Gas-Phase Formation of 1,3,5,7-Cyclooctatetraene (C8H8) through Ring Expansion via the Aromatic 1,3,5-Cyclooctatrien-7-yl Radical (C8H9•) Transient.

Gas-phase 1,3,5,7-cyclooctatetraene (C8H8) and triplet aromatic 1,3,5,7-cyclooctatetraene (C8H8) were formed for the first time through bimolecular methylidyne radical (CH)-1,3,5-cycloheptatriene (C7H8) reactions under single-collision conditions on a doublet surface. The reaction involves methylidyne radical addition to the olefinic π electron system of 1,3,5-cycloheptatriene followed by isomerization and ring expansion to an aromatic 1,3,5-cyclooctatrien-7-yl radical (C8H9•). The chemically activated doublet radical intermediate undergoes unimolecular decomposition to 1,3,5,7-cyclooctatetraene. Substituted 1,3,5,7-cyclooctatetraene molecules can be prepared in the gas phase with hydrogen atom(s) in the 1,3,5-cycloheptatriene reactant being replaced by organic side groups. These findings are also of potential interest to organometallic chemists by expanding the synthesis of exotic transition-metal complexes incorporating substituted 1,3,5,7-cyclooctatetraene dianion (C8H82-) ligands and to untangle the unimolecular decomposition of chemically activated and substituted 1,3,5-cyclooctatrien-7-yl radical, eventually gaining a fundamental insight of their bonding chemistry, electronic structures, and stabilities.

Simultaneous Tunneling Control in Conformer-Specific Reactions.

We present here a new example of chemical reactivity governed by quantum tunneling, which also highlights the limitations of the classical theories. The syn and anti conformers of a triplet 2-formylphenylnitrene, generated in a nitrogen matrix, were found to spontaneously rearrange to the corresponding 2,1-benzisoxazole and imino-ketene, respectively. The kinetics of both transformations were measured at 10 and 20 K and found to be temperature-independent, providing clear evidence of concomitant tunneling reactions (heavy-atom and H-atom). Computations confirm the existence of these tunneling reaction pathways. Although the energy barrier between the nitrene conformers is lower than any of the observed reactions, no conformational interconversion was observed. These results demonstrate an unprecedented case of simultaneous tunneling control in conformer-specific reactions of the same chemical species. The product outcome is impossible to be rationalized by the conventional kinetic or thermodynamic control.

Breaking and Formation of Intramolecular Hydrogen Bonds in Dihydroxybenzaldehydes through UV-Induced Conformational Changes in a Low-Temperature Matrix.

Hydrogen bonding (HB) has been receiving attention from both experimental and theoretical researchers since its discovery in the 1920s due to its impact on many chemical and biological processes. However, despite the large number of investigations conducted on this topic, the nature of the HBs and, in particular, the estimation of intramolecular HB energies are still very active subjects of research. In this context, here we report a matrix isolation infrared spectroscopy study of 2,3-dihydroxybenzaldehyde (2,3-DHBA) and 2,4-dihydroxybenzaldehyde (2,4-DHBA), which contain two [one resonance-assisted HB (RAHB) and one conventional HB] and one (RAHB) intramolecular hydrogen bonds, respectively, in their most stable conformer. After isolation of the compounds in cryogenic (15 K) krypton matrices, ultraviolet irradiation led to the formation of higher-energy conformers (by a 180° rotation of the hydroxyl and aldehyde groups), which implies breaking of the intramolecular HBs initially existing in the isolated species and, in the case of 2,3-DHBA, to the formation of a new intramolecular HB. In this way, we were able to manipulate the structure of the molecules, allowing to characterize a diversity of intramolecular HBs in which the OH groups participate (from strong intramolecular RAHBs to weaker conventional HBs, and also no intramolecular HBs) through the corresponding vibrational signatures. The spectroscopic studies were complemented by natural bond orbital analysis and the molecular tailoring approach method, in order to estimate the relative intramolecular HB energies and explore the substitution effects on HB strength.

Directed gas phase preparation of ethynylallene (H2CCCHCCH; X1A') via the crossed molecular beam reaction of the methylidyne radical (CH; X2Π) with vinylacetylene (H2CCHCCH; X1A').

The gas-phase bimolecular reaction of the methylidyne (CH; X2Π) radical with vinylacetylene (H2CCHCCH; X1A') was conducted at a collision energy of 20.3 kJ mol-1 under single collision conditions exploiting the crossed molecular beam experimental results merged with ab initio electronic structure calculations and ab initio molecular dynamics (AIMD) simulations. The laboratory data reveal that the bimolecular reaction proceeds barrierlessly via indirect scattering dynamics through long-lived C5H5 reaction intermediate(s) ultimately dissociating to C5H4 isomers along with atomic hydrogen with the latter predominantly originating from the vinylacetylene reactant as confirmed by the isotopic substitution experiments in the D1-methylidyne-vinylacetylene reaction. Combined with ab initio calculations of the potential energy surface (PES) and statistical Rice-Ramsperger-Kassel-Marcus (RRKM) calculations, the experimental determined reaction energy of -146 ± 26 kJ mol-1 along with the distribution minimum of T(θ) at 90° and isotopic substitution experiments suggest ethynylallene (p1; ΔrG = -230 ± 4 kJ mol-1) as the dominant product. The ethynylallene (p1) may be formed with extensive rovibrational excitation, which would result in a lower maximum translational energy. Further, AIMD simulations reveal that the reaction dynamics leads to p1 (ethynylallene, 75%) plus atomic hydrogen with the dominant initial complex being i1 formed by methylidyne radical addition to the double CC bond in vinylacetylene. Overall, combining the crossed molecular beam experimental results with ab initio electronic structure calculations and ab initio molecular dynamics (AIMD) simulations, ethynylallene (p1) is expected to represent the dominant product in the reaction of the methylidyne (CH; X2Π) radical with vinylacetylene (H2CCHCCH; X1A').

Differential Tunneling-Driven and Vibrationally-Induced Reactivity in Isomeric Benzazirines.

Quantum mechanical tunneling of heavy-atoms and vibrational excitation chemistry are unconventional and scarcely explored types of reactivity. Once fully understood, they might bring new avenues to conduct chemical transformations, providing access to a new world of molecules or ways of exquisite reaction control. In this context, we present here the discovery of two isomeric benzazirines exhibiting differential tunneling-driven and vibrationally-induced reactivity, which constitute exceptional results for probing into the nature of these phenomena. The isomeric 6-fluoro- and 2-fluoro-4-hydroxy-2H-benzazirines (3-a and 3'-s) were generated in cryogenic krypton matrices by visible-light irradiation of the corresponding triplet nitrene 3 2-a, which was produced by UV-light irradiation of its azide precursor. The 3'-s was found to be stable under matrix dark conditions, whereas 3-a spontaneously rearranges (τ1/2 ∼64 h at 10 and 20 K) by heavy-atom tunneling to 3 2-a. Near-IR-light irradiation at the first OH stretching overtone frequencies (remote vibrational antenna) of the benzazirines induces the 3'-s ring-expansion reaction to a seven-member cyclic ketenimine, but the 3-a undergoes 2H-azirine ring-opening reaction to triplet nitrene 3 2-a. Computations demonstrate that 3-a and 3'-s have distinct reaction energy profiles, which explain the different experimental results. The spectroscopic direct measurement of the tunneling of 3-a to 3 2-a constitutes a unique example of an observation of a species reacting only by nitrogen tunneling. Moreover, the vibrationally-induced sole activation of the most favorable bond-breaking/bond-forming pathway available for 3-a and 3'-s provides pioneer results regarding the selective nature of such processes.

Gas-phase formation of silicon monoxide via non-adiabatic reaction dynamics and its role as a building block of interstellar silicates.

Silicon monoxide (SiO) is classified as a key precursor and fundamental molecular building block to interstellar silicate nanoparticles, which play an essential role in the synthesis of molecular building blocks connected to the Origins of Life. In the cold interstellar medium, silicon monoxide is of critical importance in initiating a series of elementary chemical reactions leading to larger silicon oxides and eventually to silicates. To date, the fundamental formation mechanisms and chemical dynamics leading to gas phase silicon monoxide have remained largely elusive. Here, through a concerted effort between crossed molecular beam experiments and electronic structure calculations, it is revealed that instead of forming highly-stable silicon dioxide (SiO2), silicon monoxide can be formed via a barrierless, exoergic, single-collision event between ground state molecular oxygen and atomic silicon involving non-adiabatic reaction dynamics through various intersystem crossings. Our research affords persuasive evidence for a likely source of highly rovibrationally excited silicon monoxide in cold molecular clouds thus initiating the complex chain of exoergic reactions leading ultimately to a population of silicates at low temperatures in our Galaxy.

A chemical dynamics study of the reaction of the methylidyne radical (CH, X2Π) with dimethylacetylene (CH3CCCH3, X1A1g).

The gas-phase reaction of the methylidyne (CH; X2Π) radical with dimethylacetylene (CH3CCCH3; X1A1g) was studied at a collision energy of 20.6 kJ mol-1 under single collision conditions with experimental results merged with ab initio calculations of the potential energy surface (PES) and ab initio molecule dynamics (AIMD) simulations. The crossed molecular beam experiment reveals that the reaction proceeds barrierless via indirect scattering dynamics through long-lived C5H7 reaction intermediate(s) ultimately dissociating to C5H6 isomers along with atomic hydrogen with atomic hydrogen predominantly released from the methyl groups as verified by replacing the methylidyne with the D1-methylidyne reactant. AIMD simulations reveal that the reaction dynamics are statistical leading predominantly to p28 (1-methyl-3-methylenecyclopropene, 13%) and p8 (1-penten-3-yne, 81%) plus atomic hydrogen with a significant amount of available energy being channeled into the internal excitation of the polyatomic reaction products. The dynamics are controlled by addition to the carbon-carbon triple bond with the reaction intermediates eventually eliminating a hydrogen atom from the methyl groups of the dimethylacetylene reactant forming 1-methyl-3-methylenecyclopropene (p28). The dominating pathways reveal an unexpected insertion of methylidyne into one of the six carbon-hydrogen single bonds of the methyl groups of dimethylacetylene leading to the acyclic intermediate, which then decomposes to 1-penten-3-yne (p8). Therefore, the methyl groups of dimethylacetylene effectively 'screen' the carbon-carbon triple bond from being attacked by addition thus directing the dynamics to an insertion process as seen exclusively in the reaction of methylidyne with ethane (C2H6) forming propylene (CH3C2H3). Therefore, driven by the screening of the triple bond, one propynyl moiety (CH3CC) acts in four out of five trajectories as a spectator thus driving an unexpected, but dominating chemistry in analogy to the methylidyne - ethane system.

Crossed Beam Experiments and Computational Studies of Pathways to the Preparation of Singlet Ethynylsilylene (HCCSiH; X1A'): The Silacarbene Counterpart of Triplet Propargylene (HCCCH; X3B).

Ethynylsilylene (HCCSiH; X1A') has been prepared in the gas phase through the elementary reaction of singlet dicarbon (C2) with silane (SiH4) under single-collision conditions. Electronic structure calculations reveal a barrierless reaction pathway involving 1,1-insertion of dicarbon into one of the silicon-hydrogen bonds followed by hydrogen migration to form the 3-sila-methylacetylene (HCCSiH3) intermediate. The intermediate undergoes unimolecular decomposition through molecular hydrogen loss to ethynylsilylene (HCCSiH; Cs; X1A'). The dicarbon-silane system defines a benchmark to explore the consequence of a single collision between the simplest "only carbon" molecule (dicarbon) with the prototype of a closed-shell silicon hydride (silane) yielding a nonclassical silacarbene, whose molecular geometry and electronic structure are quite distinct from the isovalent triplet propargylene (HCCCH; C2; 3B) carbon-counterpart. These organosilicon transients cannot be prepared through traditional organic, synthetic methods, thus opening up a versatile path to access the previously largely elusive class of silacarbenes.

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).

Nonadiabatic reaction dynamics to silicon monosulfide (SiS): A key molecular building block to sulfur-rich interstellar grains.

Sulfur- and silicon-containing molecules are omnipresent in interstellar and circumstellar environments, but their elementary formation mechanisms have been obscure. These routes are of vital significance in starting a chain of chemical reactions ultimately forming (organo) sulfur molecules-among them precursors to sulfur-bearing amino acids and grains. Here, we expose via laboratory experiments, computations, and astrochemical modeling that the silicon-sulfur chemistry can be initiated through the gas-phase reaction of atomic silicon with hydrogen sulfide leading to silicon monosulfide (SiS) via nonadiabatic reaction dynamics. The facile pathway to the simplest silicon and sulfur diatomic provides compelling evidence for the origin of silicon monosulfide in star-forming regions and aids our understanding of the nonadiabatic reaction dynamics, which control the outcome of the gas-phase formation in deep space, thus expanding our view about the life cycle of sulfur in the galaxy.

A Crossed Molecular Beams and Computational Study of the Formation of the Astronomically Elusive Thiosilaformyl Radical (HSiS, X2A').

The formation pathways to silicon- and sulfur-containing molecules are crucial to the understanding of silicon-sulfur chemistry in interstellar and circumstellar environments. While multiple silicon- and sulfur-containing species have been observed in deep space, their fundamental formation mechanisms are largely unknown. The crossed molecular beams technique combined with electronic structure and Rice-Ramsperger-Kassel-Marcus (RRKM) calculations was utilized to study the bimolecular reaction of atomic silicon (Si(3Pj)) with thiomethanol (CH3SH, X1A') leading to the thiosilaformyl radical (HSiS, X2A') via an exclusive methyl radical (CH3, X2A2″) loss via indirect scattering dynamics which involves barrierless addition and hydrogen migration in an overall exoergic reaction, indicating the possibility that HSiS can form in cold molecular clouds. The astronomically elusive thiosilaformyl radical may act as a tracer of an exotic silicon-sulfur chemistry to be deciphered toward, for example, the star-forming region SgrB2, thus leading to a better understanding of the formation of silicon-sulfur bonds in deep space.

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.

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.

Non-Adiabatic Reaction Dynamics in the Gas-Phase Formation of Phosphinidenesilylene, the Isovalent Counterpart of Hydrogen Isocyanide, under Single-Collision Conditions.

The phosphinidenesilylene (HPSi; X1A') molecule was prepared via a directed gas-phase synthesis in the bimolecular reaction of ground-state atomic silicon (Si; 3P) with phosphine (PH3; X1A1) under single-collision conditions. The chemical dynamics are initiated on the triplet surface via addition of a silicon atom to the non-bonding electron pair of phosphine, followed by non-adiabatic dynamics and surface hopping to the singlet manifold, accompanied by isomerization via atomic hydrogen shift and decomposition to phosphinidenesilylene (HPSi, X1A') along with molecular hydrogen. Statistical calculations predict that silylidynephosphine (HSiP, X1Σ+) is also formed, albeit with lower yields. The barrier-less route to phosphinidenesilylene opens up a multipurpose mechanism to access the hitherto obscure class of phosphasilenylidenes through silicon-phosphorus coupling via reactions of atomic silicon with alkylphosphines under single-collision conditions in the absence of successive reactions of the reaction products, which are not feasible to prepare by traditional synthetic routes.

Gas-phase synthesis of corannulene - a molecular building block of fullerenes.

Fullerenes (C60, C70) detected in planetary nebulae and carbonaceous chondrites have been implicated to play a key role in the astrochemical evolution of the interstellar medium. However, the formation mechanism of even their simplest molecular building block-the corannulene molecule (C20H10)-has remained elusive. Here we demonstrate via a combined molecular beams and ab initio investigation that corannulene can be synthesized in the gas phase through the reactions of 7-fluoranthenyl (C16H9˙) and benzo[ghi]fluoranthen-5-yl (C18H9˙) radicals with acetylene (C2H2) mimicking conditions in carbon-rich circumstellar envelopes. This reaction sequence reveals a reaction class in which a polycyclic aromatic hydrocarbon (PAH) radical undergoes ring expansion while simultaneously forming an out-of-plane carbon backbone central to 3D nanostructures such as buckybowls and buckyballs. These fundamental reaction mechanisms are critical in facilitating an intimate understanding of the origin and evolution of the molecular universe and, in particular, of carbon in our galaxy.

Low-temperature gas-phase formation of indene in the interstellar medium.

Polycyclic aromatic hydrocarbons (PAHs) are fundamental molecular building blocks of fullerenes and carbonaceous nanostructures in the interstellar medium and in combustion systems. However, an understanding of the formation of aromatic molecules carrying five-membered rings-the essential building block of nonplanar PAHs-is still in its infancy. Exploiting crossed molecular beam experiments augmented by electronic structure calculations and astrochemical modeling, we reveal an unusual pathway leading to the formation of indene (C9H8)-the prototype aromatic molecule with a five-membered ring-via a barrierless bimolecular reaction involving the simplest organic radical-methylidyne (CH)-and styrene (C6H5C2H3) through the hitherto elusive methylidyne addition-cyclization-aromatization (MACA) mechanism. Through extensive structural reorganization of the carbon backbone, the incorporation of a five-membered ring may eventually lead to three-dimensional PAHs such as corannulene (C20H10) along with fullerenes (C60, C70), thus offering a new concept on the low-temperature chemistry of carbon in our galaxy.

Gas-Phase Formation of C5H6 Isomers via the Crossed Molecular Beam Reaction of the Methylidyne Radical (CH; X2Π) with 1,2-Butadiene (CH3CHCCH2; X1A').

The bimolecular gas-phase reaction of the methylidyne radical (CH; X2Π) with 1,2-butadiene (CH2CCHCH3; X1A') was investigated at a collision energy of 20.6 kJ mol-1 under single collision conditions. Combining our laboratory data with high-level electronic structure calculations, we reveal that this bimolecular reaction proceeds through the barrierless addition of the methylidyne radical to the carbon-carbon double bonds of 1,2-butadiene leading to doublet C5H7 intermediates. These collision adducts undergo a nonstatistical unimolecular decomposition through atomic hydrogen elimination to at least the cyclic 1-vinyl-cyclopropene (p5/p26), 1-methyl-3-methylenecyclopropene (p28), and 1,2-bis(methylene)cyclopropane (p29) in overall exoergic reactions. The barrierless nature of this bimolecular reaction suggests that these cyclic C5H6 isomers might be viable targets to be searched for in cold molecular clouds like TMC-1.

Directed Gas Phase Formation of the Elusive Silylgermylidyne Radical (H3 SiGe, X2 A'').

The previously unknown silylgermylidyne radical (H3 SiGe; X2 A'') was prepared via the bimolecular gas phase reaction of ground state silylidyne radicals (SiH; X2 Π) with germane (GeH4 ; X1 A1 ) under single collision conditions in crossed molecular beams experiments. This reaction begins with the formation of a van der Waals complex followed by insertion of silylidyne into a germanium-hydrogen bond forming the germylsilyl radical (H3 GeSiH2 ). A hydrogen migration isomerizes this intermediate to the silylgermyl radical (H2 GeSiH3 ), which undergoes a hydrogen shift to an exotic, hydrogen-bridged germylidynesilane intermediate (H3 Si(µ-H)GeH); this species emits molecular hydrogen forming the silylgermylidyne radical (H3 SiGe). Our study offers a remarkable glance at the complex reaction dynamics and inherent isomerization processes of the silicon-germanium system, which are quite distinct from those of the isovalent hydrocarbon system (ethyl radical; C2 H5 ) eventually affording detailed insights into an exotic chemistry and intriguing chemical bonding of silicon-germanium species at the microscopic level exploiting crossed molecular beams.

A chemical dynamics study on the gas phase formation of thioformaldehyde (H2CS) and its thiohydroxycarbene isomer (HCSH).

Complex organosulfur molecules are ubiquitous in interstellar molecular clouds, but their fundamental formation mechanisms have remained largely elusive. These processes are of critical importance in initiating a series of elementary chemical reactions, leading eventually to organosulfur molecules-among them potential precursors to iron-sulfide grains and to astrobiologically important molecules, such as the amino acid cysteine. Here, we reveal through laboratory experiments, electronic-structure theory, quasi-classical trajectory studies, and astrochemical modeling that the organosulfur chemistry can be initiated in star-forming regions via the elementary gas-phase reaction of methylidyne radicals with hydrogen sulfide, leading to thioformaldehyde (H2CS) and its thiohydroxycarbene isomer (HCSH). The facile route to two of the simplest organosulfur molecules via a single-collision event affords persuasive evidence for a likely source of organosulfur molecules in star-forming regions. These fundamental reaction mechanisms are valuable to facilitate an understanding of the origin and evolution of the molecular universe and, in particular, of sulfur in our Galaxy.

Gas Phase Synthesis of the Elusive Trisilacyclopropyl Radical (Si3H5) via Unimolecular Decomposition of Chemically Activated Doublet Trisilapropyl Radicals (Si3H7).

The gas phase reaction of the simplest silicon-bearing radical silylidyne (SiH; X2Π) with disilane (Si2H6; X1A1g) was investigated in a crossed molecular beams machine. Combined with electronic structure calculations, our data reveal the synthesis of the previously elusive trisilacyclopropyl radical (Si3H5)-the isovalent counterpart of the cyclopropyl radical (C3H5)-along with molecular hydrogen via indirect scattering dynamics through long-lived, acyclic trisilapropyl (i-Si3H7) collision complex(es). Possible hydrogen-atom roaming on the doublet surface proceeds to molecular hydrogen loss accompanied by ring closure. The chemical dynamics are quite distinct from the isovalent methylidyne (CH)-ethane (C2H6) reaction, which leads to propylene (C3H6) radical plus atomic hydrogen but not to cyclopropyl (C3H5) radical plus molecular hydrogen. The identification of the trisilacyclopropyl radical (Si3H5) opens up preparative pathways for an unusual gas phase chemistry of previously inaccessible ring-strained (inorgano)silicon molecules as a result of single-collision events.