Rovibrational Quantum Chemical Treatment of Inorganic and Organometallic Astrochemicals

Theoretical Rovibrational Spectroscopy of Magnesium Tricarbide-Multireference Character Thwarts a Full Analysis of All Isomers

Magnesium tricarbide isomers are studied herein with coupled cluster theory and multireference configuration interaction to support their possible detection in astrochemical environments such as the circumstellar envelope surrounding the star IRC +10216 or in terrestrial laboratories. Magnesium-bearing species may abound in the interstellar medium (ISM), but only eight (MgNC, MgCN, HMgNC, MgC₂H, MgC₃N, MgC₄H, MgC₅N, and MgC₆H) have been directly identified thus far. Several possible isomers for the related MgC₃ system are explored in their singlet and triplet spin multiplicities. Overall, this work offers quantum chemical insight of rovibrational spectroscopic data for MgC₃ using quartic force fields (QFFs) based on the CCSD(T) and CCSD(T)-F12 levels of theory at the complete basis set (CBS) limit. Additional corrections with small basis set CCSDT(Q) and scalar relativistic effects are also included in the analysis. Salient multireference character is found in the singlet diamond electronic state, which makes a definitive assignment of the ground state challenging. Nevertheless, coupled cluster-based composite energies and multireference configuration interaction both predict that the ¹A₁ diamond isomer is 1.6-2.2 kcal mol^-1 lower in energy than the ³A₁ diamond isomer. Furthermore, highly accurate binding energies of various isomers MgC₃ are provided for comparison to photodetachment experiments. Dipole moments along with harmonic infrared intensities will guide efforts for astronomical and spectroscopic characterization.

Pathways for the Formation of Formamide, a Prebiotic Biomonomer: Metal-Ions in Interstellar Gas-Phase Chemistry

The chemistry occurring in the interstellar medium (ISM) is an active area of contemporary research. New aspects of interstellar chemistry are getting unraveled regularly. In this context, the role of metal-ions in the chemistry occurring in the ISM is not well-studied so far. Herein, we highlight the role of metal-ions in interstellar chemistry. For this purpose, we choose the problem of gas-phase formamide formation in interstellar molecular clouds. Formamide is a key biomonomer and contains the simplest peptide [-(C═O)-NH-] linkage. With its two electronegative atoms ("O" and "N"), it provides an excellent platform to probe the role of the metal-ions. The metal-ions chosen are Na⁺, K⁺, Al⁺, Mg⁺, and Mg²⁺-all of them present in the ISM. The metal-ions are studied in three different forms as bare positively charged ions, as hydrated metal-ions co-ordinated with a molecule of water, and when the metal-ions are part of a neutral covalent molecule. With the aid of electronic structure calculations [CCSD(T) and DFT methods], we study different gas-phase pathways which result in the generation of interstellar formamide. Throughout our study, we find that metal-ions lower the barriers (with Mg⁺, Mg⁺⁺, and Al⁺ offering maximal stabilization of the transition states) and facilitate the reactions. The chemical factors influencing the reactions, how we consider the putative conditions in the ISM, the astrochemical implications of this study, and its connection with terrestrial prebiotic chemistry and refractory astrochemistry are subsequently presented. Based on our results, we also recommend the detection of two new closed-shell molecules, NH₂CH₂OH (aminomethanol) and CH₂NH₂⁺ (iminium ion), and two open-shell molecules, CONH₂ (carbamyl radical) and HCONH (an isomer of carbamyl radical), in the ISM.