Publications by authors named "Ralf I Kaiser"

231 Publications

Identification of a prismatic PN molecule formed from electron irradiated phosphine-nitrogen ices.

Nat Commun 2021 Sep 15;12(1):5467. Epub 2021 Sep 15.

Department of Chemistry, University of Hawaii at Manoa, 2545 McCarthy Mall, Honolulu, HI, 96822, USA.

Polyhedral nitrogen containing molecules such as prismatic PN - a hitherto elusive isovalent species of prismane (CH) - have attracted particular attention from the theoretical, physical, and synthetic chemistry communities. Here we report on the preparation of prismatic PN [1,2,3-triaza-4,5,6-triphosphatetracyclo[2.2.0.0.0]hexane] by exposing phosphine (PH) and nitrogen (N) ice mixtures to energetic electrons. Prismatic PN was detected in the gas phase and discriminated from its isomers utilizing isomer selective, tunable soft photoionization reflectron time-of-flight mass spectrometry during sublimation of the ices along with an isomer-selective photochemical processing converting prismatic PN to 1,2,4-triaza-3,5,6-triphosphabicyclo[2.2.0]hexa-2,5-diene (PN). In prismatic PN, the P-P, P-N, and N-N bonds are lengthened compared to those in, e.g., diphosphine (PH), di-anthracene stabilized phosphorus mononitride (PN), and hydrazine (NH), by typically 0.03-0.10 Å.  These findings advance our fundamental understanding of the chemical bonding of poly-nitrogen and poly-phosphorus systems and reveal a versatile pathway to produce exotic, ring-strained cage molecules.
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http://dx.doi.org/10.1038/s41467-021-25775-1DOI Listing
September 2021

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

J Am Chem Soc 2021 Sep 25;143(35):14227-14234. Epub 2021 Aug 25.

Department of Chemistry and Biochemistry, Florida International University, Miami, Florida 33199, United States.

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.e., thermodynamical stabilities of the carbene versus silylene are reversed by 220 kJ mol. Hence, the isovalency of the main group XIV element silicon was found to exhibit little similarities with the atomic carbon revealing a remarkable effect not only on the reactivity but also on the thermochemistry and chemical bonding.
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http://dx.doi.org/10.1021/jacs.1c05510DOI Listing
September 2021

Identification of Glycolaldehyde Enol (HOHC═CHOH) in Interstellar Analogue Ices.

J Am Chem Soc 2021 Sep 18;143(34):14009-14018. Epub 2021 Aug 18.

Department of Chemistry, University of Hawaii at Ma̅noa, Honolulu, Hawaii 96822, United States.

Glycolaldehyde is considered the entry point in the aqueous prebiotic formose (Butlerow) reaction although it mainly exists in its unreactive hydrated form in aqueous solution. The characterization of the more reactive nucleophilic enol form under interstellar conditions has remained elusive to date. Here we report on the identification of glycolaldehyde enol (1,2-ethenediol, HOHC═CHOH) in low temperature methanol-bearing ices at temperatures as low as 5 K. Exploiting isotope labeling and isomer-selective photoionization coupled with reflectron time-of-flight mass spectrometry, our results unravel distinct reaction pathways to 1,2-ethenediol, thus demonstrating the kinetic stability, availability for prebiotic sugar formation, and potential detectability in deep space.
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http://dx.doi.org/10.1021/jacs.1c07978DOI Listing
September 2021

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

Sci Adv 2021 Jun 25;7(26). Epub 2021 Jun 25.

School of Mathematics and Physics, Queen's University Belfast, Belfast BT7 1NN, Northern Ireland, UK.

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.
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http://dx.doi.org/10.1126/sciadv.abg7003DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8232914PMC
June 2021

Identification of Elusive Keto and Enol Intermediates in the Photolysis of 1,3,5-Trinitro-1,3,5-Triazinane.

J Phys Chem Lett 2021 Jul 25;12(26):6062-6069. Epub 2021 Jun 25.

Department of Chemistry, University of Hawaii at Manoa, Honolulu, Hawaii 96822, United States.

Enols have emerged as critical reactive intermediates in combustion processes and in fundamental molecular mass growth processes in the interstellar medium, but the elementary reaction pathways to enols in extreme environments, such as during the decomposition of molecular energetic materials, are still elusive. Here, we report on the original identification of the enol and keto isomers of oxy--triazine, as well as its deoxygenated derivative 1,3,5-triazine, formed in the photodecomposition processes of 1,3,5-trinitro-1,3,5-triazinane (RDX)-a molecular energetic material. The identification was facilitated by exploiting isomer-selective tunable photoionization reflectron time-of-flight mass spectrometry (PI-ReTOF-MS) in conjunction with quantum chemical calculations. The present study reports the first experimental evidence of an enol intermediate in the dissociation domain of a nitramine-based energetic material. Our investigations suggest that the enols like 1,3,5-triazine-2-ol could be the source of hydroxyl radicals, and their inclusion in the theoretical models is important to understand the unprecedented chemistry of explosive materials.
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http://dx.doi.org/10.1021/acs.jpclett.1c01610DOI Listing
July 2021

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

J Phys Chem Lett 2021 Jul 23;12(25):5979-5986. Epub 2021 Jun 23.

Department of Chemistry, University of Hawai'i at Ma̅noa, Honolulu, Hawaii 96822, United States.

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. 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.
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http://dx.doi.org/10.1021/acs.jpclett.1c01706DOI Listing
July 2021

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

Phys Chem Chem Phys 2021 Jun;23(24):13647-13661

School of Maths and Physics, Queen's University Belfast, University Road, Belfast BT7 1NN, Northern Ireland, UK.

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.
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http://dx.doi.org/10.1039/d1cp01629fDOI Listing
June 2021

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

J Phys Chem A 2021 Jun 7;125(23):5040-5047. Epub 2021 Jun 7.

Department of Chemistry, University of Hawai'i at Manoa, Honolulu, Hawaii 96822, United States.

The bimolecular gas-phase reactions involving ground-state atomic silicon (Si; P) and 1- and 2-methyl-1,3-butadiene were studied via crossed molecular beam experiments. Our data revealed indirect scattering dynamics through long-lived SiCH collision complex(es) along with molecular hydrogen loss pathways, leading to facile formation of SiCH isomer(s). We propose that the reactions of silicon with 1- and 2-methyl-1,3-butadiene possess reaction dynamics in an analogy to the silicon-1,3-butadiene system. This leads to cyclic methyl-substituted 2-methylene-1-silacyclobutene isomers via nonadiabatic reaction dynamics through intersystem crossing (ISC) from the triplet to the singlet surface in overall exoergic reactions through tight exit transition states and molecular hydrogen loss. Our study also suggests that the methyl group-although a spectator from the chemical viewpoint-can influence the disposal of the angular momentum into the rotational excitation of the final product.
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http://dx.doi.org/10.1021/acs.jpca.1c03023DOI Listing
June 2021

Catalytic Effects of Zeolite Socony Mobil-5 (ZSM-5) on the Oxidation of Acoustically Levitated -Tetrahydrodicyclopentadiene (JP-10) Droplets.

J Phys Chem A 2021 Jun 27;125(22):4896-4909. Epub 2021 May 27.

Department of Chemistry, Chonnam National University, Buk-gu, Gwangju 61186, South Korea.

Jet propulsion 10 (JP-10) droplets with and without aluminum nanoparticles in conjunction with HZSM-5 zeolite and surfactants were ultrasonically levitated, and their oxidation processes were explored to identify how the oxidation process of JP-10 is catalytically affected by the HZSM-5 zeolites and how the surfactant and Al NPs in the system impacted the key experimental parameters of the ignition such as ignition delay time, burn rate, and the maximum temperatures. Singly levitated droplets were ignited using a carbon dioxide laser under an oxygen-argon atmosphere. Pure JP-10 droplets and JP-10 droplets with silicon dioxide of an identical size distribution as the zeolite HZSM-5 did not ignite in strong contrast to HZSM-5-doped droplets. Acidic sites were found to be critical in the ignition of the JP-10. With the addition of the surfactant, the characteristic features of the JP-10 ignition were improved, so the ignition delay time of the zeolite-JP-10 samples were decreased by 2-3 ms and the burn rates were increased by 1.3 to 1.6 × 10 K s. The addition of Al NPs increased the maximum temperatures during the combustion of the systems by 300-400 K. Intermediates and end products of the JP-10 oxidation over HZSM-5 were characterized by UV-vis emission and Fourier-transform infrared transmission spectroscopies, revealing key reactive intermediates (OH, CH, C, O, and HCO) along with the HO molecules in highly excited rovibrational states. Overall, this work revealed that acetic sites in HZSM-5 are critical in the catalytic ignition of JP-10 droplets with the addition of the surfactant and Al NPs, enhancing the oxidation process of JP-10 over HZSM-5 zeolites.
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http://dx.doi.org/10.1021/acs.jpca.1c02892DOI Listing
June 2021

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

Sci Adv 2021 May 21;7(21). Epub 2021 May 21.

Department of Chemistry, University of Hawaii at Manoa, Honolulu, HI 96822, USA.

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. These results uncover molecular growth pathways not only in interstellar, circumstellar, and solar systems environments but also in combustion systems, which help us gain a better understanding of the hydrocarbon chemistry of our universe.
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http://dx.doi.org/10.1126/sciadv.abf0360DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8139581PMC
May 2021

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

Chemphyschem 2021 Jul 10;22(14):1497-1504. Epub 2021 Jun 10.

Department of Chemistry, University of Hawai'i at Mānoa, Honolulu, HI 96822, USA.

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. 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 (HSiGeH ; i1). This intermediate underwent facile intersystem crossing (ISC) to the singlet surface (HSiGeH ; i1). The latter isomerized via at least three hydrogen atom migrations involving exotic, hydrogen bridged reaction intermediates eventually leading to the H SiGeH isomer i5. This intermediate could undergo unimolecular decomposition yielding the dibridged butterfly-structured isomer p1 (Si(μ-H )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 p1 (Si(μ-H )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.
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http://dx.doi.org/10.1002/cphc.202100235DOI Listing
July 2021

Origin of ammoniated phyllosilicates on dwarf planet Ceres and asteroids.

Nat Commun 2021 May 11;12(1):2690. Epub 2021 May 11.

Department of Chemistry, University of Hawaii, Honolulu, HI, USA.

The surface mineralogy of dwarf planet Ceres is rich in ammonium (NH) bearing phyllosilicates. However, the origin and formation mechanisms of ammoniated phyllosilicates on Ceres's surface are still elusive. Here we report on laboratory simulation experiments under astrophysical conditions mimicking Ceres' physical and chemical environments with the goal to better understand the source of ammoniated minerals on Ceres' surface. We observe that thermally driven proton exchange reactions between phyllosilicates and ammonia (NH) could trigger at low temperature leading to the genesis of ammoniated-minerals. Our study revealed the thermal (300 K) and radiation stability of ammoniated-phyllosilicates over a timescale of at least some 500 million years. The present experimental investigations corroborate the possibility that Ceres formed at a location where ammonia ices on the surface would have been stable. However, the possibility of Ceres' origin near to its current location by accreting ammonia-rich material cannot be excluded.
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http://dx.doi.org/10.1038/s41467-021-23011-4DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8113531PMC
May 2021

Interstellar Enolization-Acetaldehyde (CH CHO) and Vinyl Alcohol (H CCH(OH)) as a Case Study.

Chemphyschem 2021 06 28;22(12):1229-1236. Epub 2021 May 28.

Department of Chemistry and W. M. Keck Research Laboratory in Astrochemistry, University of Hawai'i at Manoa, 2545 McCarthy Mall, Honolulu, HI, 96822, USA.

Owing to the unique conditions in cold molecular clouds, enols-the thermodynamically less stable tautomers of aldehydes and ketones-do not undergo tautomerization to their more stable tautomers in the gas phase because they cannot overcome tautomerization barriers at the low temperatures. Laboratory studies of interstellar analog ices have demonstrated the formation of several keto-enol tautomer pairs in astrochemically relevant ice mixtures over the last years. However, so far only one of them, acetaldehyde-vinyl alcohol, has been detected in deep space. Due to their reactivity with electrophiles, enols can play a crucial role in our understanding of the molecular complexity in the interstellar medium and in comets and meteorites. To study the enolization of aldehydes in interstellar ices by interaction with galactic cosmic rays (GCRs), we irradiated acetaldehyde ices with energetic electrons as proxies of secondary electrons generated in the track of GCRs while penetrating interstellar ices. The results indicate that GCRs can induce enolization of acetaldehyde and that intra- as well as intermolecular processes are relevant. Therefore, enols should be ubiquitous in the interstellar medium and could be searched for using radio telescopes such as ALMA. Once enols are detected and abundances are established, they can serve as tracers for the non-equilibrium chemistry in interstellar ices thus eventually constraining fundamental reaction mechanisms deep inside interstellar ices.
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http://dx.doi.org/10.1002/cphc.202100111DOI Listing
June 2021

Formation of phosphine imide (HN[double bond, length as m-dash]PH) and its phosphinous amide (HN-PH) isomer.

Chem Commun (Camb) 2021 May;57(40):4958-4961

Department of Chemistry, National Dong Hwa University, Shoufeng, Hualien 974, Taiwan.

We present the first formation of the previously elusive phosphine imide (HN[double bond, length as m-dash]PH3) along with its phosphinous amide (H2N-PH2) isomer via exposure of phosphine (PH3) and ammonia (NH3) ices to ionizing radiation. Our approach may be extended to prepare, separate, and detect highly reactive compounds such as intermediates of Wittig reactions.
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http://dx.doi.org/10.1039/d0cc08411eDOI Listing
May 2021

An Aromatic Universe-A Physical Chemistry Perspective.

J Phys Chem A 2021 May 7;125(18):3826-3840. Epub 2021 Apr 7.

Combustion Research Facility, Sandia National Laboratories, Livermore, California 94551, United States.

This Perspective presents recent advances in our knowledge of the fundamental elementary mechanisms involved in the low- and high-temperature molecular mass growth processes to polycyclic aromatic hydrocarbons in combustion systems and in extraterrestrial environments (hydrocarbon-rich atmospheres of planets and their moons, cold molecular clouds, circumstellar envelopes). Molecular beam studies combined with electronic structure calculations extracted five key elementary mechanisms: Hydrogen Abstraction-Acetylene Addition, Hydrogen Abstraction-Vinylacetylene Addition, Phenyl Addition-DehydroCyclization, Radical-Radical Reactions, and Methylidyne Addition-Cyclization-Aromatization. These studies, summarized here, provide compelling evidence that key classes of aromatic molecules can be synthesized in extreme environments covering low temperatures in molecular clouds (10 K) and hydrocarbon-rich atmospheres of planets and their moons (35-150 K) to high-temperature environments like circumstellar envelopes of carbon-rich Asymptotic Giant Branch Stars stars and combustion systems at temperatures above 1400 K thus shedding light on the aromatic universe we live in.
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http://dx.doi.org/10.1021/acs.jpca.1c00606DOI Listing
May 2021

A Photoionization Reflectron Time-of-Flight Mass Spectrometric Study on the Detection of Ethynamine (HCCNH ) and 2H-Azirine (c-H CCHN).

Chemphyschem 2021 05 19;22(10):985-994. Epub 2021 Apr 19.

Department of Chemistry, University of Hawaii at Manoa, 2545 McCarthy Mall, Honolulu, HI 96822, USA.

Ices of acetylene (C H ) and ammonia (NH ) were irradiated with energetic electrons to simulate interstellar ices processed by galactic cosmic rays in order to investigate the formation of C H N isomers. Supported by quantum chemical calculations, experiments detected product molecules as they sublime from the ices using photoionization reflectron time-of-flight mass spectrometry (PI-ReTOF-MS). Isotopically-labeled ices confirmed the C H N assignments while photon energies of 8.81 eV, 9.80 eV, and 10.49 eV were utilized to discriminate isomers based on their known ionization energies. Results indicate the formation of ethynamine (HCCNH ) and 2H-azirine (c-H CCHN) in the irradiated C H :NH ices, and the energetics of their formation mechanisms are discussed. These findings suggest that these two isomers can form in interstellar ices and, upon sublimation during the hot core phase, could be detected using radio astronomy.
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http://dx.doi.org/10.1002/cphc.202100064DOI Listing
May 2021

Effects of Nitrogen Dioxide on the Oxidation of Levitated -Tetrahydrodicyclopentadiene (JP-10) Droplets Doped with Aluminum Nanoparticles.

J Phys Chem A 2021 Apr 26;125(13):2727-2742. Epub 2021 Mar 26.

Department of Chemistry, University of Hawaii at Manoa, Honolulu, Hawaii 96822, United States.

Nitrogen dioxide (NO) can significantly improve the combustion of hydrocarbon fuels, but the effect of NO on the ignition of fuels with energy densities enhanced by aluminum (Al) nanoparticles has not been studied. We therefore investigated the effects of NO on the ignition of JP-10 droplets containing Al nanoparticles initially acoustically levitated in an oxygen-argon mixture. A carbon dioxide laser ignited the droplet and the resulting combustion processes were traced in real time using Raman, ultraviolet-visible (UV-vis), and Fourier-transform infrared (FTIR) spectroscopies simultaneously with a high-speed optical or thermal imaging camera. Temperature temporal profiles of the ignition processes revealed that a 5% concentration of NO did not cause measurable differences in the ignition delay time or the initial rate of temperature rise, but the maximum flame temperature was reduced from 2930 ± 120 K to 2520 ± 160 K. The relative amplitudes of the UV-vis emission bands were used to deduce how NO affected the composition of the radical pool during the oxidation process; for example, the radicals NO, NH, and CN were detected and the OH (A Σ-X Π) band at 310 nm was less prominent with NO. Localized heating from a tightly focused infrared laser beam provided sufficient energy to activate chemical reactions between the JP-10 and NO without igniting the droplet. Raman spectra of the residue produced give information about the initial oxidation mechanisms and suggest that organic nitro compounds formed. Thus, in contrast to previous studies of hydrocarbon combustion without Al nanoparticles, NO was found not to enhance the ignition of an Al-doped JP-10 droplet ignited by a CO laser.
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http://dx.doi.org/10.1021/acs.jpca.0c10155DOI Listing
April 2021

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

J Phys Chem A 2021 Apr 18;125(12):2472-2479. Epub 2021 Mar 18.

Department of Chemistry, University of Hawai'i at Manoa, Honolulu, Hawaii 96822, United States.

Small silicon hydrides have attracted extensive interest because of their role in the chemical evolution of circumstellar envelopes of evolved carbon stars and applications in surface growth processes and as transients in semiconductor manufacturing. Combined with electronic structure calculations, we demonstrate that monobridged silylidynesilylenes [(Si(μ-D)SiH, Si(μ-H)SiHD, Si(μ-H)SiH] and silylsilylidyne [HSiSi, HDSiSi], which are nearly isoenergetic, can be prepared via molecular hydrogen loss channels in the crossed molecular beam study of the reaction of D1-silylidyne (SiD; XΠ) with silane (SiH; XA) in a crossed molecular beams machine. Compared to the dynamics of the isovalent methylidyne (CH) - methane (CH) system, our study delivers a unique view at the intriguing isomerization processes and reaction dynamics of dinuclear silicon hydride transients, thus contributing to our knowledge on the chemical bonding of silicon hydrides at the molecular level.
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http://dx.doi.org/10.1021/acs.jpca.0c11538DOI Listing
April 2021

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

J Phys Chem Lett 2021 Mar 5;12(10):2489-2495. Epub 2021 Mar 5.

Centro Federal de Educação Tecnológica de Minas Gerais, CEFET-MG, Av. Amazonas 5253, 30421-169 Belo Horizonte, Minas Gerais, Brazil.

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. 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.
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http://dx.doi.org/10.1021/acs.jpclett.1c00085DOI Listing
March 2021

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

Phys Chem Chem Phys 2021 Mar 17;23(10):5740-5749. Epub 2021 Feb 17.

Department of Chemistry, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA.

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. 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.
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http://dx.doi.org/10.1039/d0cp06537dDOI Listing
March 2021

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

Sci Adv 2021 Jan 1;7(1). Epub 2021 Jan 1.

Department of Chemistry, University of Hawai'i at Mānoa, Honolulu, HI 96822, USA.

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. 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 (CH) along with fullerenes (C, C), thus offering a new concept on the low-temperature chemistry of carbon in our galaxy.
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http://dx.doi.org/10.1126/sciadv.abd4044DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7775774PMC
January 2021

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

J Phys Chem A 2021 Jan 4;125(1):126-138. Epub 2021 Jan 4.

Department of Chemistry, University of Hawai'i at Manoa, Honolulu, Hawaii 96822, United States.

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. These collision adducts undergo a nonstatistical unimolecular decomposition through atomic hydrogen elimination to at least the cyclic 1-vinyl-cyclopropene (/), 1-methyl-3-methylenecyclopropene (), and 1,2-bis(methylene)cyclopropane () in overall exoergic reactions. The barrierless nature of this bimolecular reaction suggests that these cyclic CH isomers might be viable targets to be searched for in cold molecular clouds like TMC-1.
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http://dx.doi.org/10.1021/acs.jpca.0c08731DOI Listing
January 2021

A Photoionization Study on the Detection of 1-Sila Glycolaldehyde (HSiOCH OH), 2-Sila Acetic Acid (H SiCOOH), and 1,2-Disila Acetaldehyde (HSiOSiH ).

Chemistry 2021 Mar 16;27(15):4939-4945. Epub 2021 Feb 16.

Department of Chemistry, University of Hawaii at Manoa, 2545 McCarthy Mall, Honolulu, HI, 96822, USA.

The identification of silicon-substituted, complex organics carrying multiple functional groups by classical infrared spectroscopy is challenging because the group frequencies of functional groups often overlap. Photoionization (PI) reflectron time-of-fight mass spectrometry (ReTOF-MS) in combination with temperature-programmed desorption (TPD) holds certain advantages because molecules are identified after sublimation from the matrix into in the gas phase based on distinct ionization energies and sublimation temperatures. In this study, we reveal the detection of 1-silaglycolaldehyde (HSiOCH OH), 2-sila-acetic acid (H SiCOOH), and 1,2-disila-acetaldehyde (H SiSiHO)-the silicon analogues of the well-known glycolaldehyde (HCOCH OH), acetic acid (H CCOOH), and acetaldehyde (H CCHO), in the gas phase after preparation in silane (SiH )-carbon dioxide ices exposed to energetic electrons and subliming the neutral reaction products formed within the ices into the gas phase.
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http://dx.doi.org/10.1002/chem.202004863DOI Listing
March 2021

Gas phase identification of the elusive oxaziridine (cyclo-HCONH) - an optically active molecule.

Chem Commun (Camb) 2020 Dec 7;56(100):15643-15646. Epub 2020 Dec 7.

Department of Chemistry, University of Hawaii, 2545 McCarthy Mall, Honolulu, HI 96822, USA.

The hitherto elusive oxaziridine molecule (cyclo-HCONH) - an optically active, high energy isomer of nitrosomethane (CHNO) - is prepared in processed methane-nitrogen monoxide ices and detected upon sublimation in the gas phase. Electronic structure calculations reveal likely routes via addition of carbene (CH) to the nitrogen-oxygen double bond of nitrosyl hydride (HNO). Our findings provide a fundamental framework to explore the preparation and stability of racemic oxaziridines exploited in chiral substrate-controlled diastereoselective preparation such as Sharpless asymmetric epoxidation, thus advancing our fundamental understanding of the preparation and chemical bonding of strained rings in small organic molecules.
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http://dx.doi.org/10.1039/d0cc06760aDOI Listing
December 2020

Exploiting Photoionization Reflectron Time-of-Flight Mass Spectrometry to Explore Molecular Mass Growth Processes to Complex Organic Molecules in Interstellar and Solar System Ice Analogs.

Acc Chem Res 2020 Dec 1;53(12):2791-2805. Epub 2020 Dec 1.

Department of Chemistry and W.M. Keck Research Laboratory in Astrochemistry, University of Hawai'i at Manoa, Honolulu, Hawaii 96822, United States.

ConspectusThis Account presents recent advances in our understanding on the formation pathways of complex organic molecules (COMs) within interstellar analog ices on ice-coated interstellar nanoparticles upon interaction with ionizing radiation exploiting reflectron time-of-flight mass spectrometry (ReTOF-MS) coupled with tunable vacuum ultraviolet (VUV) single photon ionization (PI) and resonance enhanced multiphoton ionization (REMPI) of the subliming molecules during the temperature-programmed desorption (TPD) phase. Laboratory simulation experiments provided compelling evidence that key classes of complex organics (aromatic hydrocarbons, alcohols, ethers, aldehydes, enols, ketones, and carboxylic acids) can be synthesized upon exposure of astrophysically relevant model ices to ionizing radiation and the ices at temperatures as low as 5 K.Molecular mass growth processes can be initiated by suprathermal or electronically excited reactants along with barrierless radical-radical recombination if both radicals hold a proper recombination geometry. Methyl (CH), amino (NH), hydroxyl (OH), ethyl (CH), vinyl (CH), ethynyl (CH), formyl (HCO), hydroxycarbonyl (HOCO), hydroxymethyl (CHOH), methoxy (CHO), and acetyl (CHCO) represent readily available reactants for the ices. Reactive singlet species were found to without barrier into carbon-hydrogen and carbon-carbon single bonds (carbene) leading to an extension of the carbon chain and may to carbon-carbon double bonds (carbene, atomic oxygen) forming cyclic reaction products. These galactic cosmic ray-triggered nonequilibrium pathways overcome previous obstacles of hypothesized thermal grain-surface processes and operate at 5 K. Our investigations discriminate between multiple structural isomers such as alcohols/ethers, aldehydes/enols, and cyclic/acyclic carbonyls. These data provide quantitative, input parameters for a cosmic ray-dictated formation of complex organics in interstellar ices and are fully able to replicate the astronomical observations of complex organics over typical lifetimes of molecular clouds of a few 10 to 10 years. Overall, PI-ReTOF-MS revealed that the processing of astrophysically relevant ices can lead to multifaceted mixtures of organics reaching molecular weights of up to 200 amu. Further advances in laboratory techniques beyond the FTIR-QMS limit are clearly desired not only to confidently assign detection in laboratory ice analog experiments of increasingly more complex molecules of interest but also from the viewpoint of future astronomical searches in the age of the Atacama Large Millimeter/submillimeter Array (ALMA).
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http://dx.doi.org/10.1021/acs.accounts.0c00584DOI Listing
December 2020

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

Chemphyschem 2021 Jan 16;22(2):184-191. Epub 2020 Dec 16.

Department of Chemistry and Biochemistry, Florida International University, Miami, FL 33199, USA.

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). 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; C H ) eventually affording detailed insights into an exotic chemistry and intriguing chemical bonding of silicon-germanium species at the microscopic level exploiting crossed molecular beams.
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http://dx.doi.org/10.1002/cphc.202000913DOI Listing
January 2021

A chemical dynamics study on the gas-phase formation of triplet and singlet CH carbenes.

Proc Natl Acad Sci U S A 2020 Dec 16;117(48):30142-30150. Epub 2020 Nov 16.

Department of Chemistry, University of Hawai'i at Manoa, Honolulu, HI 96822;

Since the postulation of carbenes by Buchner (1903) and Staudinger (1912) as electron-deficient transient species carrying a divalent carbon atom, carbenes have emerged as key reactive intermediates in organic synthesis and in molecular mass growth processes leading eventually to carbonaceous nanostructures in the interstellar medium and in combustion systems. Contemplating the short lifetimes of these transient molecules and their tendency for dimerization, free carbenes represent one of the foremost obscured classes of organic reactive intermediates. Here, we afford an exceptional glance into the fundamentally unknown gas-phase chemistry of preparing two prototype carbenes with distinct multiplicities-triplet pentadiynylidene (HCCCCCH) and singlet ethynylcyclopropenylidene (c-CH) carbene-via the elementary reaction of the simplest organic radical-methylidyne (CH)-with diacetylene (HCCCCH) under single-collision conditions. Our combination of crossed molecular beam data with electronic structure calculations and quasi-classical trajectory simulations reveals fundamental reaction mechanisms and facilitates an intimate understanding of bond-breaking processes and isomerization processes of highly reactive hydrocarbon intermediates. The agreement between experimental chemical dynamics studies under single-collision conditions and the outcome of trajectory simulations discloses that molecular beam studies merged with dynamics simulations have advanced to such a level that polyatomic reactions with relevance to extreme astrochemical and combustion chemistry conditions can be elucidated at the molecular level and expanded to higher-order homolog carbenes such as butadiynylcyclopropenylidene and triplet heptatriynylidene, thus offering a versatile strategy to explore the exotic chemistry of novel higher-order carbenes in the gas phase.
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http://dx.doi.org/10.1073/pnas.2019257117DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7720239PMC
December 2020

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

Phys Chem Chem Phys 2020 Oct;22(39):22493-22500

Department of Chemistry, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA.

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. All the barriers involved in the formation of 3H-cyclopenta[a]naphthalene, 1H-cyclopenta[b]naphthalene and 1H-cyclopenta[a]naphthalene are submerged with respect to the reactants indicating that the mechanisms are in fact barrierless, potentially forming PAHs via the hydrogen abstraction - vinylacetylene addition (HAVA) pathway in the cold molecular clouds such as Taurus Molecular Cloud-1 (TMC-1) at temperatures as low as 10 K.
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http://dx.doi.org/10.1039/d0cp03846fDOI Listing
October 2020

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

Proc Natl Acad Sci U S A 2020 09 28;117(37):22712-22719. Epub 2020 Aug 28.

School of Mathematics and Physics, Queen's University Belfast, Belfast BT7 1NN, Northern Ireland, United Kingdom

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). 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.
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http://dx.doi.org/10.1073/pnas.2004881117DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7502777PMC
September 2020

The elusive cyclotriphosphazene molecule and its Dewar benzene-type valence isomer (PN).

Sci Adv 2020 Jul 22;6(30):eaba6934. Epub 2020 Jul 22.

Department of Chemistry, University of Hawaii at Manoa, 2545 McCarthy Mall, Honolulu, HI 96822, USA.

Although the chemistry of phosphorus and nitrogen has fascinated chemists for more than 350 years, the Hückel aromatic cyclotriphosphazene (PN, ) molecule-a key molecular building block in phosphorus chemistry-has remained elusive. Here, we report a facile, versatile pathway producing cyclotriphosphazene and its Dewar benzene-type isomer (PN, ) in ammonia-phosphine ices at 5 K exposed to ionizing radiation. Both isomers were detected in the gas phase upon sublimation via photoionization reflectron time-of-flight mass spectrometry and discriminated via isomer-selective photochemistry. Our findings provide a fundamental framework to explore the preparation of inorganic, isovalent species of benzene (CH) by formally replacing the C─H moieties alternatingly through phosphorus and nitrogen atoms, thus advancing our perception of the chemical bonding of phosphorus systems.
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http://dx.doi.org/10.1126/sciadv.aba6934DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7439403PMC
July 2020
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