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

Interstellar Formation of Nitrogen Heteroaromatics [Indole, C8H7N; Pyrrole, C4H5N; Aniline, C6H5NH2]: Key Precursors to Amino Acids and Nucleobases

Nitrogen-substituted polycyclic aromatic hydrocarbons (NPAHs) are not only fundamental building blocks in the prebiotic synthesis of vital biomolecules such as amino acids and nucleobases of DNA and RNA but also a potential source of the prominent unidentified 6.2 μm interstellar absorption band. Although NPAHs have been detected in meteorites and are believed to be ubiquitous in the universe, their formation mechanisms in deep space have remained largely elusive. Here, we report the first bottom-up formation pathways to the simplest prototype of NPAHs, indole (C8H7N), along with its building blocks pyrrole (C4H5N) and aniline (C6H5NH2) in low-temperature model interstellar ices composed of acetylene (C2H2) and ammonia (NH3). Utilizing the isomer-selective techniques of resonance-enhanced multiphoton ionization and tunable vacuum ultraviolet photoionization reflectron time-of-flight mass spectrometry, indole, pyrrole, and aniline were identified in the gas phase, suggesting that they are promising candidates for future astronomical searches in star-forming regions. Our laboratory experiments utilizing infrared spectroscopy and mass spectrometry in tandem with electronic structure calculations reveal critical insights into the reaction pathways toward NPAHs and their precursors, thus advancing our fundamental understanding of the interstellar formation of aromatic proteinogenic amino acids and nucleobases, key classes of molecules central to the Origins of Life.

Bridging the gap: viable reaction pathways from tetrahedrane to benzyne

The addition of sp-carbon-containing molecules to polycyclic sp3 tetrahedrane (c-C4H4) results in the formation of both o-benzyne (c-C6H4) and benzene (c-C6H6). Since both c-C6H4 and c-C6H6 have been detected in the interstellar medium (ISM), providing additional pathways for their possible astrochemical formation mechanisms can lead to the discovery of other molecules, such as c-C4H4, benzvalyne, and vinylidene (:CCH2). Addition of diatomic carbon (C2), the ethynyl radical (C2H), vinylidene, and acetylene (HCCH) to c-C4H4 is undertaken in individual pathways through high-level quantum chemical computations at the CCSD(T)-F12b/cc-pVTZ-F12 level of theory. The resulting C2 addition pathway proceeds barrierlessly through benzvalyne as an intermediate and reaches a true minimum at c-C6H4, but no leaving groups are produced which is required to dissipate excess energy within an interstellar chemical scheme. Similarly, the C2H addition to c-C4H4 produces benzvalyne as well as its related isomers. This pathway allows for the loss of a hydrogen leaving group to dissipate the resulting energy. Lastly, the HCCH and :CCH2 addition pathways follow through both benzvalene and benzvalyne in order to reach c-C6H6 (benzene) and c-C6H4 (o-benzyne) as well as H2 as the required leaving group. Although there is a barrier to the HCCH addition, the :CCH2 addition presents the contrary with only submerged barriers. These proposed mechanisms provide alternative possibilities for the formation of complex organic molecules in space.

Quantum Chemistry and Astrochemistry: A Match Made in the Heavens

Quantum chemistry can uniquely answer astrochemical questions that no other technique can provide. Computations can be parallelized, automated, and left to run continuously providing exceptional molecular throughput that cannot be done through experimentation. Additionally, the granularity of the individual computations that are required of potential energy surfaces, reaction mechanism pathways, or other quantum chemically derived observables produces a unique mosaic that make up the larger whole. These pieces can be dissected for their individual contributions or evaluated in an ad hoc fashion for each of their roles in generating the larger whole. No other scientific approach is capable of reporting such fine-grained insights. Quantum chemistry also works from a bottom-up approach in providing properties directly from the desired molecule instead of a top-down perspective as required of experiment where molecules have to be linked to observed phenomena. Furthermore, modern quantum chemistry is well within the range of "chemical accuracy" and is approaching "spectroscopic accuracy." As such, the seemingly difficult questions asked by astrochemistry that would not be asked initially for any other application require quantum chemical reference data. While the results of quantum chemical computations are needed to interpret astrochemical observation, modeling, or laboratory experimentation, such hard questions, regardless of the original need to answer them, produce unique solutions. While questions in astrochemistry often require novel developments in and implementations of quantum chemistry as outlined herein, the applications of these solutions will stretch beyond astrochemistry and may yet impact fields much closer to Earth.

3-Fluoro-2H-azirine: Generation, Characterization, and Photochemistry

The elusive 3-fluoro-2H-azirine, cyclic NCH2CF, has been generated through the stepwise decomposition of the acryloyl azide CH2CFC(O)N3 in an N2-matrix at 10 K. The characterization of cyclic NCH2CF with matrix-isolation IR spectroscopy is supported by 15N isotope labeling and the calculations with density functional theory (DFT) at the B3LYP/6-311++G(3df,3pd) level of theory. Upon irradiation at 193 nm, cyclic NCH2CF undergoes ring opening by forming the more stable nitrile isomer CH2FCN. In contrast to the photodecomposition reactions, the high-vacuum flash pyrolysis of CH2CFC(O)N3 in the gas phase at 500 °C yields the Curtius rearrangement product CH2CFNCO along with secondary fragmentation to the atmospherically relevant fluorocarbonyl radical (FCO) and cyanomethyl radical (CH2CN). Calculations on the potential energy profile for the decomposition reactions of CH2CFC(O)N3 demonstrate that the excessive energy, arising from the highly exothermic Curtius rearrangement of the azide, plays a key role in driving further dissociation reactions of CH2CFNCO by overcoming the formidable barriers (>50 kcal mol-1) under the pyrolysis conditions.

Formation of the elusive tetrahedral P3N molecule

The tetrahedral 1,2,3-triphospha-4-azatricyclo [1.1.0.0^2,4] butane (P₃N) molecule-an isovalent species of phosphorus (P₄)-was prepared in low-temperature (5 K) phosphine-nitrogen ices and was identified in the gas phase through isomer-selective, tunable, soft photoionization reflectron time-of-flight mass spectrometry. Theoretical calculations reveal that the substitution of a single phosphorus atom by nitrogen in the P₄ molecule results in enhanced spherical aromaticity while simultaneously increasing the strain energy from 74 to 195 kJ mol^-1. In P₃N, the P─P bond is shortened compared to those in P₄ by 3.6 pm, while the P─N─P bond angle of 73.0° is larger by 13.0° compared to the P─P─P bond angle of 60.0° in P₄. The identification of tetrahedral P₃N enhances our fundamental understanding of the chemical bonding, electronic structure, and stability of binary, interpnictide tetrahedral molecules and reveals a universal route to prepare ring strained cage molecules in extreme environments.

A Search for Heterocycles in GOTHAM Observations of TMC-1

We have conducted an extensive search for nitrogen-, oxygen-, and sulfur-bearing heterocycles toward Taurus Molecular Cloud 1 (TMC-1) using the deep, broadband centimeter-wavelength spectral line survey of the region from the GOTHAM large project on the Green Bank Telescope. Despite their ubiquity in terrestrial chemistry, and the confirmed presence of a number of cyclic and polycyclic hydrocarbon species in the source, we find no evidence for the presence of any heterocyclic species. Here, we report the derived upper limits on the column densities of these molecules obtained by Markov Chain Monte Carlo (MCMC) analysis and compare this approach to traditional single-line upper limit measurements. We further hypothesize why these molecules are absent in our data, how they might form in interstellar space, and the nature of observations that would be needed to secure their detection.

Experimental identification of aminomethanol (NH2CH2OH)-the key intermediate in the Strecker Synthesis

The Strecker Synthesis of (a)chiral α-amino acids from simple organic compounds, such as ammonia (NH₃), aldehydes (RCHO), and hydrogen cyanide (HCN) has been recognized as a viable route to amino acids on primordial earth. However, preparation and isolation of the simplest hemiaminal intermediate – the aminomethanol (NH₂CH₂OH)– formed in the Strecker Synthesis to even the simplest amino acid glycine (H₂NCH₂COOH) has been elusive. Here, we report the identification of aminomethanol prepared in low-temperature methylamine (CH₃NH₂) – oxygen (O₂) ices upon exposure to energetic electrons. Isomer-selective photoionization time-of-flight mass spectrometry (PI-ReTOF-MS) facilitated the gas phase detection of aminomethanol during the temperature program desorption (TPD) phase of the reaction products. The preparation and observation of the key transient aminomethanol changes our perception of the synthetic pathways to amino acids and the unexpected kinetic stability in extreme environments.

The Strecker synthesis is considered a viable route to amino acids formation on the primordial Earth. Here the authors succeed in observing its elusive intermediate aminomethanol, formed by insertion of an electronically excited oxygen atom in methylamine and stabilized by an icy matrix, using isomer-selective photoionization time-of-flight mass spectrometry during thermal desorption of the ice mixture.