Exploring the Chemical Dynamics of Phenylethynyl Radical (CHCC; XA) Reactions with Allene (HCCCH; XA) and Methylacetylene (CHCCH; XA).

The bimolecular gas-phase reactions of the phenylethynyl radical (CHCC, XA) with allene (HCCCH), allene- (DCCCD), and methylacetylene (CHCCH) 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 CH 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 (CHCCCHCCH) and 1-phenyl-1,3-pentadiyne (CHCCCCCH) in overall exoergic reactions (-110 kJ mol and -130 kJ mol) for the phenylethynyl-allene and phenylethynyl-methylacetylene systems, respectively.

Quantum Tunneling Mediated Low-Temperature Synthesis of Interstellar Hemiacetals.

Sugars and sugar-related molecules are ubiquitous in carbonaceous meteorites and in star-forming regions, but the underlying mechanisms of their formation have remained largely elusive. Herein, we report an unconventional synthesis of the hemiacetal, (/)-1-methoxyethanol (CHOCH(OH)CH), through quantum tunneling mediated reactions in low-temperature interstellar model ices composed of acetaldehyde (CHCHO) and methanol (CHOH). The detection of racemic 1-methoxyethanol through a bottom-up synthesis from simple, abundant precursor molecules within interstellar ices represents a vital starting point to the formation of complex interstellar hemiacetals.

Synthesis of interstellar propen-2-ol (CHC(OH)CH) - the simplest enol tautomer of a ketone.

Enols - tautomers of ketones or aldehydes - are anticipated to be ubiquitous in the interstellar medium and play a key role in the formation of complex organic molecules in deep space, but their fundamental formation mechanisms have remained largely elusive as of now. Here we present a combined experimental and computational study demonstrating the first preparation of propen-2-ol (CHC(OH)CH) and its isomer methyl vinyl ether (CHOCHCH) in low-temperature acetone (CHCOCH) ices upon exposure to energetic electrons. Propen-2-ol is the simplest enol tautomer of a ketone.

Unconventional gas-phase preparation of the prototype polycyclic aromatic hydrocarbon naphthalene (CH) the reaction of benzyl (CH) and propargyl (CH) radicals coupled with hydrogen-atom assisted isomerization.

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in the interstellar medium and in meteorites such as Murchison and Allende and signify the missing link between resonantly stabilized free radicals and carbonaceous nanoparticles (soot particles, interstellar grains). However, the predicted lifetime of interstellar PAHs of some 10 years imply that PAHs should not exist in extraterrestrial environments suggesting that key mechanisms of their formation are elusive. Exploiting a microchemical reactor and coupling these data with computational fluid dynamics (CFD) simulations and kinetic modeling, we reveal through an isomer selective product detection that the reaction of the resonantly stabilized benzyl and the propargyl radicals synthesizes the simplest representative of PAHs - the 10π Hückel aromatic naphthalene (CH) molecule - the novel Propargyl Addition-BenzAnnulation (PABA) mechanism.

Formation of Thioformic Acid (HCOSH)─The Simplest Thioacid─in Interstellar Ice Analogues.

Since the observation of the first sulfur-containing molecule, carbon monosulfide (CS), in the interstellar medium (ISM) half a century ago, sulfur-bearing species have attracted great attention from the astrochemistry, astrobiology, and planetary geology communities. Nevertheless, it is still not clear in which forms most of the sulfur resides in molecular clouds, an unsolved problem referred to as "sulfur depletion". Reported herein is the formation of thioformic acid (HCOSH)─the simplest thioacid─in interstellar ice analogues containing carbon monoxide (CO) and hydrogen sulfide (HS) at 5 K.

Gas-phase synthesis of racemic helicenes and their potential role in the enantiomeric enrichment of sugars and amino acids in meteorites.

The molecular origins of homochirality on Earth is not understood well, particularly how enantiomerically enriched molecules of astrobiological significance like sugars and amino acids might have been synthesized on icy grains in space preceding their delivery to Earth. Polycyclic aromatic hydrocarbons (PAHs) identified in carbonaceous chondrites could have been processed in molecular clouds by circularly polarized light prior to the depletion of enantiomerically enriched helicenes onto carbonaceous grains resulting in chiral islands. However, the fundamental low temperature reaction mechanisms leading to racemic helicenes are still unknown.

Gas-Phase Preparation of Subvalent Germanium Monoxide (GeO, X) via Non-Adiabatic Reaction Dynamics in the Exit Channel.

The subvalent germanium monoxide (GeO, XΣ) molecule has been prepared via the elementary reaction of atomic germanium (Ge, P) and molecular oxygen (O, XΣ) with each reactant in its electronic ground state by means of single-collision conditions. The merging of electronic structure calculations with crossed beam experiments suggests that the formation of germanium monoxide (GeO, XΣ) commences on the singlet surface through unimolecular decomposition of a linear singlet collision complex (GeOO, , C, Σ) via intersystem crossing (ISC) yielding nearly exclusively germanium monoxide (GeO, XΣ) along with atomic oxygen in its electronic ground state [, O(P)]. These results provide a sophisticated reaction mechanism of the germanium-oxygen system and demonstrate the efficient "heavy atom effect" of germanium in ISC yielding (nearly) exclusive singlet germanium monoxide and triplet atomic oxygen compared to similar systems (carbon dioxide and dinitrogen monoxide), in which non-adiabatic reaction dynamics represent only minor channels.

Gas-Phase Study of the Elementary Reaction of the D1-Ethynyl Radical (CD; XΣ) with Propylene (CH; XA') under Single-Collision Conditions.

The bimolecular gas-phase reactions of the D1-ethynyl radical (CD; XΣ) with propylene (CH; XA') and partially substituted D3-3,3,3-propylene (CHCD; XA') 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 CHD 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 /-3-penten-1-yne ((HCC)CH═CH(CH)) and, to a minor amount, 3-methyl-3-buten-1-yne ((HCC)C(CH)═CH) via overall exoergic reactions.

Unconventional excited-state dynamics in the concerted benzyl (CH) radical self-reaction to anthracene (CH).

Polycyclic aromatic hydrocarbons (PAHs) are prevalent in deep space and on Earth as products in combustion processes bearing direct relevance to energy efficiency and environmental remediation. Reactions between hydrocarbon radicals in particular have been invoked as critical molecular mass growth processes toward cyclization leading to these PAHs. However, the mechanism of the formation of PAHs through radical - radical reactions are largely elusive.

Formation of Benzene and Naphthalene through Cyclopentadienyl-Mediated Radical-Radical Reactions.

Resonantly stabilized free radicals (RSFRs) have been contemplated as fundamental molecular building blocks and reactive intermediates in molecular mass growth processes leading to polycyclic aromatic hydrocarbons (PAHs) and carbonaceous nanoparticles on Earth and in deep space. By combining molecular beams and computational fluid dynamics simulations, we provide compelling evidence on the formation of benzene via the cyclopentadienyl-methyl reaction and of naphthalene through the cyclopentadienyl self-reaction, respectively. These systems offer benchmarks for the conversion of a five-membered ring to the 6π-aromatic (benzene) and the generation of the simplest 10π-PAH (naphthalene) at elevated temperatures.

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.

The Reaction of o-Benzyne with Vinylacetylene: An Unexplored Way to Produce Naphthalene.

The mechanism and kinetics of the reaction of ortho-benzyne with vinylacetylene have been studied by ab initio and density functional CCSD(T)-F12/cc-pVTZ-f12//B3LYP/6-311G(d,p) calculations of the pertinent potential energy surface combined with Rice-Ramsperger-Kassel-Marcus - Master Equation calculations of reaction rate constants at various temperatures and pressures. Under prevailing combustion conditions, the reaction has been shown to predominantly proceed by the biradical acetylenic mechanism initiated by the addition of C H to one of the C atoms of the triple bond in ortho-benzyne by the acetylenic end, with a significant contribution of the concerted addition mechanism. Following the initial reaction steps, an extra six-membered ring is produced and the rearrangement of H atoms in this new ring leads to the formation of naphthalene, which can further dissociate to 1- or 2-naphthyl radicals.

Ozone destruction due to the recombination of oxygen atoms.

Kinetics of ozone destruction due to the recombination of oxygen atoms produced by pulsed 266 nm laser photolysis of O/M (M = CO and/or N) mixtures was studied using the absorption and emission spectroscopy to follow time evolutions of O and electronically excited molecules O* formed in the recombination process 2O(P) + M → O* + M. An unexpected high ozone destruction rate was observed when O* was present in the system. The kinetic model developed for the oxygen nightglow on the terrestrial planets was adapted to interpret the detected temporal profiles of the ozone number density and the O* emission intensities.

Theoretical Study of the Reaction of the Methylidyne Radical (CH; XΠ) with 1-Butyne (CHCHCCH; XA').

Ab initio CCSD(T)-F12/-pVTZ-f12//ωB97X-D/6-311G(d,p) + ZPE[ωB97X-D/6-311G(d,p)] calculations were carried out to unravel the area of the CH potential energy surface accessed by the reaction of the methylidyne radical with 1-butyne. The results were utilized in Rice-Ramsperger-Kassel-Marcus calculations of the product branching ratios at the zero pressure limit. The preferable reaction mechanism has been shown to involve (nearly) instantaneous decomposition of the initial reaction adducts, whose structures are controlled by the isomeric form of the CH reactant.

A molecular beam and computational study on the barrierless gas phase formation of (iso)quinoline in low temperature extraterrestrial environments.

Despite remarkable progress toward the understanding of the formation pathways leading to polycyclic aromatic hydrocarbons (PAHs) in combustion systems and in deep space, the complex reaction pathways leading to nitrogen-substituted PAHs (NPAHs) at low temperatures of molecular clouds and hydrocarbon-rich, nitrogen-containing atmospheres of planets and their moons like Titan have remained largely obscure. Here, we demonstrate through laboratory experiments and computations that the simplest prototype of NPAHs - quinoline and isoquinoline (CHN) - can be synthesized via rapid and de-facto barrier-less reactions involving o-, m- and p-pyridinyl radicals (CHN˙) with vinylacetylene (CH) under low-temperature conditions.

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

The aminosilylene molecule (HSiNH, XA')-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 (NH). The reaction is initiated by the barrierless addition of the silylidyne radical to the nonbonding electron pair of nitrogen forming an HSiNH collision complex, which then undergoes unimolecular decomposition to aminosilylene (HSiNH) via atomic hydrogen loss from the nitrogen atom. Compared to the isovalent aminomethylene carbene (HCNH, XA'), by replacing a single carbon atom with silicon, a profound effect on the stability and chemical bonding of the isovalent methanimine (HCNH)-aminomethylene (HNCH) and aminosilylene (HSiNH)-silanimine (HSiNH) isomer pairs is shown; i.

Gas-phase synthesis of benzene via the propargyl radical self-reaction.

Polycyclic aromatic hydrocarbons (PAHs) have been invoked in fundamental molecular mass growth processes in our galaxy. We provide compelling evidence of the formation of the very first ringed aromatic and building block of PAHs-benzene-via the self-recombination of two resonantly stabilized propargyl (CH) radicals in dilute environments using isomer-selective synchrotron-based mass spectrometry coupled to theoretical calculations. Along with benzene, three other structural isomers (1,5-hexadiyne, fulvene, and 2-ethynyl-1,3-butadiene) and -benzyne are detected, and their branching ratios are quantified experimentally and verified with the aid of computational fluid dynamics and kinetic simulations.

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.

Theoretical Study of the Phenoxy Radical Recombination with the O(P) Atom, Phenyl plus Molecular Oxygen Revisited.

Quantum chemical calculations of the CHO potential energy surface (PES) were carried out to study the mechanism of the phenoxy + O(P) and phenyl + O reactions. CASPT2(15e,13o)/CBS//CASSCF(15e,13o)/DZP multireference calculations were utilized to map out the minimum energy path for the entrance channels of the phenoxy + O(P) reaction. Stationary points on the CHO PES were explored at the CCSD(T)-F12/cc-pVTZ-f12//B3LYP/6-311++G** level for the species with a single-reference character of the wave function and at the CASPT2(15e,13o)/CBS//B3LYP/6-311++G** level of theory for the species with a multireference character of the wave function.

Mechanism and kinetics of the oxidation of 1,3-butadien-1-yl (-CH): a theoretical study.

Ab initio CCSD(T)-F12/cc-pVTZ-f12//B3LYP/6-311G(d,p) calculations of the C4H5O2 potential energy surface have been combined with Rice-Ramsperger-Kassel-Marcus Master Equation (RRKM-ME) calculations of temperature- and pressure-dependent rate constants and product branching ratios to unravel the mechanism and kinetics of the n-C4H5 + O2 reaction. The results indicate that the reaction is fast, with the total rate constant being in the range of 3.4-5.

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.

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).

Gas phase formation of cyclopentanaphthalene (benzindene) isomers reactions of 5- and 6-indenyl radicals with vinylacetylene.

The tricyclic polycyclic aromatic hydrocarbons (PAHs) 3H-cyclopenta[a]naphthalene (C13H10), 1H-cyclopenta[b]naphthalene (C13H10) and 1H-cyclopenta[a]naphthalene (C13H10) along with their indene-based bicyclic isomers (E)-5-(but-1-en-3-yn-1-yl)-1H-indene, (E)-6-(but-1-en-3-yn-1-yl)-1H-indene, 5-(but-3-ene-1-yn-1-yl)-1H-in-dene, and 6-(but-3-ene-1-yn-1-yl)-1H-indene were formed via a "directed synthesis" in a high-temperature chemical micro reactor at the temperature of 1300 ± 10 K through the reactions of the 5- and 6-indenyl radicals (C9H7˙) with vinylacetylene (C4H4). The isomer distributions were probed utilizing tunable vacuum ultraviolet light by recording the photoionization efficiency curves at mass-to-charge of m/z = 166 (C13H10) and 167 (13CC12H10) of the products in a supersonic molecular beam. The underlying reaction mechanisms involve the initial formation of van-der-Waals complexes followed by addition of the 5- and 6-indenyl radicals to vinylacetylene via submerged barriers, followed by isomerization (hydrogen shifts, ring closures), and termination via atomic hydrogen elimination accompanied by aromatization.

Gas phase formation of phenalene via 10π-aromatic, resonantly stabilized free radical intermediates.

For the last few decades, the Hydrogen-Abstraction/aCetylene-Addition (HACA) mechanism has been fundamental in aiding our understanding of the source of polycyclic aromatic hydrocarbons (PAHs) in combustion processes and in circumstellar envelopes of carbon rich stars. However, the reaction mechanisms driving high temperature molecular mass growth beyond triphenylene (CH) along with the link between PAHs and graphene-type nanostructures as identified in carbonaceous meteorites such as in Murchison and Allende has remained elusive. By exploring the reaction of the 1-naphthyl radical (CH˙) with methylacetylene (CHCCH) and allene (HCCCH) under conditions prevalent in carbon-rich circumstellar environments and combustion systems, we provide compelling evidence on a facile formation of 1H-phenalene (CH) - the central molecular building block of graphene-type nanostructures.