ACS Earth and Space Chemistry最新文献

The thioketenyl cation (HCCS⁺) has been recently detected in the dark cloud TMC-1 by radioastronomical observations within the QUIJOTE survey. However, the infrared (IR) spectrum of this ion is yet to be reported in the literature. Spectroscopic reference data are essential for the search of HCCS⁺ using the James Webb Space Telescope, not only in molecular clouds and star-forming regions, but also in the ionospheres and upper atmospheres of exoplanets. In this work, we demonstrate a method for the selective generation of the HCCS⁺ ion in its triplet ground state (³Σ^–) and use this method to obtain IR band positions for HCCS⁺. The IR-action spectrum of H₂-tagged HCCS⁺ has been measured in a cryogenic 22-pole ion trap via IR photodissociation (IR-PD) spectroscopy with the FELIX light source in the wavenumber regions 450–1850 and 3000–3350 cm^–1. Spectral information is complemented by theoretical calculations on the fragmentation mechanisms leading to the formation of HCCS⁺ from dissociative ionization of 2,5-dibromothiophene. The assignment of the experimental HCCS⁺ vibrational bands is aided by comparison with ab initio computed values from literature and from calculations at the UB3LYP/cc-pVQZ level of theory, for both the triplet (³Σ^–) and singlet (¹Σ⁺) states of HCCS⁺. The experimental HCCS⁺ spectra show an overall good agreement with the scaled theoretical values (to account for anharmonicity effects), facilitating assignment of the IR spectral features. These findings will enable new reactivity investigations and spectroscopic measurements to be conducted, and for HCCS⁺ to be included in astrochemical models and databases.

Surface chemistry on interstellar dust grains is recognized as a central component in astrochemical models, representing a plausible formation route for many of the observed complex molecular species. However, key parameters governing interstellar surface chemistry, such as diffusion energy barriers, remain poorly constrained. In particular, surface diffusion constitutes a fundamental step in the synthesis of complex organic molecules and plays a crucial role in understanding the desorption process. In this paper, the diffusion dynamics of carbon monoxide (CO) on amorphous solid water (ASW) surfaces, representative of interstellar ices, is modeled with quantum-chemical methods. Employing a representative ensemble of water clusters, each made of 22 molecules, diffusion energy barriers between the binding sites are computed using density functional theory. Diffusion rate coefficients are then determined by applying the harmonic approximation of the transition state theory. The results, in agreement with experimental studies, revealed a wide distribution of diffusion energies. This reflects the intrinsic topological heterogeneity of ASW surfaces and highlights how surface mobility significantly influences CO’s desorption dynamics and, as a consequence, surface-mediated reactivity in interstellar environments. We show that key parameters commonly employed in astrochemical models, such as the ratio between binding and diffusion energy, should be carefully revised.

Fluorinated esters (FESs) are emerging atmospheric species formed as secondary products from the degradation of hydrofluoroethers (HFEs) and other fluorinated precursors. Fluoroalkylfluoroformates (FAFs) are a subgroup of fluorinated esters, where a fluoroalkyl group is bonded to a fluoroformate group (C(O)F). In this work, we have done a thorough kinetic and mechanistic analysis of the atmospheric degradation of specific FAFs, including FC(O)OCHF₂, FC(O)OCH₂F, FC(O)OCH₃, and FC(O)OCH₂CF₃, with hydroxyl (OH) radical and Chlorine (Cl) atoms. Geometry optimizations and frequency analyses were performed at the M06–2X/aug-cc-pVTZ and MP2/aug-cc-pVTZ levels. The single-point (SP) energies were refined using the CCSD(T)/aug-cc-pVXZ (D and T) basis sets, and complete basis set (CBS) extrapolation was performed to obtain more reliable rate coefficient values for the reactions. Eight distinct H-abstraction pathways were identified, and the corresponding potential energy surfaces were constructed. Rate constants were computed using canonical variational transition state theory with small-curvature tunneling (CVT/SCT) over the 250–500 K range. We also examined the impact of these substances on the atmosphere.

The long theorized polycyclic aromatic hydrocarbons (PAHs) crashed onto the astrochemical scene with the radioastronomical discovery of cyanobenzene (benzonitrile) in 2018, but the astronomical observation of the CN stretch for this or any other known CN-PAHs in the IR has yet to be reported. With the wealth of James Webb Space Telescope (JWST) data being actively returned, the need for reference data for IR features of known interstellar molecules like CN-PAHs has never been greater. Due to complexities in working with PAHs in the laboratory, quantum chemistry provides the best means of high-throughput vibrational reference data generation. This work gives an overview for the state-of-the-art in quantum chemical approaches and results for computing IR spectroscopic data for CN-PAHs, specifically for the CN stretch. The use of the hybrid rDSD/junTZ + B3LYP/N07D method through quartic force fields (QFFs) and second-order vibrational perturbation theory (VPT2) has computed the CN stretch of C_2v 9-cyanoanthracene to be 2207 cm^–1 (4.531 μm), exactly the same as free electron laser experiments report. Additionally, due to the structural regularity of the attached PAHs, the CN stretch does not vary greatly (less than 20 cm^–1) from CN-PAH molecule to different CN-PAH molecule. However, this spectroscopic region is home to Mg IV and Ar VI lines that may hinder observations of the CN stretch in astronomical environments where vibrational emission is possible.

A sensitive analytical method with a run time of 17 min is presented to detect and quantify free and combined amino acids (FAA/CAA) and nucleobases in complex marine matrices. The method combines hydrophilic interaction chromatography (HILIC) with quadrupole time-of-flight mass spectrometry (Q-TOF-MS). Retention time interday relative standard deviation (RSD) was 0.09–7.1%; peak area RSD was <6%; and individual limits of quantification (LOQs) ranged between 0.5 and 5 μg/L. The optimized solid-phase extraction (SPE) desalting protocol for salt water achieved a recovery of more than 50%, except for threonine (30%) and uracil (15%). Despite lower recoveries, compounds were consistently and reliably detected in ambient samples. Recovery RSD was ≤10% for most analytes, with uracil (23%) and cystine (13%) as exceptions. Hydrolysis in polypropylene vials to quantify CAA yielded recoveries between 61 and 102%. Matrix effects were evaluated via standard addition and isotope-labeled standards, revealing ion suppression/enhancement in desalted seawater samples, thus confirming the need for matrix-matched calibration. For aerosol particle extracts, no desalting procedure was necessary. The method was applied to polar-region samples, yielding atmospheric concentrations that ranged between 0.5 pg/m³ for adenine and 3.5 ng/m³ for glutamine. In seawater samples, concentrations ranged between 1.3 μg/L for glycine and 48 μg/L for uracil. Altogether, the optimized method enables reliable analysis, without the need for a derivatization step. Thus, it is allowing for the sensitive determination of amino acids and nucleobases that are challenging to analyze and rarely reported in environmental studies to date.

Shale organic matter (OM) enrichment plays a critical role in evaluating the future prospects of shale gas worldwide. The Ordovician-Silurian transition represents a critical interval notable for extensive deposits of organic-rich shale formations. Despite this, the underlying factors controlling OM enrichment during this interval, particularly in the Wufeng-Longmaxi (WF-LMX) shale, need comprehensive reevaluation. Our study compiles major and trace element concentrations for 1376 samples and TOC values for 3143 samples from the WF-LMX shale in South China to define quantitative thresholds for redox environment and paleo-productivity. Paleoenvironmental characteristics associated with the deposition of shales containing varying OM abundances (TOC <2%, 2–4%, and >4%) were clarified. Random Forest (RF) and Artificial Neural Network (ANN) models were applied to identify the primary factors controlling OM enrichment of the WF-LMX succession. Results of this work show that Mo_EF, U_EF, V_EF, and Cr_EF values of 10, 10, 1.5, and 1.5, respectively, represent the threshold for anoxic conditions during shale deposition. SiO_2(exc), P/Ti, and Ba_bio values of approximately 30%, 0.2, and 1000 ppm, respectively, signify high paleo-productivity. RF and ANN results suggest that redox environment and paleo-productivity as the foremost influences on OM accumulation, followed by paleoclimate. Organic-rich shales (Type-a, TOC > 4%) were primarily deposited under warm and humid climate conditions. Enhanced nutrient supply promoted elevated productivity and established anoxic bottom conditions that facilitated OM preservation. Type-c shales (TOC < 2%) accumulated under low-moderate productivity and oxygenated bottom conditions, leading to extensive degradation and dilution of organic matter. Type-b shales (TOC = 2–4%) exhibit intermediate characteristics between Type-a and Type-c. This study offers new perspectives on OM enrichment mechanisms in the WF-LMX shale succession and provides a theoretical basis for shale gas exploration within this stratigraphic interval.

The sulfur depletion problem in the interstellar medium motivates a detailed characterization of sulfur-bearing molecules such as CH₄S₂, whose isomers may act as potential reservoirs of interstellar sulfur. Using high-level ab initio methods, we identified eight structural isomers of CH₄S₂, with 1-methylhydrodisulfide (CH₃SSH) as the global minimum and methanedithiol lying 3.1 kcal·mol^–1 higher in energy. These low-energy isomers show both kinetic and thermodynamic stability against decomposition into H₂CS and H₂S, even in the presence of water. In contrast, hypervalent sulfur isomers are more than 60 kcal·mol^–1 higher in energy. Rotational constants obtained with the jun-Cheap Scheme (jun-ChS), including anharmonic corrections reproduce experimental values within 0.17%, ensuring reliable spectral predictions. The stability of CH₄S₂ isomers under interstellar conditions and their dipole moments (up to 1.9 D) suggest that they are promising targets for radioastronomical detection and may help clarify the chemical pathways of sulfur in cold molecular clouds.

Isotopic fractionation is a very powerful tool to follow the evolution of material from one stage to the next in the star-formation process. Prestellar cores exhibit some of the highest levels of deuteration because their physical conditions (T ≤ 10 K and n(H₂) ≥ 10⁵ cm^–3) greatly favor deuteration processes. Deuteration maps are a measure of the effectiveness of the deuteration across the core, and they are useful to study both the deuteration and the formation mechanism (either in the gas-phase or on grain surfaces) of the main species. Methanol is the simplest complex organic molecule (COM) that is O-bearing and detected in the interstellar medium (ISM). It represents the beginning of molecular complexity in star-forming regions; thus, a complete understanding of its formation and deuteration is a necessary step to understand the development of further chemical complexity. In this paper, we use single-dish observations with the IRAM 30 m telescope and state-of-the-art chemical models to investigate the deuteration of methanol toward the prototypical prestellar core L1544. We also compare the results of the chemical models with previous observations of deuterated methanol toward the presttellar cores HMM1 and L694-2. The spectra extracted from the CHD₂OH map show that the emission is concentrated in the center and toward the northwest of the core. Using deep observations toward the dust and the methanol peaks of the core, we derive a very large deuterium fraction for methanol (∼20%) toward both peaks. The comparison of our observational results with chemical models has highlighted the importance of H-abstraction processes in the formation and deuteration of methanol. Deep observations combined with state-of-the-art chemical models are of fundamental importance in understanding the development of molecular complexity in the ISM. Our analysis also shows the importance of non-LTE effects when measuring the D/H ratios in methanol.

Understanding diffusion on interstellar ices is key to modeling the chemical evolution of cold molecular clouds, where low temperatures severely limit molecular mobility. In this study, we introduce a robust and fully automated multiscale computational framework to quantify diffusion processes of adsorbates at the surface of amorphous solid water (ASW). Using H₂S as a test case, whose binding sites were previously studied at the ab initio level, we constructed a detailed network of 141 adsorption sites connected by over 270 transition states. All density functional energetics were benchmarked against DLPNO–CCSD(T), achieving chemical accuracy in the description of diffusion barriers, which span from 0.1 to 27 kJ mol^–1 with a median value of 5.4 kJ mol^–1. An off-lattice kinetic Monte Carlo (kMC) model adopting both the ab initio diffusion barriers and binding energies for the desorption processes was carried out to compute temperature-dependent diffusion coefficients and to reconstruct the temperature-programmed desorption (TPD) curve. Our simulations reveal that thermal diffusion of H₂S is negligible below 20 K, with diffusion coefficients as low as 10^–48 cm² s^–1 at 10 K, thus excluding Langmuir–Hinshelwood surface encounters under typical dense cloud conditions. Moreover, under submonolayer conditions, diffusion was found to have negligible influence on the reconstructed TPD peak position. Furthermore, our results demonstrate that a universal scaling factor f to guess the diffusion barriers (ΔE_diff) from the sole knowledge of BE: f = ΔE_diff/BE does not apply as it exhibits wide variability across the sampled configurations. These findings highlight the need for incorporating statistically meaningful distributions of binding energies and diffusion barriers in astrochemical models to more accurately capture diffusion and surface reactivity on interstellar ices.