHCCH2) and methyldiacetylene (HCCCCCH3), respectively, suggesting that in the aforementioned reactions the phenyl group acts as a spectator. These molecular mass growth processes are accessible in low-temperature environments such as cold molecular clouds (TMC-1) or Saturn's moon Titan, efficiently incorporating a benzene ring into unsaturated hydrocarbons.

Gas-Phase Preparation of Silyl Cyanide (SiH3CN) via a Radical Substitution Mechanism.

The silyl cyanide (SiH3CN) molecule, the simplest representative of a fully saturated silacyanide, was prepared in the gas phase under single-collision conditions via a radical substitution mechanism. The chemical dynamics were direct and revealed a pronounced backward scattering as a consequence of a transition state with a pentacoordinated silicon atom and almost colinear geometry of the attacking cyano radical and leaving hydrogen. Compared to the isovalent cyano (CN)-methane (CH4) system, the CN-SiH4 system dramatically reduces the energy of the transition state to silyl cyanide by nearly 100 kJ mol-1, which reveals a profound effect on the chemical bonding and reaction mechanism. In extreme high-temperature environments including circumstellar envelopes of IRC +10216, this versatile radical substitution mechanism may synthesize organosilicon molecules via reactions of silane with doublet radicals. Overall, this study provides rare insights into the exotic reaction mechanisms of main-group XIV elements in extreme environments and affords deeper insights into fundamental molecular mass growth processes involving silicon in our universe.

A crossed molecular beams and computational study on the unusual reactivity of banana bonds of cyclopropane (c-C3H6; ) through insertion by ground state carbon atoms (C(3Pj)).

The mechanism and chemical dynamics of the reaction of ground electronic state atomic carbon C(3Pj) with cyclopropane c-C3H6 have been explored by combining crossed molecular beams experiments with electronic structure calculations of the pertinent triplet C4H6 potential energy surface and statistical computations of product branching ratios under single-collision conditions. The experimental findings suggest that the reaction proceeds via indirect scattering dynamics through triplet C4H6 reaction intermediate(s) leading to C4H5 product(s) plus atomic hydrogen via a tight exit transition state, with the overall reaction exoergicity evaluated as 231 ± 52 kJ mol-1. The calculations indicate that C(3Pj) can easily insert into one of the three equivalent C-C 'banana' bonds of cyclopropane overcoming a low barrier of only 2 kJ mol-1 following the formation of a van der Waals reactant complex stabilized by 15 kJ mol-1. The carbon atom insertion into one of the six C-H bonds is also feasible via a slightly higher barrier of 5 kJ mol-1. These results highlight an unusual reactivity of cyclopropane's banana C-C bonds, which behave more like unsaturated C-C bonds with a π-character than saturated σ C-C bonds, which are known to be generally unreactive toward the ground electronic state atomic carbon such as in ethane (C2H6). The statistical theory predicts the overall product branching ratios at the experimental collision energy as 50% for 1-butyn-4-yl, 33% for 1,3-butadien-2-yl, i-C4H5, and 11% for 1,3-butadien-1-yl, n-C4H5, with i-C4H5 (230 kJ mol-1 below the reactants) favored by the C-C insertion providing the best match with the experimentally observed reaction exoergicity. The C(3Pj) + c-C3H6 reaction is predicted to be a source of C4H5 radicals under the conditions where its low entrance barriers can be overcome, such as in planetary atmospheres or in circumstellar envelopes but not in cold molecular clouds. Both i- and n-C4H5 can further react with acetylene eventually producing the first aromatic ring and hence, the reaction of the atomic carbon with c-C3H6 can be considered as an initial step toward the formation of benzene.

Gas-Phase Study of the Elementary Reaction of the D1-Ethynyl Radical (C2D; X2Σ+) with Propylene (C3H6; X1A') under Single-Collision Conditions.

The bimolecular gas-phase reactions of the D1-ethynyl radical (C2D; X2Σ+) with propylene (C3H6; X1A') and partially substituted D3-3,3,3-propylene (C2H3CD3; X1A') were studied under single collision conditions utilizing the crossed molecular beams technique. Combining our laboratory data with electronic structure and statistical calculations, the D1-ethynyl radical is found to add without barrier to the C1 and C2 carbons of the propylene reactant, resulting in doublet C5H6D intermediate(s) with lifetime(s) longer than their rotational period(s). These intermediates undergo isomerization and unimolecular decomposition via atomic hydrogen loss through tight exit transition states forming predominantly cis/trans-3-penten-1-yne ((HCC)CH═CH(CH3)) and, to a minor amount, 3-methyl-3-buten-1-yne ((HCC)C(CH3)═CH2) via overall exoergic reactions. Although the title reaction does not lead to the cyclopentadiene molecule (c-C5H6, X1A1), high-temperature environments can convert the identified acyclic C5H6 isomers through hydrogen atom assisted isomerization to cyclopentadiene (c-C5H6, X1A1). Since both the ethynyl radical and propylene reactants have been observed in cold interstellar environments such as TMC-1 and the reaction is exoergic and all barriers lie below the energy of the separated reactants, these C5H6 product isomers are predicted to form in those low-temperature regions.

Photodissociation Dynamics of Astrophysically Relevant Propyl Derivatives (C3H7X; X = CN, OH, HCO) at 157 nm Exploiting an Ultracompact Velocity Map Imaging Spectrometer: The (Iso)Propyl Channel.

The photodissociation dynamics of astrophysically relevant propyl derivatives (C3H7X; X = CN, OH, HCO) at 157 nm exploiting an ultracompact velocity map imaging (UVMIS) setup has been reported. The successful operation of UVMIS allowed the exploration of the 157 nm photodissociation of six (iso)propyl systems─n/i-propyl cyanide (C3H7CN), n/i-propyl alcohol (C3H7OH), and (iso)butanal (C3H7CHO)─to explore the C3H7 loss channel. The distinct center-of-mass translational energy distributions for the i-C3H7X (X= CN, OH, HCO) could be explained through preferential excitation of the low frequency C-H bending modes of the formyl moiety compared to the higher frequency stretching of the cyano and hydroxy moieties. Although the ionization energy of the n-C3H7 radical exceeds the energy of a 157 nm photon, C3H7+ was observed in the n-C3H7X (X = CN, OH, HCO) systems as a result of photoionization of vibrationally "hot" n-C3H7 fragments, photoionization of i-C3H7 after a hydrogen shift in vibrationally "hot" n-C3H7 radicals, and/or two-photon ionization. Our experiments reveal that at least the isopropyl radical (i-C3H7) and possibly the normal propyl radical (n-C3H7) should be present in the interstellar medium and hence searched for by radio telescopes.

Directed Gas-Phase Formation of Aminosilylene (HSiNH2; X1A'): The Simplest Silicon Analogue of an Aminocarbene, under Single-Collision Conditions.

The aminosilylene molecule (HSiNH2, X1A')-the simplest representative of an unsaturated nitrogen-silylene-has been formed under single collision conditions via the gas phase elementary reaction involving the silylidyne radical (SiH) and ammonia (NH3). The reaction is initiated by the barrierless addition of the silylidyne radical to the nonbonding electron pair of nitrogen forming an HSiNH3 collision complex, which then undergoes unimolecular decomposition to aminosilylene (HSiNH2) via atomic hydrogen loss from the nitrogen atom. Compared to the isovalent aminomethylene carbene (HCNH2, X1A'), by replacing a single carbon atom with silicon, a profound effect on the stability and chemical bonding of the isovalent methanimine (H2CNH)-aminomethylene (HNCH2) and aminosilylene (HSiNH2)-silanimine (H2SiNH) isomer pairs is shown; i.e., thermodynamical stabilities of the carbene versus silylene are reversed by 220 kJ mol-1. Hence, the isovalency of the main group XIV element silicon was found to exhibit little similarities with the atomic carbon revealing a remarkable effect not only on the reactivity but also on the thermochemistry and chemical bonding.

Combined Crossed Molecular Beams and Ab Initio Study of the Bimolecular Reaction of Ground State Atomic Silicon (Si; 3 P) with Germane (GeH4 ; X1 A1 ).

The chemical dynamics of the elementary reaction of ground state atomic silicon (Si; 3 P) with germane (GeH4 ; X1 A1 ) were unraveled in the gas phase under single collision condition at a collision energy of 11.8±0.3 kJ mol-1 exploiting the crossed molecular beams technique contemplated with electronic structure calculations. The reaction follows indirect scattering dynamics and is initiated through an initial barrierless insertion of the silicon atom into one of the four chemically equivalent germanium-hydrogen bonds forming a triplet collision complex (HSiGeH3 ; 3 i1). This intermediate underwent facile intersystem crossing (ISC) to the singlet surface (HSiGeH3 ; 1 i1). The latter isomerized via at least three hydrogen atom migrations involving exotic, hydrogen bridged reaction intermediates eventually leading to the H3 SiGeH isomer i5. This intermediate could undergo unimolecular decomposition yielding the dibridged butterfly-structured isomer 1 p1 (Si(µ-H2 )Ge) plus molecular hydrogen through a tight exit transition state. Alternatively, up to two subsequent hydrogen shifts to i6 and i7, followed by fragmentation of each of these intermediates, could also form 1 p1 (Si(µ-H2 )Ge) along with molecular hydrogen. The overall non-adiabatic reaction dynamics provide evidence on the existence of exotic dinuclear hydrides of main group XIV elements, whose carbon analog structures do not exist.

Chemical dynamics study on the gas-phase reaction of the D1-silylidyne radical (SiD; X2Π) with deuterium sulfide (D2S) and hydrogen sulfide (H2S).

The reactions of the D1-silylidyne radical (SiD; X2Π) with deuterium sulfide (D2S; X1A1) and hydrogen sulfide (H2S; X1A1) were conducted utilizing a crossed molecular beams machine under single collision conditions. The experimental work was carried out in conjunction with electronic structure calculations. The elementary reaction commences with a barrierless addition of the D1-silylidyne radical to one of the non-bonding electron pairs of the sulfur atom of hydrogen (deuterium) sulfide followed by possible bond rotation isomerization and multiple atomic hydrogen (deuterium) migrations. Unimolecular decomposition of the reaction intermediates lead eventually to the D1-thiosilaformyl radical (DSiS) (p1) and D2-silanethione (D2SiS) (p3) via molecular and atomic deuterium loss channels (SiD-D2S system) along with the D1-thiosilaformyl radical (DSiS) (p1) and D1-silanethione (HDSiS) (p3) through molecular and atomic hydrogen ejection (SiD-H2S system) via indirect scattering dynamics in barrierless and overall exoergic reactions. Our study provides a look into the complex dynamics of the silicon and sulfur chemistries involving multiple deuterium/hydrogen shifts and tight exit transition states, as well as insight into silicon- and sulfur-containing molecule formation pathways in deep space. Although neither of the non-deuterated species - the thiosilaformyl radical (HSiS) and silanethione (H2SiS) - have been observed in the interstellar medium (ISM) thus far, astrochemical models presented here predict relative abundances in the Orion Kleinmann-Low nebula to be sufficiently high enough for detection.

Directed gas-phase preparation of the elusive phosphinosilylidyne (SiPH2, X2A'') and cis/trans phosphinidenesilyl (HSiPH; X2A') radicals under single-collision conditions.

The reaction of the D1-silylidyne radical (SiD; X2Π) with phosphine (PH3; X1A1) was conducted in a crossed molecular beams machine under single collision conditions. Merging of the experimental results with ab initio electronic structure and statistical Rice-Ramsperger-Kassel-Marcus (RRKM) calculations indicates that the reaction is initiated by the barrierless formation of a van der Waals complex (i0) as well as intermediate (i1) formed via the barrierless addition of the SiD radical with its silicon atom to the non-bonding electron pair of phosphorus of the phosphine. Hydrogen shifts from the phosphorous atom to the adjacent silicon atom yield intermediates i2a, i2b, i3; unimolecular decomposition of these intermediates leads eventually to the formation of trans/cis-phosphinidenesilyl (HSiPH, p2/p4) and phosphinosilylidyne (SiPH2, p3) via hydrogen deuteride (HD) loss (experiment: 80 ± 11%, RRKM: 68.7%) and d-trans/cis-phosphinidenesilyl (DSiPH, p2'/p4') plus molecular hydrogen (H2) (experiment: 20 ± 7%, RRKM: 31.3%) through indirect scattering dynamics via tight exit transition states. Overall, the study reveals branching ratios of p2/p4/p2'/p4' (trans/cis HSiPH/DSiPH) to p3 (SiPH2) of close to 4 : 1. The present study sheds light on the complex reaction dynamics of the silicon and phosphorous systems involving multiple atomic hydrogen migrations and tight exit transition states, thus opening up a versatile path to access the previously elusive phosphinidenesilyl and phosphinosilylidyne doublet radicals, which represent potential targets of future astronomical searches toward cold molecular clouds (TMC-1), star forming regions (Sgr(B2)), and circumstellar envelopes of carbon rich stars (IRC + 10216).

Reaction Dynamics Study of the Molecular Hydrogen Loss Channel in the Elementary Reactions of Ground-State Silicon Atoms (Si(3P)) With 1- and 2-Methyl-1,3-Butadiene (C5H8).

The bimolecular gas-phase reactions involving ground-state atomic silicon (Si; 3P) and 1- and 2-methyl-1,3-butadiene were studied via crossed molecular beam experiments. Our data revealed indirect scattering dynamics through long-lived SiC5H8 collision complex(es) along with molecular hydrogen loss pathways, leading to facile formation of SiC5H6 isomer(s). We propose that the reactions of silicon with 1- and 2-methyl-1,3-butadiene possess reaction dynamics in an analogy to the silicon-1,3-butadiene system. This leads to cyclic methyl-substituted 2-methylene-1-silacyclobutene isomers via nonadiabatic reaction dynamics through intersystem crossing (ISC) from the triplet to the singlet surface in overall exoergic reactions through tight exit transition states and molecular hydrogen loss. Our study also suggests that the methyl group-although a spectator from the chemical viewpoint-can influence the disposal of the angular momentum into the rotational excitation of the final product.

Combined Experimental and Computational Study on the Reaction Dynamics of the D1-Silylidyne(SiD) - Silane (SiH4) System.

Small silicon hydrides have attracted extensive interest because of their role in the chemical evolution of circumstellar envelopes of evolved carbon stars and applications in surface growth processes and as transients in semiconductor manufacturing. Combined with electronic structure calculations, we demonstrate that monobridged silylidynesilylenes [(Si(µ-D)SiH2, Si(µ-H)SiHD, Si(µ-H)SiH2] and silylsilylidyne [H3SiSi, H2DSiSi], which are nearly isoenergetic, can be prepared via molecular hydrogen loss channels in the crossed molecular beam study of the reaction of D1-silylidyne (SiD; X2Π) with silane (SiH4; X1A1) in a crossed molecular beams machine. Compared to the dynamics of the isovalent methylidyne (CH) - methane (CH4) system, our study delivers a unique view at the intriguing isomerization processes and reaction dynamics of dinuclear silicon hydride transients, thus contributing to our knowledge on the chemical bonding of silicon hydrides at the molecular level.

Gas-phase preparation of the dibenzo[e,l]pyrene (C24H14) butterfly molecule via a phenyl radical-mediated ring annulation.

A high temperature phenyl-mediated addition-cyclization-dehydrogenation mechanism to form peri-fused polycyclic aromatic hydrocarbon (PAH) derivatives-illustrated through the formation of dibenzo[e,l]pyrene (C24H14)-is explored through a gas-phase reaction of the phenyl radical (C6H5˙) with triphenylene (C18H12) utilizing photoelectron photoion coincidence spectroscopy (PEPICO) combined with electronic structure calculations. Low-lying vibrational modes of dibenzo[e,l]pyrene exhibit out-of-plane bending and are easily populated in high temperature environments such as combustion flames and circumstellar envelopes of carbon stars, thus stressing dibenzo[e,l]pyrene as a strong target for far-IR astronomical surveys.

Low-Temperature Gas-Phase Formation of Methanimine (CH2NH; X1A')─the Simplest Imine─under Single-Collision Conditions.

The D1-methanimine molecule (CHDNH; X1A')─the simplest (deuterated) imine─has been prepared through the elementary reaction of the D1-methylidyne (CD; X2Π) with ammonia (NH3; X1A1) under single collision conditions. As a highly reactive species with a carbon-nitrogen double bond and a key building block of biomolecules such as amino acids and nucleobases, methanimine is of particular significance in coupling the nitrogen and carbon chemistries in the interstellar medium and in hydrocarbon-rich atmospheres of planets and their moons. However, the underlying formation mechanisms of methanimine in these extreme environments are still elusive. The directed, low-temperature gas-phase formation of D1-methanimine will deepen our fundamental understanding of low-temperature molecular growth processes via carbon-nitrogen bond coupling. Considering the recent detection of the interstellar D1-methylidyne radical, the investigation of the CD-NH3 system also suggests a promising pathway for future astronomical observations of D1-methanimine as a molecular tracer of gas phase deuterium enrichment in deep space.

Gas-phase formation of the resonantly stabilized 1-indenyl (C9H7•) radical in the interstellar medium.

The 1-indenyl (C9H7•) radical, a prototype aromatic and resonantly stabilized free radical carrying a six- and a five-membered ring, has emerged as a fundamental molecular building block of nonplanar polycyclic aromatic hydrocarbons (PAHs) and carbonaceous nanostructures in deep space and combustion systems. However, the underlying formation mechanisms have remained elusive. Here, we reveal an unconventional low-temperature gas-phase formation of 1-indenyl via barrierless ring annulation involving reactions of atomic carbon [C(3P)] with styrene (C6H5C2H3) and propargyl (C3H3•) with phenyl (C6H5•). Macroscopic environments like molecular clouds act as natural low-temperature laboratories, where rapid molecular mass growth to 1-indenyl and subsequently complex PAHs involving vinyl side-chained aromatics and aryl radicals can occur. These reactions may account for the formation of PAHs and their derivatives in the interstellar medium and carbonaceous chondrites and could close the gap of timescales of their production and destruction in our carbonaceous universe.

Gas phase synthesis of the C40 nano bowl C40H10.

Nanobowls represent vital molecular building blocks of end-capped nanotubes and fullerenes detected in combustion systems and in deep space such as toward the planetary nebula TC-1, but their fundamental formation mechanisms have remained elusive. By merging molecular beam experiments with electronic structure calculations, we reveal a complex chain of reactions initiated through the gas-phase preparation of benzocorannulene (C24H12) via ring annulation of the corannulenyl radical (C20H9•) by vinylacetylene (C4H4) as identified isomer-selectively in situ via photoionization efficiency curves and photoion mass-selected threshold photoelectron spectra. In silico studies provided compelling evidence that the benzannulation mechanism can be expanded to pentabenzocorannulene (C40H20) followed by successive cyclodehydrogenation to the C40 nanobowl (C40H10) - a fundamental building block of buckminsterfullerene (C60). This high-temperature pathway opens up isomer-selective routes to nanobowls via resonantly stabilized free-radical intermediates and ring annulation in circumstellar envelopes of carbon stars and planetary nebulae as their descendants eventually altering our insights of the complex chemistry of carbon in our Galaxy.