ACS Earth and Space Chemistry最新文献

Reactions of low-energy cations with constituents adsorbed on icy grain mantles result in new pathways to significant compounds in the interstellar medium and protostellar nebulae. The present work characterizes reactions that can occur when the C⁺ cation in its ground state reacts with five nitrogen-containing molecules adsorbed onto amorphous ice cluster models: ammonia (NH₃) and four NH₂X amides: cyanamide (NH₂CN), formamide (NH₂CHO), hydroxylamine (NH₂OH), and carbamic acid (NH₂COOH), which are all either known or plausible astromolecules. Density functional theory calculations were performed using clusters containing 15–26 water molecules plus reactants. Vibrational spectra were generated for the five nitrogen-containing molecules adsorbed on amorphous ice, and reactions with C⁺ were characterized. In addition to identifying the variety of compounds that initially form from C⁺ addition, subsequent reactions of hydrogen atoms with radical intermediates were also characterized. Some of the products include known astromolecules or derivatives thereof, including H₂NC, methanimine (CH₂NH), protonated methanimine (CH₂NH₂⁺), 2-aminoketene, 2-iminoacetaldehyde, NH₂, isocyanic acid (HNCO), hydrogen isocyanide (HNC), and others. In addition, a number of the H adducts are exotic carbene species that may potentially be detectable in astrophysical environments.

The current work presented the investigation of the atmospheric repercussions of reactions of 1,3,3,4,4-pentafluorocyclobutene (cyc-CF₂CF₂CF═CH–, HFO-c1325yz) with the ^•OH radical using quantum chemical methods. To understand the reaction mechanism, we first explored the potential energy surface of the title reaction using M11/6-311++G(d,p) and CCSD(T)//M11/6-311++G(d,p) methods. The estimated relative energy and thermodynamic parameters for both methods reveal that the ^•OH addition reactions to the C═C bond of cyc-CF₂CF₂CF═CH– are more favorable than the H- and F-abstraction reactions by the ^•OH radical. Additionally, kinetic analysis demonstrates that these two ^•OH addition pathways are faster than the H- and F-abstraction reaction pathways. The calculated overall rate coefficient (3.68 × 10^–14 cm³ molecule^–1s^–1) using variational transition state theory (VTST) at 298.15 K agrees with the experimental result. Additionally, we evaluated the atmospheric lifetime, global warming potential (GWP), and photochemical ozone creation potential (POCP) of this compound. Furthermore, the degradation pathways of the product radicals are analyzed by using the same quantum chemical methods in the presence of O₂ and HO₂^•. The key products formed in the degradation pathways include stable compounds like CF₂═CHOH, CF₂═CFOH, F(O)CCH(OH)CF₂COF, HOF, and COF₂, along with radicals such as F(O)CCF₂(·), H(O)CCF₂(·), F(O)CCH(OH)(·), and H(O)CCF(OH)(·). Moreover, to understand the photolytic degradation mechanism, a time-dependent density functional theory (TD-DFT) calculation by the M11/6-311++G(d,p) method was performed for cyc-CF₂CF₂CF═CH– as well as for all intermediates and products. The TD-DFT calculations suggest that most products are susceptible to photodissociation, except F(O)CCH(OH)CF₂COF and COF₂, which exhibit photolytic stability under sunlight.

Copper (Cu) oxidation-state distribution controls its toxicity and mobility in natural systems and is affected by its interaction with redox-reactive mineral surfaces such as magnetite. By combining aqueous chemical analysis, X-ray absorption spectroscopy and aqueous speciation modeling, this study provides detailed mechanistic insights regarding Cu redox speciation in solution and at the surface of 10 nm-sized magnetite nanoparticles with varying stoichiometries (0.1 ≤ R = Fe(II)/Fe(III) ≤ 0.5), versus pH (4 to 9) and initial Cu(II) concentration ([Cu(II)]_ini = 25 and 500 μM), with 10 mM NaCl. Cu(II) generally prevailed at the magnetite surface, with co-occurring Cu(I) from a smaller extent to an equal extent. Conversely, in the aqueous solution (i.e., after filtration), Cu(I) prevailed at pH > 5, hence evidencing that solution and surface redox speciation may differ. Reduction to Cu(0) was only partial and detected under the most reducing condition (for R = 0.5) at high [Cu(II)]_ini, because it was limited by Cu(II)- and Cu(I)-magnetite binding. Significant oxidation Fe(II) to Fe(III) was shown to be responsible for this partial reduction process, which could be overcome by adding Fe(II) ions to recharge the magnetite surface, hence promoting further copper reduction to Cu(0). This study provides fundamental insights into copper–magnetite interactions and enables improved predictions of copper speciation and fate in environmental systems.

In the Middle Jurassic, whether the Bangong-Nujiang Tethys Ocean extended eastward from the central Tibetan Plateau into western Yunnan, China, thereby separating the Tengchong and Baoshan blocks, remains unresolved. Middle Jurassic carbonate rocks of the Liuwan Formation of the Baoshan Block provide essential insights. This study examines the petrography, rare earth element and yttrium (REE+Y) geochemistry, and strontium isotopic composition of the Liuwan Formation limestones to determine their depositional environment and tectonic setting. Results indicate that these carbonate rocks show no significant recrystallization or dissolution. REE+Y patterns show light-REE enrichment, slightly positive La and Ce anomalies, a minimal Eu anomaly, and a Y anomaly, all indicating open marine conditions. The ⁸⁷Sr^/86Sr ratios range from 0.707287 to 0.707554 (averaging 0.707365) and are slightly higher than those of coeval seawater. These geochemical signatures suggest a depositional environment between open marine and restricted settings, influenced by freshwater influx and volcanic activity, indicating an ocean separating the Baoshan and Tengchong blocks. Tectonic proxies suggest a stable passive margin for the Baoshan Block and a subduction-related tectonic/depositional setting for the Tengchong Block. Subduction of the Bangong-Nujiang Ocean beneath the Tengchong Block explains these distinct tectonic settings. Correlations among ⁸⁷Sr/⁸⁶Sr values and REE+Y geochemistry of the Liuwan Formation reveal paleoceanographic characteristics of the ancient subducted ocean and the contributions from multiple sources. These findings support the existence of the Bangong-Nujiang Ocean between the Tengchong and Baoshan blocks during the Middle Jurassic, enhancing our understanding of eastern Tethys paleogeography and evolution.

Methyl cyanide, CH₃CN, is present in diverse regions in space, in particular in the warm parts of star-forming regions where it is a common molecule. Rotational transitions of ¹³CH₃CN and CH₃¹³CN in their v₈ = 1 lowest excited vibrational states (E_vib ≈520 K) are quite prominent in Sagittarius B2(N). In order to be able to search for transitions of the next higher vibrational state v₈ = 2, we recorded spectra of samples enriched in ¹³CH₃CN and CH₃¹³CN up to v₈ = 2 in the 35–1091 GHz region and reinvestigated existing spectra of CH₃CN in its natural isotopic composition between 1085 and 1200 GHz. Perturbations caused by near-degeneracies in K = 4 of v₈ = 2⁰ and K = 2 of v₈ = 2^–2 yielded accurate information on the energy spacing of 22.93 and 21.79 cm^–1 between the l components of ¹³CH₃CN and CH₃¹³CN, respectively. Fermi-type interaction between K = 13 and 14 of v₈ = 1^–1 and v₈ = 2⁺² probe the energy differences between the two states of both isotopomers. In addition, a ΔK ± 2, Δl ∓ 1 interaction between the ground vibrational state of ¹³CH₃CN and v₈ = 1⁺¹ provides information on their energy spacing. Furthermore, we obtained improved or extended ground-state rotational transition frequencies of ¹³CH₃¹³CN and extensive data for ¹³CH₃C¹⁵N and CH₃¹³C¹⁵N. Finally, we report the results of our search for transitions of ¹³CH₃CN and CH₃¹³CN in their v₈ = 2 states toward Sagittarius B2(N).

In the interstellar medium, cosmic rays (CRs) generate a field of ultraviolet (UV) photons via excitation and subsequent radiative decay of H₂ molecules. This UV field is a major agent of ionization and dissociation in the inner regions of molecular clouds that are shielded from the effects of the interstellar radiation field. In particular, the dissociation of H₂, by far the most abundant molecule in interstellar clouds, leads to the production of atomic hydrogen, which then takes part in the production of a multitude of molecules, in particular complex organics on the surfaces of interstellar dust grains. Precise knowledge of the rates of CR-induced dissociation processes is thus crucial for constructing reliable chemical models. For the present paper, we have derived a new value of k_diss,CR(H₂) = 0.831ζ for the rate of H₂ dissociation, where ζ is the CR ionization rate of H₂. This prediction contrasts a previous value from the Leiden database which overestimated the rate due to an inconsistent treatment of the H₂ abundances and photodissociation cross sections. By running a series of chemical models, we show that the overestimated dissociation rate has a large effect on the results of chemical simulations, with the abundance of methanol being overestimated by over 1 order of magnitude. Hence, we strongly recommend the adoption of our new estimate, k_diss,CR(H₂) = 0.831ζ in all chemical models that include this process. Our newly derived value corresponds to H₂ being purely in the para form (J″ = 0). However, in the interiors of molecular clouds, the H₂ ortho-to-para ratio is low and using the rate for para-H₂ is an adequate approximation.

The prebiotic formation of nucleobases remains a fundamental question in the field of the origins of life. The reaction between 4-amino-5-cyanoimidazole (AICN) and hydrogen cyanide (HCN) has been proposed as a possible step in the HCN oligomerization pathway, forming adenine, a nucleobase found in RNA and DNA. In this work, we employ machine learning interatomic potentials combined with enhanced sampling simulations to investigate this reaction in aqueous solution. This approach allows the identification of reaction pathways and a detailed exploration of the associated free energy landscape, explicitly accounting for solvent effects. The data efficiency of the employed machine learning architecture enables modeling at a hybrid DFT level of theory, which would otherwise be computationally too expensive using conventional ab initio methods. Our results show that, although the overall transformation is thermodynamically favorable, the high free-energy barriers make it difficult to proceed efficiently in bulk water. This observation agrees with the low yields typically reported in experimental syntheses of adenine from HCN.

For decades, nuclear magnetic resonance (NMR) spectroscopy has been utilized as a powerful tool in various scientific disciplines, most prominently in chemistry, to determine molecular structures or monitor reactions. While well established in various fields, NMR applications in astrobiology are still unclear. This work aims to explore the potential of NMR in astrobiology, highlighting strengths but also weaknesses. We illustrate the capabilities of NMR with two applications: (1) recently developed methods for position-specific carbon isotope analysis of complex organics; and (2) well-established tools for performing quantitative compositional analysis of complex organic mixtures. By utilizing samples relevant to astrobiology, specifically the amino acid valine and analogue mixtures of organics, we showcase that molecules retain a source-dependent and distinct intramolecular carbon isotope fingerprint. We demonstrate that compositional sample analysis provides an independent and complementary line of evidence pointing toward the origin of a molecule or mixture. Together, these NMR tools have the potential to support life detection efforts and aid in distinguishing between biotic and abiotic samples. Finally, we discuss sensitivity, detection limits, and the portability of NMR, and propose how integration with mass-spectrometry techniques will be imperative to enable more targeted and comprehensive analyses relevant to astrobiology, including in situ analysis, but also sample return missions.

Ferrihydrite (Fh), which plays a critical role in element cycling and contaminant sequestration in the environment, is susceptible to transformation. However, understanding and predicting its transformation process are challenging due to the multitude of influencing factors. Herein, we developed six machine learning (ML) models and selected the best performance model (Random Forest) to predict the transformation and fate of Fh. Key influencing factors for Fh transformation, including the C/Fe ratio and Fe(II)/Fe(III) ratio, were identified. ML further implied a distinct transformation route between biological-induced and chemical-induced Fh transformation; that is, the former one resulted primarily in lepidocrocite (Lp), while the latter tended to more readily yield goethite (Gt) or magnetite (Mt) via intermediate phases. Linking with the fixation capacity of different Fe minerals, our ML model indicated the risk of emerging pollutants (e.g., perfluoroalkyl and polyfluoroalkyl substances, PFAS) and global warming during the Fh transformation under different environmental conditions. The predicted release risk of PFAS and soil organic carbon could reach up to 67.8% in paddy soil and 181.6 Pg globally, respectively. Through a data-driven approach, this study provides new insights into the transformation, fate, and implications of poorly crystalline iron minerals, highlighting their roles in pollutant turnover and carbon cycling.