Min Xie

The abiotic formation of prebiotic molecules with peptide or peptide-like bonds in the cold gaseous interstellar medium (ISM) and (exo)planetary atmospheric environments remains a critical unsolved question for elucidating cosmic chemical evolution and the origin of life. To explore the potential formation pathway, we combined vacuum ultraviolet–infrared (VUV-IR) photodissociation spectroscopy with ab initio quantum chemical calculations to investigate the ion–molecule reactions of three unsaturated nitriles (acrylonitrile, AN; 3-butenenitrile, 3BN; and 4-pentenenitrile, 4PN) with water under 118 nm photoionization. Experimental spectra reveal that the reaction products show a pronounced dependence on the alkyl chain length of the nitrile precursors. When short-chain AN reacts with water, it exclusively forms a kinetically trapped imine-alcohol cation (H+–N≡C–R–OH). In contrast, longer-chain 3BN and 4PN undergo efficient barrierless cyclization upon reaction with water, forming stable five- or six-membered cyclic cations with peptide-like bonds, respectively. The conclusions were supported by theoretical calculations of reaction pathways, which further reveal that all these reactions initially proceed via a mechanism analogous to Michael addition under basic conditions. These findings provide direct evidence for an ionization-induced intracluster covalent bond formation (ICBF) pathway to prebiotic peptide-like bonds under astrochemically relevant conditions, highlighting the critical role of molecular structure in governing its reaction selectivity.

Nitrogen-substituted polycyclic aromatic hydrocarbons (PANHs) are critical intermediates in combustion pollutant formation and key prebiotic building blocks in interstellar chemistry. However, the fundamental mechanisms by which a nitrogen atom is directly incorporated into an aromatic ring—bypassing pre-existing nitrogenous structures—have remained elusive, hindering both predictive combustion models and astronomical searches. Here, we establish a previously unrecognized, barrierless pathway for the formation of quinoline (C ₉ H ₇ N), the simplest PANH prototype, via the radical–radical reaction between benzyl (•C ₇ H ₇ ) and cyanomethyl (•CH ₂ CN)—two species of well-established relevance in combustion and interstellar chemistry. High-level electronic structure calculations reveal that the reaction proceeds on a potential energy surface entirely submerged below the reactant asymptote, with the initial adduct formation exothermic by 72.5 kcal/mol and all subsequent cyclization and dehydrogenation steps facing no intrinsic barrier. This process is predicted to be efficient kinetically across a wide range of astrophysical and combustion temperatures. These predictions are corroborated experimentally using a SiC microreactor coupled with synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) supported by comprehensive two-dimensional gas chromatography–mass spectrometry (GC×GC-MS) analysis, which unequivocally identify quinoline and its elusive C ₉ H ₉ N intermediates and isomers. Beyond resolving a long-standing mechanistic puzzle, this work provides a mechanistic paradigm for direct nitrogen incorporation into aromatic networks, with profound implications for prebiotic chemistry and nitrogen-containing pollutant formation.

Enols, the tautomeric forms of carbonyl compounds, are considered to play key roles in the formation of prebiotic molecules in the interstellar medium. Herein, we investigated the possibility of nitrogen-containing molecules in cationic acetone enolization by measuring the infrared spectra of acetone-acetonitrile, acetone-acrylonitrile, and acetone-ammonia binary cationic complexes in the range of 2400-3800 cm^-1 using a vacuum ultraviolet photoionization and infrared dissociation (VUV-IR) scheme. To determine the structures of the complex isomers corresponding to the observed spectra, ab initio electronic structure calculations and infrared spectral simulations were employed. The results suggest that ammonia effectively catalyzes cationic acetone enolization, whereas acetonitrile and acrylonitrile do not exhibit observable catalytic activity in this process. Through reaction pathway searching and microcanonical rate constant calculations, the lack of catalytic activity in the nitrile-containing complexes stems from their low-energy lone-pair electrons and insufficient excess energy acquired during photoionization. These factors collectively hinder the key proton-transfer reaction underlying cationic acetone enolization. These findings provide insights into the multiscale regulatory mechanisms that control enolization pathways in interstellar chemistry.

Hydrogen bonds, crucial for the formation and stabilization of complex organic molecules, present intriguing possibilities for the existence of such molecular complexes in extraterrestrial environments. To investigate the microscopic hydrogen-bonding networks in nitrile–methanol clusters—potential candidates for interstellar organic complexes—we recorded infrared spectra of neutral XN-M (XN = nitriles; M = methanol) heterodimers in the 2600-3900 cm−1 range using infrared-vacuum ultraviolet (IR-VUV) spectroscopy. Quantum chemical calculations based on harmonic approximations are typically used to simulate such spectral peaks; however, they often fail to fully reproduce the experimental features. To overcome these limitations, we employed ab initio anharmonic algorithms and ab initio molecular dynamics (AIMD) simulations. Anharmonic algorithms successfully assigned elusive spectral features by accounting for vibrational couplings (e.g., Fermi resonances and combination bands), and AIMD proved essential for accurately assigning characteristically broad hydrogen-bonded OH bands unresolved by harmonic methods. Applying this integrated approach, we confirmed the distinct H-bonding preferences in the XN-M heterodimers: acetonitrile (ACN) and unsaturated aliphatic nitriles (e.g., 3-butene nitrile, 3BN; 4-pentene nitrile, 4PN) predominantly adopt linear N⋯H–O configurations, whereas acrylonitrile and benzonitrile favor cyclic double H-bonded structures. This study establishes a powerful framework that integrates advanced computational methodologies to decipher hydrogen-bonding motifs from complex spectral data, enabling the direct interpretation of both laboratory and astronomical infrared observations.

In this study, we tackle the challenge of synthesizing nitrogen-containing bicyclic compounds in the gas phase, a topic of considerable interest to interstellar chemistry and combustion science. We specifically explore the bimolecular reaction between the o-pyridinyl radical (C₅H₄N•), a quintessential nitrogen-containing monocyclic carbon-centered aromatic radical, and the interstellar molecule acrylonitrile (C₃H₃N) in a resistance-heated silicon carbide (SiC) microreactor. Utilizing advanced molecular beam-sampled synchrotron VUV photoionization mass spectrometry (SVUV-PIMS), we detected the formation of C₈H₆N₂ species at 1100 K. Photoionization efficiency (PIE) curve analysis suggested the exclusive production of a ring-tail structure, rather than the desired bicyclic 1,5-naphthyridine structure. High-level ab initio calculations revealed a detailed reaction mechanism that addition, cyclization, and subsequent dehydrogenation reactions between C₅H₄N• and C₃H₃N can lead to the formation of either 3-(2-pyridinyl)-2-propenenitrile or 1,5-naphthyridine. However, the formation of 1,5-naphthyridine under the present experimental conditions was not efficient, as supported by rate coefficient calculations. The complex mechanisms of radical-neutral reactions were revealed in this work, guiding the gas-phase synthesis of nitrogen-containing aromatic compounds.

Ion-molecule reactions are a crucial form of reaction in space that can reveal the formation mechanisms and evolutionary processes of some complex interstellar molecules and even prebiotic molecules. In this study, infrared spectra of pyridine (Pyd) and methanol (CH₃OH) or deuterated methanol (CH₃OD) clusters were measured in the spectral range of 2400-3800 cm^-1 using an infrared-vacuum ultraviolet (IR-VUV) scheme. The geometric conformer, infrared spectra of the possible products, and reaction paths were investigated utilizing quantum chemical calculations. By comparing the experimental and theoretically calculated spectra, we found that only a unique hydrogen-bonded structure exists within the Pyd-CH₃OH complex, characterized by a linear hydrogen bond conformer between the two constituent molecules. On the other hand, interestingly, we observed a relatively weak NH vibrational peak at 3383 cm^-1 in the infrared spectra of both cationic Pyd-CH₃OH and Pyd-CH₃OD, suggesting that proton transfer has occurred via at least two distinct pathways. These pathways likely involve a proton transfer reaction from either the OH group or the CH₃ group of CH₃OH to the nitrogen atom of Pyd, with the transfer from the OH group being the predominant route. Through the theoretical calculations, we also found that a new C-C bond can be formed between the two moieties after the proton transfer reactions.

Our spectroscopic study identified cyclic H-bonded structures in benzonitrile–ammonia (BN–NH ₃ ) and benzonitrile–methylamine (BN–MA) complexes. After ionization, the (BN–NH ₃ ) ⁺ forms a C–N bond, while the (BN–MA) ⁺ develops a proton-shared structure.

2-Butenenitrile (2-Bu) is a recently discovered crucial interstellar molecule. Herein, an abnormal NH band was observed in the infrared spectrum of the 2-Bu dimer cation, suggestive of a proton transfer reaction within the cluster. Through a comprehensive theoretical analysis of the IR spectrum of (2-Bu)₂⁺, we discovered not only the formation of a new C-N bond through the attachment of one 2-Bu to another but also the occurrence of a proton transfer reaction in the cluster. This proton was identified as originating from the methyl group of the attaching 2-Bu in the cluster based on the analysis of IR spectra of (2-Bu)⁺ and [2-Bu-acrylonitrile (AN)]⁺. Furthermore, the detailed reaction process of this ion-molecule reaction is examined with theoretical calculation. This finding contributes significantly to our deeper understanding of ion-molecule reactions in the gas phase and the formation of nitrogen-containing prebiotic molecules in the interstellar medium.

The formation of nitrogen-containing organic compounds is crucial for understanding chemical evolution and the origin of life in the interstellar medium (ISM). In this study, we explore whether acrylonitrile (AN) and pyrrole (Py) can form new nitrogen-containing compounds after single-photon ionization in their gaseous clusters by vacuum ultraviolet (VUV)-infrared (IR) spectroscopy and theoretical calculations. The results show that a strong linear H-bond is formed in neutral AN-Py, while cyclic or bicyclic H-bonded networks are formed in the neutral AN-Py₂ cluster. It is found that the structure containing a new C-C covalent bond between two moieties in (AN-Py)⁺ is formed besides the formation of H-bonded structures after AN-Py is ionized by VUV light. In (AN-Py₂)⁺ cluster cations, new C-C or C-N covalent bonds tend to be formed between two Py, with (Py)₂⁺ as the core in the cluster. The results reveal that new covalent bonds are more likely to be formed between two Py species when AN and Py are present in the cationic clusters. These results provide spectroscopic evidence of the formation of new nitrogen-containing organic compounds from AN and Py induced by VUV, which are helpful for our understanding of the formation of diverse prebiotic molecules in interstellar space.