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

Gas-phase 1,3,5,7-cyclooctatetraene (CH) and triplet aromatic 1,3,5,7-cyclooctatetraene (CH) were formed for the first time through bimolecular methylidyne radical (CH)-1,3,5-cycloheptatriene (CH) 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 (CH). The chemically activated doublet radical intermediate undergoes unimolecular decomposition to 1,3,5,7-cyclooctatetraene.

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.

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 and 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).

Directed gas phase preparation of ethynylallene (HCCCHCCH; XA') the crossed molecular beam reaction of the methylidyne radical (CH; XΠ) with vinylacetylene (HCCHCCH; XA').

The gas-phase bimolecular reaction of the methylidyne (CH; XΠ) radical with vinylacetylene (HCCHCCH; XA') was conducted at a collision energy of 20.3 kJ mol under single collision conditions exploiting the crossed molecular beam experimental results merged with electronic structure calculations and molecular dynamics (AIMD) simulations. The laboratory data reveal that the bimolecular reaction proceeds barrierlessly indirect scattering dynamics through long-lived CH reaction intermediate(s) ultimately dissociating to CH 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.

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.

Gas-phase formation of silicon monoxide 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.

A chemical dynamics study of the reaction of the methylidyne radical (CH, XΠ) with dimethylacetylene (CHCCCH, XA).

The gas-phase reaction of the methylidyne (CH; XΠ) radical with dimethylacetylene (CHCCCH; XA) was studied at a collision energy of 20.6 kJ mol under single collision conditions with experimental results merged with calculations of the potential energy surface (PES) and molecule dynamics (AIMD) simulations. The crossed molecular beam experiment reveals that the reaction proceeds barrierless indirect scattering dynamics through long-lived CH reaction intermediate(s) ultimately dissociating to CH 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.

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

Ethynylsilylene (HCCSiH; XA') has been prepared in the gas phase through the elementary reaction of singlet dicarbon (C) with silane (SiH) 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 (HCCSiH) intermediate. The intermediate undergoes unimolecular decomposition through molecular hydrogen loss to ethynylsilylene (HCCSiH; C; XA').

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

The reaction of the D1-silylidyne radical (SiD; XΠ) with phosphine (PH; XA) 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 (SiPH, p3) via hydrogen deuteride (HD) loss (experiment: 80 ± 11%, RRKM: 68.

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.

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

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(P)) with thiomethanol (CHSH, XA') leading to the thiosilaformyl radical (HSiS, XA') via an exclusive methyl radical (CH, XA″) 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.

Chemical dynamics study on the gas-phase reaction of the D1-silylidyne radical (SiD; XΠ) with deuterium sulfide (DS) and hydrogen sulfide (HS).

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.

Combined Crossed Molecular Beams and Ab Initio Study of the Bimolecular Reaction of Ground State Atomic Silicon (Si; P) with Germane (GeH ; X A ).

The chemical dynamics of the elementary reaction of ground state atomic silicon (Si; P) with germane (GeH ; X A ) were unraveled in the gas phase under single collision condition at a collision energy of 11.8±0.3 kJ mol exploiting the crossed molecular beams technique contemplated with electronic structure calculations.

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

The phosphinidenesilylene (HPSi; XA') molecule was prepared via a directed gas-phase synthesis in the bimolecular reaction of ground-state atomic silicon (Si; P) with phosphine (PH; XA) 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, XA') along with molecular hydrogen. Statistical calculations predict that silylidynephosphine (HSiP, XΣ) is also formed, albeit with lower yields.

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

Fullerenes (C, C) 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 (CH)-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 (CH˙) and benzo[ghi]fluoranthen-5-yl (CH˙) radicals with acetylene (CH) mimicking conditions in carbon-rich circumstellar envelopes.

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 (CH)-the prototype aromatic molecule with a five-membered ring-via a barrierless bimolecular reaction involving the simplest organic radical-methylidyne (CH)-and styrene (CHCH) through the hitherto elusive methylidyne addition-cyclization-aromatization (MACA) mechanism.

Gas-Phase Formation of CH Isomers via the Crossed Molecular Beam Reaction of the Methylidyne Radical (CH; XΠ) with 1,2-Butadiene (CHCHCCH; XA').

The bimolecular gas-phase reaction of the methylidyne radical (CH; XΠ) with 1,2-butadiene (CHCCHCH; XA') was investigated at a collision energy of 20.6 kJ mol 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 CH intermediates.

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

The previously unknown silylgermylidyne radical (H SiGe; X A'') was prepared via the bimolecular gas phase reaction of ground state silylidyne radicals (SiH; X Π) with germane (GeH ; X A ) 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 (H GeSiH ). A hydrogen migration isomerizes this intermediate to the silylgermyl radical (H GeSiH ), which undergoes a hydrogen shift to an exotic, hydrogen-bridged germylidynesilane intermediate (H Si(μ-H)GeH); this species emits molecular hydrogen forming the silylgermylidyne radical (H SiGe).

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

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

The gas phase reaction of the simplest silicon-bearing radical silylidyne (SiH; XΠ) with disilane (SiH; XA) was investigated in a crossed molecular beams machine. Combined with electronic structure calculations, our data reveal the synthesis of the previously elusive trisilacyclopropyl radical (SiH)-the isovalent counterpart of the cyclopropyl radical (CH)-along with molecular hydrogen via indirect scattering dynamics through long-lived, acyclic trisilapropyl (i-SiH) collision complex(es). Possible hydrogen-atom roaming on the doublet surface proceeds to molecular hydrogen loss accompanied by ring closure.

Gas Phase Formation of Methylgermylene (HGeCH3).

The methylgermylene species (HGeCH ; X A') has been synthesized via the bimolecular gas phase reaction of ground state methylidyne radicals (CH) with germane (GeH ) under single collision conditions in crossed molecular beams experiments. Augmented by electronic structure calculations, this elementary reaction was found to proceed through barrierless insertion of the methylidyne radical in one of the four germanium-hydrogen bonds on the doublet potential energy surface yielding the germylmethyl (CH GeH ; X A') collision complex. This insertion is followed by a hydrogen shift from germanium to carbon and unimolecular decomposition of the methylgermyl (GeH CH ; X A') intermediate by atomic hydrogen elimination leading to singlet methylgermylene (HGeCH ; X A').

Directed Gas Phase Formation of Silene (H SiCH ).

The silene molecule (H SiCH ; X A ) has been synthesized under single collision conditions via the bimolecular gas phase reaction of ground state methylidyne radicals (CH) with silane (SiH ). Exploiting crossed molecular beams experiments augmented by high-level electronic structure calculations, the elementary reaction commenced on the doublet surface through a barrierless insertion of the methylidyne radical into a silicon-hydrogen bond forming the silylmethyl (CH SiH ; X A') complex followed by hydrogen migration to the methylsilyl radical (SiH CH ; X A'). Both silylmethyl and methylsilyl intermediates undergo unimolecular hydrogen loss to silene (H SiCH ; X A ).

Gas-Phase Synthesis of 3-Vinylcyclopropene via the Crossed Beam Reaction of the Methylidyne Radical (CH; X Π) with 1,3-Butadiene (CH CHCHCH ; X A ).

The crossed molecular beam reactions of the methylidyne radical (CH; X Π) with 1,3-butadiene (CH CHCHCH ; X A ) along with their (partially) deuterated counterparts were performed at collision energies of 20.8 kJ mol under single collision conditions. Combining our laboratory data with ab initio calculations, we reveal that the methylidyne radical may add barrierlessly to the terminal carbon atom and/or carbon-carbon double bond of 1,3-butadiene, leading to doublet C H intermediates with life times longer than the rotation periods.

A Barrierless Pathway Accessing the CH and CH Potential Energy Surfaces via the Elementary Reaction of Benzene with 1-Propynyl.

The crossed molecular beams reactions of the 1-propynyl radical (CHCC; XA) with benzene (CH; XA) and D6-benzene (CD; XA) were conducted to explore the formation of CH isomers under single-collision conditions. The underlying reaction mechanisms were unravelled through the combination of the experimental data with electronic structure and statistical RRKM calculations. These data suggest the formation of 1-phenyl-1-propyne (CHCCCH) via the barrierless addition of 1-propynyl to benzene forming a low-lying doublet CH intermediate that dissociates by hydrogen atom emission via a tight transition state.