Molecular growth upon ionization of van der Waals clusters containing HCCH and HCN is a pathway to prebiotic molecules

Structures and Stabilities of Carbon Chain Clusters Influenced by Atomic Antimony

The C-C bond lengths of the linear magnetic neutral C_nSb, C_nSb⁺ cations and C_nSb^- anions are within 1.255-1.336 Å, which is typical for cumulene structures with moderately strong double-bonds. In this report, we found that the adiabatic ionization energy (IE) of C_nSb decreased with n. When comparing the IE~n relationship of C_nSb with that of pure C_n, we found that the latter exhibited a stair-step pattern (n ≥ 6), but the IE~n relationship of C_nSb chains took the shape of a flat curve. The IEs of C_nSb were lower than those of corresponding pure carbon chains. Different from pure carbon chains, the adiabatic electron affinity of C_nSb does not exhibit a parity effect. There is an even-odd alternation for the incremental binding energies of the open chain C_nSb (for n = 1-16) and C_nSb⁺ (n = 1-10, when n > 10, the incremental binding energies of odd (n) chain of C_nSb⁺ are larger than adjacent clusters). The difference in the incremental binding energies between the even and odd chains of both C_nSb and pure C_n diminishes with the increase in n. The incremental binding energies for C_nSb^- anions do not exhibit a parity effect. For carbon chain clusters, the most favorable binding site of atomic antimony is the terminal carbon of the carbon cluster because the terminal carbon with a large spin density bonds in an unsaturated way. The C-Sb bond is a double bond with Wiberg bond index (WBI) between 1.41 and 2.13, which is obviously stronger for a carbon chain cluster with odd-number carbon atoms. The WBI of all C-C bonds was determined to be between 1.63 and 2.01, indicating the cumulene character of the carbon chain. Generally, the alteration of WBI and, in particular, the carbon chain cluster is consistent with the bond length alteration. However, the shorter C-C distance did not indicate a larger WBI. Rather than relying on the empirical comparison of bond distance, the WBI is a meaningful quantitative indicator for predicting the bonding strength in the carbon chain.

The Binary-Encounter-Bethe Model for Computation of Singly Differential Cross Sections Due to Electron-Impact Ionization

In the present work, we assess the effectiveness of singly differential cross sections (SDCS) due to electron-impact ionization by invoking the binary-encounter-Bethe (BEB) model on various atomic and molecular targets. The computed results were compared with the experimental and theoretical data. A good agreement was observed between the present and the available results. This agreement improves as the incident energy of the projectile increases. The model can be applied to compute the SDCS for the ions produced due to the electron-impact dissociative ionization process and the average energy due to the secondary electrons. Both these quantities are of interest in plasma processing and radiation physics.

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.

The Effect of Cluster Size on the Intra-Cluster Ionic Polymerization Process

Polyaromatic hydrocarbons (PAHs) are widespread in the interstellar medium (ISM). The abundance and relevance of PAHs call for a clear understanding of their formation mechanisms, which, to date, have not been completely deciphered. Of particular interest is the formation of benzene, the basic building block of PAHs. It has been shown that the ionization of neutral clusters can lead to an intra-cluster ionic polymerization process that results in molecular growth. Ab-initio molecular dynamics (AIMD) studies in clusters consisting of 3-6 units of acetylene modeling ionization events under ISM conditions have shown maximum aggregation of three acetylene molecules forming bonded C₆H₆⁺ species; the larger the number of acetylene molecules, the higher the production of C₆H₆⁺. These results lead to the question of whether clusters larger than those studied thus far promote aggregation beyond three acetylene units and whether larger clusters can result in higher C₆H₆⁺ production. In this study, we report results from AIMD simulations modeling the ionization of 10 and 20 acetylene clusters. The simulations show aggregation of up to four acetylene units producing bonded C₈H₈⁺. Interestingly, C₈H₈⁺ bicyclic species were identified, setting a precedent for their astrochemical identification. Comparable reactivity rates were shown with 10 and 20 acetylene clusters.

Molecular dynamics reveals formation path of benzonitrile and other molecules in conditions relevant to the interstellar medium

Polycyclic aromatic hydrocarbons and polycyclic aromatic nitrogen heterocycles are believed to be widespread in different areas of the interstellar medium. However, the astronomical detection of specific aromatic molecules is extremely challenging. As a result, only a few aromatic molecules have been identified, and very little is known about how they are formed in different areas of the interstellar medium. Recently, McGuire et al. [Science 359, 202-205 (2018)] detected the simple aromatic molecule benzonitrile in Taurus Molecular Cloud-1. Although benzonitrile has been observed, the molecular mechanism for its formation is still unknown. In this study, we use quantum chemistry and ab initio molecular dynamics to model ionization processes of van der Waals clusters containing cyanoacetylene and acetylene molecules. We demonstrate computationally that the clusters' ionization leads to molecular formation. For pure cyanoacetylene clusters, we observe bond formation among two and three monomer units, whereas in mixed clusters, bond formation can also occur in up to four units. We show that the large amount of energy available to the system after ionization can lead to barrier crossing and the formation of complex molecules. Our study reveals the rich chemistry that is observed upon ionization of the clusters, with a wide variety of molecules being formed. Benzonitrile is among the observed molecules, and we study the potential energy path for its formation. These results also offer insights that can guide astronomers in their search for aromatic molecules in the interstellar medium.

Formation of covalently bound C4H4 + upon electron-impact ionization of acetylene dimer

We investigate the formation mechanisms of covalently bound C₄H₄ ⁺ cations from direct ionization of hydrogen bonded dimers of acetylene molecules through fragment ion and electron coincident momentum spectroscopy and quantum chemistry calculations. The measurements of momenta and energies of two outgoing electrons and one ion in triple-coincidence allow us to assign the ionization channels associated with different ionic fragments. The measured binding energy spectra show that the formation of C₄H₄ ⁺ can be attributed to the ionization of the outermost 1π_u orbital of acetylene. The kinetic energy distributions of the ionic fragments indicate that the C₄H₄ ⁺ ions originate from direct ionization of acetylene dimers while ions resulting from the fragmentation of larger clusters would obtain significantly larger momenta. The formation of C₄H₄ ⁺ through the evaporation mechanism in larger clusters is not identified in the present experiments. The calculated potential energy curves show a potential well for the electronic ground state of (C₂H₂)2⁺, supporting that the ionization of (C₂H₂)₂ dimers can form stable C₂H₂⋅C₂H₂ ⁺(1π_u ^-1) cations. Further transition state analysis and ab initio molecular dynamics simulations reveal a detailed picture of the formation dynamics. After ionization of (C₂H₂)₂, the system undergoes a significant rearrangement of the structure involving, in particular, C-C bond formation and hydrogen migrations, leading to different C₄4⁺ isomers.