Brant M. Jones

MECHANISTICAL STUDIES ON THE PRODUCTION OF FORMAMIDE (H2NCHO) WITHIN INTERSTELLAR ICE ANALOGS

Formamide, H2NCHO, represents the simplest molecule containing the peptide bond. Consequently, the formamide molecule is of high interest as it is considered an important precursor in the abiotic synthesis of amino acids, and thus significant to further prebiotic chemistry, in more suitable environments. Previous experiments have demonstrated that formamide is formed under extreme conditions similar throughout the interstellar medium via photolysis and the energetic processing of ultracold interstellar and solar system ices with high-energy protons; however, no clear reaction mechanism has been identified. Utilizing a laboratory apparatus capable of simulating the effects of galactic cosmic radiation on ultralow temperature ice mixtures, we have examined the formation of formamide starting from a variety of carbon monoxide (CO) to ammonia (NH3) ices of varying composition. Our results suggest that the primary reaction step leading to the production of formamide in low-temperature ices involves the cleavage of the nitrogen-hydrogen bond of ammonia forming the amino radical (NH2) and atomic hydrogen (H), the latter of which containing excess kinetic energy. These suprathermal hydrogen atoms can then add to the carbon-oxygen triple bond of the carbon monoxide (CO) molecule, overcoming the entrance barrier, and ultimately producing the formyl radical (HCO). From here, the formyl radical may combine without an entrance barrier with the neighboring amino radical if the proper geometry for these two species exists within the matrix cage.

Formation of benzene in the interstellar medium

Polycyclic aromatic hydrocarbons and related species have been suggested to play a key role in the astrochemical evolution of the interstellar medium, but the formation mechanism of even their simplest building block—the aromatic benzene molecule—has remained elusive for decades. Here we demonstrate in crossed molecular beam experiments combined with electronic structure and statistical calculations that benzene (C6H6) can be synthesized via the barrierless, exoergic reaction of the ethynyl radical and 1,3-butadiene, C2H + H2CCHCHCH2 → C6H6 + H, under single collision conditions. This reaction portrays the simplest representative of a reaction class in which aromatic molecules with a benzene core can be formed from acyclic precursors via barrierless reactions of ethynyl radicals with substituted 1,3-butadiene molecules. Unique gas-grain astrochemical models imply that this low-temperature route controls the synthesis of the very first aromatic ring from acyclic precursors in cold molecular clouds, such as in the Taurus Molecular Cloud. Rapid, subsequent barrierless reactions of benzene with ethynyl radicals can lead to naphthalene-like structures thus effectively propagating the ethynyl-radical mediated formation of aromatic molecules in the interstellar medium.

Application of Reflectron Time-of-Flight Mass Spectroscopy in the Analysis of Astrophysically Relevant Ices Exposed to Ionization Radiation: Methane (CH4) and D4-Methane (CD4) as a Case Study

Methane ices have been detected on ice-coated interstellar grains and on the surface of Kuiper belt objects. These ices are chemically altered by ionizing radiation in the form of energetic photons and charged particles, leading to complex organic molecules. Despite decades of research, the chemical makeup of these newly synthesized molecules has not been completely understood to date. Here, we present a novel application of reflectron time-of-flight mass spectrometry coupled to soft photoionization to probe the molecular formulas of the molecules formed upon interaction of ionizing radiation with simple methane and D4-methane ices. Our study depicts clear evidence of high-molecular-weight hydrocarbons of up to C22, among them alkanes, alkenes, and alkynes/dienes, with those product classes in italics identified for the first time on line and in situ. These studies are particular timely as they provide laboratory data of methane-processed ices, which can be compared to actual data from the New Horizons mission on route to Pluto.

Formation of the Phenyl Radical [C6H5(X2A1)] under Single Collision Conditions: A Crossed Molecular Beam and ab Initio Study

Reactions of dicarbon molecules (C(2)) with C(4)H(6) isomers such as 1,3-butadiene represent a potential, but hitherto unnoticed, route to synthesize the first aromatic C(6) ring in hydrocarbon flames and in the interstellar medium where concentrations of dicarbon transient species are significant. Here, crossed molecular beams experiments of dicarbon molecules in their X(1)Sigma(g)(+) electronic ground state and in the first electronically excited a(3)Pi(u) state have been conducted with 1,3-butadiene and two partially deuterated counterparts (1,1,4,4-D4-1,3-butadiene and 2,3-D2-1,3-butadiene) at two collision energies of 12.7 and 33.7 kJ mol(-1). Combining these scattering experiments with electronic structure and RRKM calculations on the singlet and triplet C(6)H(6) surfaces, our investigation reveals that the aromatic phenyl radical is formed predominantly on the triplet surface via indirect scattering dynamics through a long-lived reaction intermediate. Initiated by a barrierless addition of triplet dicarbon to one of the terminal carbon atoms of 1,3-butadiene, the collision complex undergoes trans-cis isomerization followed by ring closure and hydrogen migration prior to hydrogen atom elimination, ultimately forming the phenyl radical. The latter step emits the hydrogen atom almost perpendicularly to the rotational plane of the decomposing intermediate and almost parallel to the total angular momentum vector. On the singlet surface, smaller contributions of phenyl radical could not be excluded; experiments with partially deuterated 1,3-butadiene indicate the formation of the thermodynamically less stable acyclic H(2)CCHCCCCH(2) isomer. This study presents the very first experimental evidence, contemplated by theoretical studies, that under single collision conditions an aromatic hydrocarbon molecule can be formed in a bimolecular gas-phase reaction via reaction of two acyclic molecules involving cyclization processes at collision energies highly relevant to combustion flames.

FORMATION OF KETENE (H2CCO) IN INTERSTELLAR ANALOGOUS METHANE (CH4)-CARBON MONOXIDE (CO) ICES: A COMBINED FTIR AND REFLECTRON TIME-OF-FLIGHT MASS SPECTROSCOPIC STUDY

The formation of ketene (H2CCO) in methane-carbon monoxide (CH4-CO) ices was investigated upon its exposure to ionizing radiation in the form of energetic electrons at 5.5 K. The radiation-induced nonthermal equilibrium processing of these ices was monitored online and in situ via infrared spectroscopy complimented with post-irradiation temperature programmed desorption studies exploiting highly sensitive reflectron time-of-flight mass spectrometry (ReTOF) coupled with single photon fragment-free photo ionization (PI) at 10.49 eV. The detection of ketene in irradiated (isotopically labeled) methane-carbon monoxide ices was confirmed via the ν2 infrared absorption band and substantiated during the warm-up phase based on sublimation profiles obtained from the ReTOF-PI spectra of the corresponding isotopic masses. The experiments conducted with the mixed isotopic ices of 12CD4-13CO provide clear evidence of the formation of at least two ketene isotopomers (D2 12C13CO and D2 13C13CO), allowing for the derivation of two competing formation pathways. We have also proposed underlying reaction mechanisms to the formation of ketene based on kinetic fitting of the temporal evolution of the ketene isotopomers.

Crossed Molecular Beam Study on the Formation of Phenylacetylene and Its Relevance to Titan’s Atmosphere

The crossed molecular beam experiment of the deuterated ethynyl radical (C(2)D; X(2)Sigma(+)) with benzene [C(6)H(6)(X(1)A(1g))] and its fully deuterated analog [C(6)D(6)(X(1)A(1g))] was conducted at a collision energy of 58.1 kJ mol(-1). Our experimental data suggest the formation of the phenylacetylene-d(6) via indirect reactive scattering dynamics through a long-lived reaction intermediate; the reaction is initiated by a barrierless addition of the ethynyl-d(1) radical to benzene-d(6). This initial collision complex was found to decompose via a tight exit transition state located about 42 kJ mol(-1) above the separated products; here, the deuterium atom is ejected almost perpendicularly to the rotational plane of the decomposing intermediate and almost parallel to the total angular momentum vector. The overall experimental exoergicity of the reaction is shown to be 121 +/- 10 kJ mol(-1); this compares nicely with the computed reaction energy of -111 kJ mol(-1). Even though the experiment was conducted at a collisional energy higher than equivalent temperatures typically found in the atmosphere of Titan (94 K and higher), the reaction may proceed in Titans atmosphere as it involves no entrance barrier, all transition states involved are below the energy of the separated reactants, and the reaction is exoergic. Further, the phenylacetylene was found to be the sole reaction product.

ON THE FORMATION AND ISOMER SPECIFIC DETECTION OF PROPENAL (C2H3CHO) AND CYCLOPROPANONE (c-C3H4O) IN INTERSTELLAR MODEL ICES—A COMBINED FTIR AND REFLECTRON TIME-OF-FLIGHT MASS SPECTROSCOPIC STUDY

The formation routes of two structural isomers—propenal (C2H3CHO) and cyclopropanone (c-C3H4O)—were investigated experimentally by exposing ices of astrophysical interest to energetic electrons at 5.5 K thus mimicking the interaction of ionizing radiation with interstellar ices in cold molecular clouds. The radiationinduced processing of these ices was monitored online and in situ via Fourier Transform Infrared spectroscopy and via temperature programmed desorption exploiting highly sensitive reflectron time-of-flight mass spectrometry coupled with single photon ionization in the post irradiation phase. To selectively probe which isomer(s) is/are formed, the photoionization experiments were conducted with 10.49 and 9.60 eV photons. Our studies provided compelling evidence on the formation of both isomers—propenal (C2H3CHO) and cyclopropanone (c-C3H4O)—in ethylene (C2H4)—carbon monoxide (CO) ices forming propenal and cyclopropanone at a ratio of (4.5± 0.9):1. Based on the extracted reaction pathways, the cyclopropanone molecule can be classified as a tracer of a low temperature non-equilibrium chemistry within interstellar ices involving most likely excited triplet states, whereas propenal can be formed at ultralow temperatures, but also during the annealing phase via non-equilibrium as well as thermal chemistry (radical recombination). Since propenal has been detected in the interstellar medium and our laboratory experiments demonstrate that both isomers originated from identical precursor molecules our study predicts that the hitherto elusive second isomer—cyclopropanone—should also be observable toward those astronomical sources such as Sgr B2(N) in which propenal has been detected.

Detection of the Elusive Triazane Molecule (N3H5) in the Gas Phase

We report the detection of triazane (N3 H5 ) in the gas phase. Triazane is a higher order nitrogen hydride of ammonia (NH3 ) and hydrazine (N2 H4 ) of fundamental importance for the understanding of the stability of single-bonded chains of nitrogen atoms and a potential key intermediate in hydrogen-nitrogen chemistry. The experimental results along with electronic-structure calculations reveal that triazane presents a stable molecule with a nitrogen-nitrogen bond length that is a few picometers shorter than that of hydrazine and has a lifetime exceeding 6±2 μs at a sublimation temperature of 170 K. Triazane was synthesized through irradiation of ammonia ice with energetic electrons and was detected in the gas phase upon sublimation of the ice through soft vacuum ultraviolet (VUV) photoionization coupled with a reflectron-time-of-flight mass spectrometer. Isotopic substitution experiments exploiting [D3 ]-ammonia ice confirmed the identification through the detection of its fully deuterated counterpart [D5 ]-triazane (N3 D5 ).

High-sensitivity Raman spectrometer to study pristine and irradiated interstellar ice analogs.

We discuss the novel design of a sensitive, normal-Raman spectrometer interfaced to an ultra-high vacuum chamber (5 × 10(-11) Torr) utilized to investigate the interaction of ionizing radiation with low temperature ices relevant to the solar system and interstellar medium. The design is based on a pulsed Nd:YAG laser which takes advantage of gating techniques to isolate the scattered Raman signal from the competing fluorescence signal. The setup incorporates innovations to achieve maximum sensitivity without detectable heating of the sample. Thin films of carbon dioxide (CO2) ices of 10 to 396 nm thickness were prepared and characterized using both Fourier transform infrared (FT-IR) spectroscopy and HeNe interference techniques. The ν+ and ν- Fermi resonance bands of CO2 ices were observed by Raman spectroscopy at 1385 and 1278 cm(-1), respectively, and the band areas showed a linear dependence on ice thickness. Preliminary irradiation experiments are conducted on a 450 nm thick sample of CO2 ice using energetic electrons. Both carbon monoxide (CO) and the infrared inactive molecular oxygen (O2) products are readily detected from their characteristic Raman bands at 2145 and 1545 cm(-1), respectively. Detection limits of 4 ± 3 and 6 ± 4 monolayers of CO and O2 were derived, demonstrating the unique power to detect newly formed molecules in irradiated ices in situ. The setup is universally applicable to the detection of low-abundance species, since no Raman signal enhancement is required, demonstrating Raman spectroscopy as a reliable alternative, or complement, to FT-IR spectroscopy in space science applications.

FORMATION OF D2-WATER AND D2-CARBONIC ACID IN OXYGEN-RICH SOLAR SYSTEM ICES VIA D+2 IRRADIATION

Molecular oxygen (O2) and carbon dioxide (CO2) ices were irradiated with energetic D+ 2 ions to simulate the exposure of oxygen-bearing solar system ices to magnetospheric H+ 2 and H+ ions and energetic protons from the solar wind. The experiments provided evidence on the incorporation of the implanted deuterium and inherent formation of D2-water (D2O) as well as D2-carbonic acid (D2CO3). In the molecular oxygen ices, the temporal profiles inferred that D2-water formation followed successive deuterium atom addition to atomic oxygen via a D-hydroxyl radical intermediate in the matrix. In the carbon dioxide ices, D2-carbonic acid was likely formed via successive deuterium atom reaction with cyclic carbon trioxide. These chemical processes have specific relevance to water formation on outer solar system bodies, such as the icy moons of Jupiter and Saturn, as well as possible implications for the formation of water on the lunar surface.