On the formation and the isomer specific detection of methylacetylene (CH3CCH), propene (CH3CHCH2), cyclopropane (c-C3H6), vinylacetylene (CH2CHCCH), and 1,3-butadiene (CH2CHCHCH2) from interstellar methane ice analogues

On-surface chemical dynamics of monolayer, bilayer, and many-layered graphene surfaces probed with supersonic beam scattering and STM imaging

We have developed the capability to elucidate interfacial reaction dynamics using an arguably unique combination of supersonic molecular beams combined with in situ STM visualization. These capabilities have been implemented in order to reveal the complex spatiotemporal correlations that govern the oxidation of graphitic systems spanning atomic-, nano-, and meso-length scales. In this study, the 3 nm periodic moiré pattern of monolayer and bilayer graphene on Ru(0001) provides a diverse palette of potential scattering and binding sites at the interface for ground state atomic oxygen. We resolve the site-specificity of atomic oxygen placement on the moiré lattice for both monolayer and bilayer graphene on Ru(0001) with atomic resolution. Angle- and energy-controlled scattering of O(3P) on these interfaces reveals an incisive side-by-side comparison of preferential reactivity of the monolayer surface compared to a more free-standing bilayer graphene ruthenium interface. Morphologically dependent reactivity of many layered graphene (HOPG) and monolayer graphene on Ru(0001) reveal anisotropic on-surface reactivity dependent on the presence of proximal reacted sites or local regions. The kinetics of on-surface oxidation are additionally shown to influence the morphology of surface products by varying the temperature of the interface and flux of reactant species. Such correlations are important in chemisorption, catalysis, materials oxidation and erosion, and film processing-and tunable moiré templated adsorption is a route to well-ordered self-assembled 2D materials for use in next-generation platforms for quantum devices and catalysis. Taken together, these results highlight a new direction in the examination of interfacial reaction dynamics where incident beam kinetic energy and angle of incidence can be used as reaction control parameters, with outcomes such as site-specific reactivity, changes for overall time-evolving mechanisms, and the relative importance of non-adiabatic channels in adsorption all linked to the on-surface fate of chemisorbed species.

The Composition and Chemistry of Titan's Atmosphere

In this review I summarize the current state of knowledge about the composition of Titan's atmosphere and our current understanding of the suggested chemistry that leads to that observed composition. I begin with our present knowledge of the atmospheric composition, garnered from a variety of measurements including Cassini-Huygens, the Atacama Large Millimeter/submillimeter Array, and other ground- and space-based telescopes. This review focuses on the typical vertical profiles of gases at low latitudes rather than global and temporal variations. The main body of the review presents a chemical description of how complex molecules are believed to arise from simpler species, considering all known "stable" molecules-those that have been uniquely identified in the neutral atmosphere. The last section of the review is devoted to the gaps in our present knowledge of Titan's chemical composition and how further work may fill those gaps.

Thermal Synthesis of Carbamic Acid and Its Dimer in Interstellar Ices: A Reservoir of Interstellar Amino Acids

Reactions in interstellar ices are shown to be capable of producing key prebiotic molecules without energetic radiation that are necessary for the origins of life. When present in interstellar ices, carbamic acid (H2NCOOH) can serve as a condensed-phase source of the molecular building blocks for more complex proteinogenic amino acids. Here, Fourier transform infrared spectroscopy during heating of analogue interstellar ices composed of carbon dioxide and ammonia identifies the lower limit for thermal synthesis to be 62 ± 3 K for carbamic acid and 39 ± 4 K for its salt ammonium carbamate ([H2NCOO-][NH4+]). While solvation increases the rates of formation and decomposition of carbamic acid in ice, the absence of solvent effects after sublimation results in a significant barrier to dissociation and a stable gas-phase molecule. Photoionization reflectron time-of-flight mass spectrometry permits an unprecedented degree of sensitivity toward gaseous carbamic acid and demonstrates sublimation of carbamic acid from decomposition of ammonium carbamate and again at higher temperatures from carbamic acid dimers. Since the dimer is observed at temperatures up to 290 K, similar to the environment of a protoplanetary disk, this dimer is a promising reservoir of amino acids during the formation of stars and planets.

Differential Condensation of Methane Isotopologues Leading to Isotopic Enrichment under Non-equilibrium Gas-Surface Collision Conditions

We examine the initial differential sticking probability of CH4 and CD4 on CH4 and CD4 ices under nonequilibrium flow conditions using a combination of experimental methods and numerical simulations. The experimental methods include time-resolved in situ reflection-absorption infrared spectroscopy (RAIRS) for monitoring on-surface gaseous condensation and complementary King and Wells mass spectrometry techniques for monitoring sticking probabilities that provide confirmatory results via a second independent measurement method. Seeded supersonic beams are employed so that the entrained CH4 and CD4 have the same incident velocity but different kinetic energies and momenta. We found that as the incident velocity of CH4 and CD4 increases, the sticking probabilities for both molecules on a CH4 condensed film decrease systematically, but that preferential sticking and condensation occur for CD4. These observations differ when condensed CD4 is used as the target interface, indicating that the film's phonon and rovibrational densities of states, and collisional energy transfer cross sections, have a role in differential energy accommodation between isotopically substituted incident species. Lastly, we employed a mixed incident supersonic beam composed of both CH4 and CD4 in a 3:1 ratio and measured the condensate composition as well as the sticking probability. When doing so, we see the same effect in the condensed mixed film, supporting an isotopic enrichment of the heavier isotope. We propose that enhanced multi-phonon interactions and inelastic cross sections between the incident CD4 projectile and the CH4 film allow for more efficacious gas-surface energy transfer. VENUS code MD simulations show the same sticking probability differences between isotopologues as observed in the gas-surface scattering experiments. Ongoing analyses of these trajectories will provide additional insights into energy and momentum transfer between the incident species and the interface. These results offer a new route for isotope enrichment via preferential condensation of heavier isotopes and isotopologues during gas-surface collisions under specifically selected substrate, gas-mixture, and incident velocity conditions. They also yield valuable insights into gaseous condensation under non-equilibrium conditions such as occur in aircraft flight in low-temperature environments. Moreover, these results can help to explain the increased abundance of deuterium in solar system planets and can be incorporated into astrophysical models of interstellar icy dust grain surface processes.

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

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.

Thermal desorption effects on fragment ion production from multi-photon ionized uridine and selected analogues

Experiments on neutral gas-phase nucleosides are often complicated by thermal lability. Previous mass spectrometry studies of nucleosides have identified enhanced relative production of nucleobase ions (e.g. uracil⁺ from uridine) as a function of desorption temperature to be the critical indicator of thermal decomposition. On this basis, the present multi-photon ionization (MPI) experiments demonstrate that laser-based thermal desorption is effective for producing uridine, 5-methyluridine, and 2′-deoxyuridine targets without thermal decomposition. Our experiments also revealed one notable thermal dependence: the relative production of the sugar ion C₅H₉O₄⁺ from intact uridine increased substantially with the desorption laser power and this only occurred at MPI wavelengths below 250 nm (full range studied 222–265 nm). We argue that this effect can only be rationalized plausibly in terms of changing populations of different isomers, tautomers, or conformers in the target as a function of the thermal desorption conditions. Furthermore, the wavelength threshold behavior of this thermally-sensitive MPI channel indicates a critical dependence on neutral excited state dynamics between the absorption of the first and second photons. The experimental results are complemented by density functional theory (DFT) optimizations of the lowest-energy structure of uridine and two further conformers distinguished by different orientations of the hydroxymethyl group on the sugar part of the molecule. The energies of the transitions states between these three conformers are low compared with the energy required for decomposition.

This work reveals the first experimental evidence supporting isomer-dependence in the radiation response of a nucleoside.

A Photoionization Reflectron Time-of-Flight Mass Spectrometric Study on the Detection of Ethynamine (HCCNH2 ) and 2H-Azirine (c-H2 CCHN)

Ices of acetylene (C₂ H₂ ) and ammonia (NH₃ ) were irradiated with energetic electrons to simulate interstellar ices processed by galactic cosmic rays in order to investigate the formation of C₂ H₃ 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 C₂ H₃ 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 (HCCNH₂ ) and 2H-azirine (c-H₂ CCHN) in the irradiated C₂ H₂ :NH₃ 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.

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

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 within and throughout 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 (CH₃), amino (NH₂), hydroxyl (OH), ethyl (C₂H₅), vinyl (C₂H₃), ethynyl (C₂H), formyl (HCO), hydroxycarbonyl (HOCO), hydroxymethyl (CH₂OH), methoxy (CH₃O), and acetyl (CH₃CO) represent readily available reactants for radical-radical recombination within the ices. Reactive singlet species were found to insert without barrier into carbon-hydrogen and carbon-carbon single bonds (carbene) leading to an extension of the carbon chain and may add 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 throughout the ice at 5 K. Our investigations discriminate between multiple structural isomers such as alcohols/ethers, aldehydes/enols, and cyclic/acyclic carbonyls. These data provide quantitative, isomer selective 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 10⁶ to 10⁷ 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).

On the Formation of the Popcorn Flavorant 2,3-Butanedione (CH3 COCOCH3 ) in Acetaldehyde-Containing Interstellar Ices

Acetaldehyde (CH₃ CHO) is ubiquitous throughout the interstellar medium and has been observed in cold molecular clouds, star forming regions, and in meteorites such as Murchison. As the simplest methyl-bearing aldehyde, acetaldehyde constitutes a critical precursor to prebiotic molecules such as the sugar deoxyribose and amino acids via the Strecker synthesis. In this study, we reveal the first laboratory detection of 2,3-butanedione (diacetyl, CH₃ COCOCH₃ ) - a butter and popcorn flavorant - synthesized within acetaldehyde-based interstellar analog ices exposed to ionizing radiation at 5 K. Detailed isotopic substitution experiments combined with tunable vacuum ultraviolet (VUV) photoionization of the subliming molecules demonstrate that 2,3-butanedione is formed predominantly via the barrier-less radical-radical reaction of two acetyl radicals (CH₃ ĊO). These processes are of fundamental importance for a detailed understanding of how complex organic molecules (COMs) are synthesized in deep space thus constraining the molecular structures and complexity of molecules forming in extraterrestrial ices containing acetaldehyde through a vigorous galactic cosmic ray driven non-equilibrium chemistry.

Gas Phase Identification of the Elusive N-Hydroxyoxaziridine (c-H2CON(OH)): A Chiral Molecule

The hitherto elusive N-hydroxyoxaziridine molecule (c-H₂CON(OH)), a chiral, high energy isomer of nitromethane (CH₃NO₂) and one of the simplest representatives of an oxaziridine, is detected in the gas phase. Electronic structure calculations propose an impending synthesis eventually via addition of carbene (CH₂) to the nitrogen-oxygen double bond of nitrous acid (HONO). The oxaziridine ring demonstrates an unusual kinetic stability toward ring opening compared to the isoeletronic cyclopropane (C₃H₆) counterpart. This system defines a fundamental benchmark to explore the formation and stability of racemic derivatives of strained oxaziridines (c-H₂CONH) and changes our perception how we think about fundamental decomposition and isomerization mechanisms in (model compounds of) energetic materials.

Gas phase identification of the elusive oxaziridine (cyclo-H2CONH) – an optically active molecule

First detection of the chiral oxaziridine (cyclo-H₂CONH) molecule.

Low-temperature synthesis of polycyclic aromatic hydrocarbons in Titan's surface ices and on airless bodies

Titan's equatorial dunes represent the most monumental surface structures in our Solar System, but the chemical composition of their dark organics remains a fundamental, unsolved enigma, with solid acetylene detected near the dunes implicated as a key feedstock. Here, we reveal in laboratory simulation experiments that aromatics such as benzene, naphthalene, and phenanthrene-prospective building blocks of the organic dune material-can be efficiently synthesized via galactic cosmic ray exposure of low-temperature acetylene ices on Titan's surface, hence challenging conventional wisdom that aromatic hydrocarbons are formed solely in Titan's atmosphere. These processes are also of critical importance in unraveling the origin and chemical composition of the dark surfaces of airless bodies in the outer Solar System, where hydrocarbon precipitation from the atmosphere cannot occur. This finding notably advances our understanding of the distribution of carbon throughout our Solar System such as on Kuiper belt objects like Makemake.

Millimeter/Submillimeter Spectroscopic Detection of Desorbed Ices: A New Technique in Laboratory Astrochemistry

A new laboratory technique has been developed that utilizes gas-phase, direct-absorption millimeter and submillimeter spectroscopy to detect and identify desorbed species from interstellar and cometary ice analogues. Rotational spectroscopy is a powerful structure-specific technique for detecting isomers and other species possessing the same mass that are indistinguishable with mass spectrometry. Furthermore, the resultant laboratory spectra are directly comparable to observational data from far-infrared and millimeter telescopes. Here, we present the proof-of-concept measurements of the detection of thermally desorbed H₂O, D₂O, and CH₃OH originating in a solid film created at low temperature (∼12 K). The surface binding energy of H₂O is reported and compared to results from traditional techniques, including mass spectrometry and quartz-crystal microbalance measurements of mass loss. Lastly, we demonstrate that this technique can be used to derive thermodynamic values including the sublimation enthalpy and entropy of H₂O.

On the formation of complex organic molecules in the interstellar medium: untangling the chemical complexity of carbon monoxide-hydrocarbon containing ice analogues exposed to ionizing radiation via a combined infrared and reflectron time-of-flight analysis

Recently, over 200 molecules have been detected in the interstellar medium (ISM), with about one third being complex organic molecules (COMs), molecules containing six or more atoms. Over the last few decades, astrophysical laboratory experiments have shown that several COMs are formed via interaction of ionizing radiation within ices deposited on interstellar dust particles at 10 K (H₂O, CH₃OH, CO, CO₂, CH₄, NH₃). However, there is still a lack of understanding of the chemical complexity that is available through individual ice constituents. The present research investigates experimentally the synthesis of carbon, hydrogen, and oxygen bearing COMs from interstellar ice analogues containing carbon monoxide (CO) and methane (CH₄), ethane (C₂H₆), ethylene (C₂H₄), or acetylene (C₂H₂) exposed to ionizing radiation. Utilizing online and in situ techniques, such as infrared spectroscopy and tunable photoionization reflectron time-of-flight mass spectrometry (PI-ReTOF-MS), specific isomers produced could be characterized. A total of 12 chemically different groups were detected corresponding to C₂H_nO (n = 2, 4, 6), C₃H_nO (n = 2, 4, 6, 8), C₄H_nO (n = 4, 6, 8, 10), C₅H_nO (n = 4, 6, 8, 10), C₆H_nO (n = 4, 6, 8, 10, 12, 14), C₂H_nO₂ (n = 2, 4), C₃H_nO₂ (n = 4, 6, 8), C₄H_nO₂ (n = 4, 6, 8, 10), C₅H_nO₂ (n = 6, 8), C₆H_nO₂ (n = 8, 10, 12), C₄H_nO₃ (n = 4, 6, 8), and C₅H_nO₃ (n = 6, 8). More than half of these isomer specifically identified molecules have been identified in the ISM, and the remaining COMs detected here can be utilized to guide future astronomical observations. Of these isomers, three groups - alcohols, aldehydes, and molecules containing two of these functional groups - displayed varying degrees of unsaturation. Also, the detection of 1-propanol, 2-propanol, 1-butanal, and 2-methyl-propanal has significant implications as the propyl and isopropyl moieties (C₃H₇), which have already been detected in the ISM via propyl cyanide and isopropyl cyanide, could be detected in our laboratory studies. General reaction mechanisms for their formation are also proposed, with distinct follow-up studies being imperative to elucidate the complexity of COMs synthesized in these ices.

Combined Experimental and Computational Study on the Reaction Dynamics of the 1-Propynyl (CH3CC)-1,3-Butadiene (CH2CHCHCH2) System and the Formation of Toluene under Single Collision Conditions

The crossed beams reactions of the 1-propynyl radical (CH₃CC; X²A₁) with 1,3-butadiene (CH₂CHCHCH₂; X¹A_g), 1,3-butadiene- d₆ (CD₂CDCDCD₂; X¹A_g), 1,3-butadiene- d₄ (CD₂CHCHCD₂; X¹A_g), and 1,3-butadiene- d₂ (CH₂CDCDCH₂; X¹A_g) were performed under single collision conditions at collision energies of about 40 kJ mol^-1. The underlying reaction mechanisms were unraveled through the combination of the experimental data with electronic structure calculations at the CCSD(T)-F12/cc-pVTZ-f12//B3LYP/6-311G(d,p) + ZPE(B3LYP/6-311G(d,p) level of theory along with statistical Rice-Ramsperger-Kassel-Marcus (RRKM) calculations. Together, these data suggest the formation of the thermodynamically most stable C₇H₈ isomer-toluene (C₆H₅CH₃)-via the barrierless addition of 1-propynyl to the 1,3-butadiene terminal carbon atom, forming a low-lying C₇H₉ intermediate that undergoes multiple isomerization steps resulting in cyclization and ultimately aromatization following hydrogen atom elimination. RRKM calculations predict that the thermodynamically less stable isomers 1,3-heptadien-5-yne, 5-methylene-1,3-cyclohexadiene, and 3-methylene-1-hexen-4-yne are also synthesized. Since the 1-propynyl radical may be present in cold molecular clouds such as TMC-1, this pathway could potentially serve as a carrier of the methyl group incorporating itself into methyl-substituted (poly)acetylenes or aromatic systems such as toluene via overall exoergic reaction mechanisms that are uninhibited by an entrance barrier. Such pathways are a necessary alternative to existing high energy reactions leading to toluene that are formally closed in the cold regions of space and are an important step toward understanding the synthesis of polycyclic aromatic hydrocarbons (PAHs) in space's harsh extremes.