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Kaiser, Ralf I.

Publications and source records attributed to Kaiser, Ralf I..

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Gas phase synthesis of the C40 nano bowl C 40 H 10

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 (C 24 H 12 ) via ring annulation of the corannulenyl radical (C 20 H 9 • ) by vinylacetylene (C 4 H 4 ) 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 (C 40 H 20 ) followed by successive cyclodehydrogenation to the C40 nanobowl (C 40 H 10 ) – a fundamental building block of buckminsterfullerene (C 60 ). 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.

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DOE OSTI2023

Exotic Reaction Dynamics in the Gas-Phase Preparation of Anthracene (C 14 H 10 ) via Spiroaromatic Radical Transients in the Indenyl–Cyclopentadienyl Radical–Radical Reaction

The gas-phase reaction between the 1-indenyl (C 9 H 7 •) and the cyclopentadienyl (C 5 H 5 •) radical has been investigated for the first time using synchrotron-based mass spectrometry coupled with a pyrolytic reactor. Soft photo-ionization with tunable vacuum ultraviolet photons afforded for the isomer-selective identification of the production of phenanthrene, anthracene, and benzofulvalene (C 14 H 10 ). The classical theory prevalent in the literature proposing that radicals combine only at their specific radical centers is challenged by our discovery of an unusual reaction pathway that involves a barrierless combination of a resonantly stabilized hydrocarbon radical with an aromatic radical at the carbon atom adjacent to the traditional C1 radical center; this unconventional addition is followed by substantial isomerization into phenanthrene and anthracene via a category of exotic spiroaromatic intermediates. Furthermore, this result leads to a deeper understanding of the evolution of the cosmic carbon budget and provides new methodologies for the bottom-up synthesis of unique spiro-aromatics that may be relevant for the synthesis of more complex aromatic carbon skeletons in deep space.

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DOE OSTI2023

A unified reaction network on the formation of five-membered ringed polycyclic aromatic hydrocarbons (PAHs) and their role in ring expansion processes through radical–radical reactions

Exploiting a chemical microreactor in combination with an isomer-selective product identification through fragment-free photoionization utilizing tunable vacuum ultraviolet (VUV) light in tandem with the detection of the ionized molecules by a high resolution reflection time-of-flight mass spectrometer (Re-TOF-MS), the present investigation reveals molecular mass growth processes to four distinct polycyclic aromatic hydrocarbons carrying two six- and one five-membered ring (C 13 H 10 ): 3H-cyclopenta[a]naphthalene, 1H-cyclopenta[b]naphthalene, 1H-cyclopenta[a]naphthalene, and fluorene in the gas phase. Temperatures of 973 and 1023 K simulating conditions in combustion settings along with circumstellar envelopes of carbon-rich stars and planetary nebulae. These reactions highlight the importance of methyl-substituted aromatic reactants (biphenyl, naphthalene) which can be converted to the methylene (–CH 2 ˙) motive by hydrogen abstraction or photolysis. Upon reaction with acetylene, methylene-substituted aromatics carrying a hydrogen atom at the ortho position of the ring can be then converted to cyclopentadiene-annulated aromatics thus providing a versatile pathway to five-membered ring aromatics at elevated temperatures.

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DOE OSTI2023

Unconventional Pathway in the Gas‐Phase Synthesis of 9 H ‐Fluorene (C 13 H 10 ) via the Radical–Radical Reaction of Benzyl (C 7 H 7 ) with Phenyl (C 6 H 5 )

Abstract The simplest polycyclic aromatic hydrocarbon (PAH) carrying a five‐membered ring—9 H ‐fluorene (C 13 H 10 )—is produced isomer‐specifically in the gas phase by reacting benzyl (C 7 H 7 ⋅) with phenyl (C 6 H 5 ⋅) radicals in a pyrolytic reactor coupled with single photon ionization mass spectrometry. The unconventional mechanism of reaction is supported by theoretical calculations, which first produces diphenylmethane and unexpected 1‐(6‐methylenecyclohexa‐2,4‐dienyl)benzene intermediates (C 13 H 12 ) accessed via addition of the phenyl radical to the ortho position of the benzyl radical. These findings offer convincing evidence for molecular mass growth processes defying conventional wisdom that radical‐radical reactions are initiated through recombination at their radical centers. The structure of 9 H ‐fluorene acts as a molecular building block for complex curved nanostructures like fullerenes and nanobowls providing fundamental insights into the hydrocarbon evolution in high temperature settings.

He, Chao↗
DOE OSTI2022

Unconventional Pathway in the Gas-Phase Synthesis of 9 H -Fluorene (C 13 H 10 ) via the Radical–Radical Reaction of Benzyl (C 7 H 7 ) with Phenyl (C 6 H 5 )

The simplest polycyclic aromatic hydrocarbon (PAH) carrying a five-membered ring - 9H-fluorene (C 13 H 10 ) - is produced isomer-specifically in the gas phase by reacting benzyl (C 7 H 7 •) with phenyl (C 6 H 5 •) radicals in a pyrolytic reactor coupled with single photon ionization mass spectrometry. The unconventional mechanism of reaction is supported by theoretical calculations, which first produces diphenylmethane and unexpectedly 1-(6-methylenecyclohexa-2,4-dienyl)benzene intermediates (C 13 H 12 ) accessed via addition of the phenyl radical to the ortho position of the benzyl radical. These findings offer convincing evidence for molecular mass growth processes defying conventional wisdom that radical–radical reactions are initiated through recombination at their radical centers. Furthermore, the structure of 9H-fluorene acts as a molecular building block for complex curved nanostructures like fullerenes and nanobowls providing fundamental insights into the hydrocarbon evolution in high temperature settings.

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DOE OSTI2022

Gas-Phase Formation of 1,3,5,7-Cyclooctatetraene (C 8 H 8 ) through Ring Expansion via the Aromatic 1,3,5-Cyclooctatrien-7-yl Radical (C 8 H 9 • ) Transient

Gas-phase 1,3,5,7-cyclooctatetraene (C 8 H 8 ) and triplet aromatic 1,3,5,7-cyclooctatetraene (C 8 H 8 ) were formed for the first time through the bimolecular methylidyne radicals (CH) - 1,3,5-cycloheptatriene (C 7 H 8 ) reactions under single collision conditions on the doublet surface. The reaction involves methylidyne radical addition to the olefinic pi electron system of 1,3,5-cycloheptatriene followed by isomerization and ring expansion to aromatic 1,3,5-cyclooctatrien-7-yl radical (C 8 H 9 •). 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. Furthermore, 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 (C 8 H 8 2- ) 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.

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DOE OSTI2022

Directed gas phase preparation of ethynylallene (H 2 CCCHCCH; X 1 A') via the crossed molecular beam reaction of the methylidyne radical (CH; X 2 Π) with vinylacetylene (H 2 CCHCCH; X 1 A')

The gas-phase bimolecular reaction of the methylidyne (CH; X 2 Π) radical with vinylacetylene (H 2 CCHCCH; X 1 A') 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 C 5 H 5 reaction intermediate(s) ultimately dissociating to C 5 H 4 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[double bond, length as m-dash]C bond in vinylacetylene. Altogether, 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; X 2 Π) radical with vinylacetylene (H 2 CCHCCH; X 1 A').

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DOE OSTI2022

Photodissociation dynamics of xylene isomers C 6 H 4 (CH 3 ) 2 at 157nm using an ultracompact velocity map imaging spectrometer – The C 7 H 7 channel

Here, we investigated the photodissociation dynamics of three xylene isomers C 6 H 4 (CH 3 ) 2 at 157nm. The center-of-mass translational energy distributions of C 7 H 7 radicals were found to peak at 26kJmol −1 . Although the ionization energy of the C 7 H 7 tolyl fragment exceeds the energy of a 157nm photon, C 7 H 7 + was observed as a result of the photoionization of vibrationally ‘hot’ tolyl (C 7 H 7 ) radicals and/or two-photon ionization. The formation of rovibrationally excited tolyl fragments was discussed. Our experiments suggest the presence of tolyl radicals in the interstellar medium as a precursor to methylated polycyclic aromatic hydrocarbons upon reaction with vinylacetylene (C 4 H 4 ).

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DOE OSTI2022

A crossed molecular beams and computational study on the unusual reactivity of banana bonds of cyclopropane (c-C 3 H 6 ; X 1 A$^{'}_{1}$ ) through insertion by ground state carbon atoms (C( 3 P j ))

The mechanism and chemical dynamics of the reaction of ground electronic state atomic carbon C( 3 P j ) with cyclopropane c-C 3 H 6 (X 1 A$^{'}_{1}$) have been explored by combining crossed molecular beams experiments with electronic structure calculations of the pertinent triplet C 4 H 6 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 C 4 H 6 reaction intermediate(s) leading to C 4 H 5 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( 3 P j ) 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 . Here, 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 (C 2 H 6 ). 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-C 4 H 5 , and 11% for 1,3-butadien-1-yl, n-C 4 H 5 , with i-C 4 H 5 (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( 3 P j ) + c-C 3 H 6 reaction is predicted to be a source of C 4 H 5 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-C 4 H 5 can further react with acetylene eventually producing the first aromatic ring and hence, the reaction of the atomic carbon with c-C 3 H 6 can be considered as an initial step toward the formation of benzene.

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DOE OSTI2022

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. Here, by exploiting synchrotron based molecular beam photoionization mass spectrometry combined with electronic structure calculations, we provide compelling testimony on barrierless, low temperature pathways leading to racemates of [5] and [6]helicene. Astrochemical modeling advocates that gas-phase reactions in molecular clouds lead to racemates of helicenes suggesting a pathway for future astronomical observation and providing a fundamental understanding for the origin of homochirality on early Earth.

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DOE OSTI2022

Photodissociation Dynamics of Astrophysically Relevant Propyl Derivatives (C 3 H 7 X; 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 (C 3 H 7 X; 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 photo dissociation of six (iso)propyl systems - n/i-propyl cyanide (C 3 H 7 CN), n/i-propyl alcohol (C 3 H 7 OH), and (iso)butanal (C 3 H 7 CHO) – to explore the C 3 H 7 loss channel. The distinct center-of-mass translational energy distributions for the i-C 3 H 7 X (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 stretchings of the cyano and hydroxy moieties. Although the ionization energy of the n-C 3 H 7 radical exceeds the energy of a 157 nm photon, C 3 H 7 + was observed in the n-C 3 H 7 X (X= CN, OH, HCO) systems as a result of photoionization of vibrationally "hot" n-C 3 H 7 fragments, photoionization of i-C 3 H 7 after a hydrogen shift in vibrationally "hot" n-C 3 H 7 radicals, and/or two-photon ionization. Our experiments reveal that at least the isopropyl radical (i-C 3 H 7 ) and possibly the normal propyl radical (n-C 3 H 7 ) should be present in the interstellar medium and hence searched for by radio telescopes.

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DOE OSTI2022

Gas-Phase Study of the Elementary Reaction of the D1-Ethynyl Radical (C 2 D; X 2 Σ + ) with Propylene (C 3 H 6 ; X 1 A') under Single-Collision Conditions

The bimolecular gas-phase reactions of the D1-ethynyl radical (C 2 D; X 2 Σ + ) with propylene (C 3 H 6 ; X 1 A’) and partially substituted D3-3,3,3-propylene (C 2 H 3 CD 3 ; X 1 A’) 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 C 5 H 6 D 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(CH 3 )) and to a minor amount 3-methyl-3-buten-1-yne ((HCC)C(CH 3 )=CH 2 ) via overall exoergic reactions. Although the title reaction does not lead to the cyclopentadiene molecule (c-C 5 H 6 , X 1 A 1 ), high temperature environments can convert the identified acyclic C 5 H 6 isomers through hydrogen atom assisted isomerization to cyclopentadiene (c-C 5 H 6 , X 1 A 1 ). 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, the these C 5 H 6 product isomers are predicted to form in those low temperature regions.

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DOE OSTI2021

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. Here, 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. These results uncover molecular mass growth processes from the “bottom up” via RSFRs in high temperature circumstellar environments and combustion systems expanding our fundamental knowledge of the organic, hydrocarbon chemistry in our universe.

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DOE OSTI2021

A Chemical Dynamics Study of the Reaction of the Methylidyne Radical (CH, X 2 Π) with Dimethylacetylene (CH 3 CCCH 3 , X 1 A 1g )

The gas-phase reaction of the methylidyne (CH; X 2 Π) radical with dimethylacetylene (CH 3 CCCH 3 ; X 1 A 1g ) 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. Here, the crossed molecular beams experiment reveals that the reaction proceeds barrierless via indirect scattering dynamics through long-lived C 5 H 7 reaction intermediate(s) ultimately dissociating to C 5 H 6 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-methylene-cyclopropene, 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.

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DOE OSTI2021

Theoretical Study of the Reaction of the Methylidyne Radical (CH; X 2 Π) with 1-Butyne (CH 3 CH 2 CCH; X 1 A')

In this study, ab initio CCSD(T)-F12/cc-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 C 5 H 7 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 C 4 H 6 reactant. If CH adds to the triple CΞC bond in the entrance reaction channel, the reaction is predicted to predominantly form the methylenecyclopropene + methyl (CH 3 ) and cyclopropenylidene + ethyl (C 2 H 5 ) products roughly in a 2:1 ratio. CH insertion into a C-H bond in the methyl group of 1-butyne is anticipated to preferentially form ethylene + propargyl (C 3 H 3 ) by the C-C bond β-scission in the initial complex, whereas CH insertion into C-H of the CH 2 group would predominantly produce vinylacetylene + methyl (CH 3 ) also by the C-C bond β-scission in the adduct. The barrierless and highly exoergic CH + 1-butyne reaction, facile in cold molecular clouds, is not likely to lead to the carbon skeleton molecular growth but generates C 4 H 4 isomers methylenecyclopropene, vinylacetylene, and 1,2,3-butatriene and smaller C 2 and C 3 hydrocarbons such as methyl, ethyl, and propargyl radicals, ethylene, and cyclopropenylidene.

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DOE OSTI2021

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 (C 9 H 7 N) – can be synthesized via rapid and de-facto barrier-less reactions involving o-, m- and p-pyridinyl radicals (C 5 H 4 N˙) with vinylacetylene (C 4 H 4 ) under low-temperature conditions.

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DOE OSTI2021

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 (C 3 H 3 ) 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, 2-ethynyl-1,3-butadiene) and o-benzyne are detected and their branching ratios quantified experimentally and verified with the aid of computational fluid dynamics and kinetic simulations. Furthermore, these results uncover molecular growth pathways not only in interstellar, circumstellar, and solar systems environments, but also in combustion systems gaining us a better understanding of the hydrocarbon chemistry of our universe.

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