arXiv subjects

Germán Molpeceres

Publications and source records attributed to Germán Molpeceres.

At least 19 recordsLinked to original sources

arXiv2026

Thermally irreversible sulfur chemisorption on silicate grains as a major sulfur reservoir in molecular clouds

Sulfur (S) chemistry in the interstellar medium remains poorly constrained due to the longstanding "missing sulfur" problem, where a significant fraction of cosmic sulfur in molecular clouds remains unidentified. To gain a comprehensive understanding of S chemistry in the interstellar medium, its behavior on interstellar dust grains must be clarified. Here, we experimentally investigated the surface elementary processes of S atoms under conditions mimicking interstellar molecular clouds. Our in-situ laser-based detection method enabled the direct tracking of S atoms adsorbed on astrophysically relevant silicate substrates and amorphous solid water (ASW). We found that sulfur atoms strongly chemisorbs onto silicate surfaces and remains bound even after ASW sublimation, rendering it observationally invisible. An astrochemical model coupled with these results suggests that at least 10 percent of the total sulfur budget in molecular clouds must be locked onto silicate dust grains. Our results identify sulfur chemisorption on silicate as a key mechanism driving sulfur depletion and provide a physical explanation for the missing sulfur problem.

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arXiv2026

Thioacetaldehyde (CH3CHS) on interstellar ices: a key molecule to unravel two chemical dichotomies in the ISM

Thioacetaldehyde (CH3CHS), recently detected in TMC-1, has an abundance approximately 36 times lower than its oxygen analog, acetaldehyde (CH3CHO). This makes the CH3CHS/CH3CHO pair the one with the largest column density difference among the detected oxygen/sulfur analogue pairs in this cloud. We investigate the hydrogenation pathways of CH3CHS to address two chemical dichotomies in the ISM: (i) the differenciation between CH3CHS and CH3CHO, and (ii) the apparent absence of both CH3CHS in the G+0.693-0.027 molecular cloud and ethyl mercaptan (CH3CH2SH), in TMC-1. Our results reveal a complex scheme that involves multiple competing reactions, highlighting an efficient sequence of consecutive hydrogenations that can lead to CH3CH2SH. This finding suggests that the high S/O ratio observed for thioacetaldehyde in TMC-1 (~36), and even more pronounced in G+0.693-0.027 ($\geq$112), may result from its conversion via hydrogenation on the ice surface, contrary to the case of CH3CHO, which is more resistant to that chemical process. The straightforward hydrogenation of CH3CHS on ices, which can also take place even in the gas-phase at 150 K, provides a reliable explanation for its non-detection in G+0.693-0.027, where grain-surface chemistry is expected to play an important role, favoring the conversion of CH3CHS into CH3CH2SH, which is indeed detected in G+0.693-0.027. In contrast, TMC-1 represents a more pristine gas-phase environment, where grain-surface chemistry has a lower impact. Under these conditions, CH3CHS can persist, while CH3CH2SH remains undetected. Overall, our results show the entirely different reactivity that contributes to the chemical complexity of two of the largest interstellar sulfur factories.

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arXiv2026

Laboratory spectroscopy, theoretical characterization, and astronomical search for syn-propenethial (CH2CHCHS)

We report the laboratory characterization of the higher-energy isomer of propenethial, syn-CH2CHCHS. While the lower-energy isomer, anti-CH2CHCHS, was detected in the interstellar medium during the QUIJOTE line survey of TMC-1, we report the non-detection of the syn isomer in the same source, deriving an upper limit to its column density of 1.5 $\times$ 10$^{10}$ cm$^{-2}$. A subsequent theoretical investigation into the origin of this non-detection reveals that the most plausible formation pathway for CH2CHCHS is highly isomer-specific, producing nearly 95\% of the anti isomer. This predicted branching ratio allows us to infer an even lower effective upper limit for the abundance of the syn isomer. In addition, calculations of tunneling-mediated unimolecular isomerization in the gas phase show that syn-CH2CHCHS converts to the lower-energy anti isomer on timescales of the order of 10$^{3}$ years. Overall, we conclude that the detection of syn-CH2CHCHS is highly challenging in both cold and warm interstellar environments. These results underscore the importance of electronic-structure and kinetic effects in determining isomer abundances across diverse interstellar environments.

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arXiv2026

Can cyanide radicals drive molecular backbone growth on interstellar icy grains?

Motivated by the value of CN-bearing molecules as tracers of interstellar physical conditions, we investigate the reactions of adsorbed CN radicals with acetylene and ethylene (C2H2 and C2H4) on interstellar dust-grain analogues using quantum chemical calculations. We find that reactivity is strongly controlled by the relative orientation of the reactants, with specific geometries either promoting or inhibiting reaction. We further show that, on ice, these reactions differ qualitatively from their gas-phase counterparts, stalling at the formation of the adduct complexes C2H2CN and C2H4CN and exhibiting newly emerged kinetic barriers for the neutral-radical association. We contextualize our calculations in the same reaction-diffusion framework that would be employed in astrochemical models, finding that, depending on the diffusion energy of the hydrocarbons, these reactions can be either negligible or efficient, highlighting the importance of the local ice structure in interstellar grain chemistry. These findings caution against the use of CN-based tracers that assume barrierless, bimolecular surface reactions involving CN radicals.

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arXiv2026

Experimental and computational studies of the hydrogenation of carbon disulfide (CS2) on ice analogues

Carbon disulfide (CS$_2$) is one of the sulfur-bearing species expected to be present in the interstellar medium (ISM). In this study, we investigated the surface reactions of solid CS$_2$ with hydrogen (H) atoms on amorphous solid water (ASW) using laboratory experiments supported by computational calculations. Our results show that CS$_2$ reacts with H atoms through quantum tunneling in the initial step, followed by successive H addition reactions, with or without activation barriers, on icy surfaces. These processes lead to the formation of several sulfur-bearing species, including hydrogen sulfide (H$_2$S), methyl mercaptan (CH$_3$SH), and small amounts of dithioformic acid (HC(S)SH) and methanedithiol (CH$_2$(SH)$_2$). The observed reactivity of CS$_2$ with H atoms provides a plausible explanation for the non-detection of CS$_2$ in interstellar ices. Furthermore, the efficient hydrogenation of the complex molecules derived from CS$_2$, namely HC(S)SH and CH$_2$(SH)$_2$, suggests that these species could be easily undergone with H atoms to produce other S-bearing species under ISM conditions.

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arXiv2026

Atom Addition Formation of Thionylimide (HNSO) on Interstellar Dust Grains: Chemical routes requiring oxygen and nitrogen atom surface diffusion

We investigate the formation of the recently detected HNSO molecule using quantum chemical calculations on ices and astrochemical models in tandem. Our results indicate that HNSO is efficiently produced on grain surfaces through reactions involving atomic oxygen and nitrogen atoms with the radicals NS and SO, forming NSO as a key intermediate. Subsequent hydrogenation of NSO leads to HNSO, with a clear preference for the lowest energy cis conformer, while the trans form is metastable and may be short-lived under typical interstellar conditions. The models predict that solid HNSO can reach abundances comparable to icy OCS, placing it among the major sulfur-bearing species in interstellar ices. Gas-phase abundances, in contrast, remain lower than those of OCS. The implementation of a multibinding scheme in our models clarifies the role of diffusive chemistry in the production of HNSO at early times, improving agreement with observations. These findings suggest that reactions involving diffusing O and N atoms on icy grains contribute significantly to sulfur chemistry and beyond in dense clouds and motivate further searches for molecules containing simultaneously H, N, O and S in other astronomical environments.

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arXiv2026

A gas-phase "top-down" chemical link between aldehydes and alcohols

Alcohols and aldehydes represent two key classes of interstellar complex organic molecules (COMs). This work seeks to better understand their possible chemical connections, with a focus on such molecules in the sources of the star-forming region Sgr B2 (N). The gas-phase reaction between ethanol (CH3CH2OH) and the halogens fluorine and chlorine was investigated via DFT calculations, with the goal of determining whether astrochemically viable chemical pathways leading to acetaldehyde (CH3CHO) exist. The studied reactions were then included in an astrochemical model of Sgr B2 (N) to determine their significance under real interstellar conditions. Our DFT calculations revealed that both chlorine and fluorine can react barrierlessly with ethanol to abstract a hydrogen atom. We further found that, following this initial step, the resulting ethanol radicals can undergo further reactions with atomic hydrogen, with some routes leading to acetaldehyde. Incorporation of these novel reactions in astrochemical models of hot cores suggest that they are indeed efficient under those conditions, and can lead to modest increases in the abundance of CH3CHO during model times where gas-phase ethanol is abundant. Of the ethanol radicals included in our chemical network, we found CH3CHOH to have the highest abundances in our simulations comparable to that of ethanol at some model times. Overall, this work reveals a novel gas-phase ``top-down'' link from alcohols to aldehydes that compliments the better studied ``bottom-up'' routes involving grain-surface H-addition reactions yielding alcohols from aldehydes. Moreover, results from our astrochemical models suggest that the ethanol radical CH3CHOH may be detectable in the interstellar medium.

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arXiv2025

Ion-molecule routes towards cycles in TMC-1. An automated study of the C2H4 + CH2CCH+ reaction

Cyclopentadiene (c-C5H6) is considered a key molecule in the formation of polycyclic aromatic hydrocarbons (PAHs) in the interstellar medium (ISM). The synthesis of PAHs from simpler precursors is known as the "bottom-up" theory, which, so far, has been dominated by reactions between organic radicals. However, this mechanism struggles to account for the origin of the smallest cycles themselves. Ion-molecule reactions emerge as promising alternative pathways to explain the formation of these molecules. In the present work, we investigate the reaction network of the main ionic precursor of cyclopentadiene c-C5H7+ . To this end, we establish an integrated protocol that combines astrochemical modelling to identify viable formation routes under cold interstellar medium conditions, automated reaction path search and kinetic simulations to obtain accurate descriptions of the reaction pathways and reliable rate constants. In particular, we examine the reaction between ethylene (C2H4) and the linear propargyl cation (CH2CCH+). Our results reveal that the formation of c-C5H7+ by radiative association turns out to be inefficient, contrary to our initial expectations. Instead, the system predominantly evolves through bimolecular channels yielding c-C5H5+ and CH3CCH2+ with the formation of c-C5H5+ offering new insights into reactivity that supports molecular growth in the ISM.

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arXiv2025

Formation of unsaturated carbon chains through carbon chemisorption on solid CO

The interaction of carbon atoms with solid carbon monoxide (CO) is a fundamental process in astrochemistry, influencing the formation of complex organic molecules in interstellar environments. This study investigates the adsorption and reaction mechanisms of carbon atoms on solid CO under cryogenic conditions, employing a combination of experimental techniques, including photostimulated desorption and resonance-enhanced multiphoton ionization (PSD-REMPI) and infrared spectroscopy, alongside quantum chemical calculations. The results reveal the formation of oxygenated carbon chains, such as CCO, C$_3$O$_2$, and C$_5$O$_2$, as well as CO$_2$. The findings highlight the role of chemisorption and subsequent reactions in driving molecular complexity on solid CO, with implications for the chemical evolution of interstellar ices and the potential formation of prebiotic molecules.

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arXiv2025

CO Adsorption Sites on Interstellar Water Ices Explored with Machine Learning Potentials. Binding energy distributions and snowline

Context. Carbon monoxide (CO) is arguably the most important molecule for interstellar organic chemistry. Its binding to amorphous solid water (ASW) ice regulates both diffusion and desorption processes. Accurately characterizing the CO binding energy (BE) is essential for realistic astrochemical modeling. Aims. We aim to derive a statistically robust and physically accurate distribution of CO BEs on ASW surfaces, and to evaluate its implications for laboratory temperature-programmed desorption experiments and interstellar chemistry, with a focus on protoplanetary disks. Methods. We trained a machine-learned potential (MLP) on 8321 density functional theory (DFT) energies and gradients of CO interacting with differently-sized water clusters (22-60 water molecules). The DFT method was selected after extensive benchmark. With this potential we built realistic non-porous and porous ASW surfaces, and computed a BE distribution. We used symmetry-adapted perturbation theory to rationalize the interaction of CO on the different binding sites. Results. We find that both ASW morphologies yield similar Gaussian-like BE distributions with mean values near 900 K. However, the nature of the binding interactions is rather different and critically depends on surface roughness and dangling-OH bonds. Simulated TPD curves reproduce experimental trends across several coverage regimes. From an astrochemical point of view, the application of the full BE distribution has a dramatic influence on the CO distribution in protoplanetary disks, leading to a broader CO snowline region, improving predictions of CO gas-ice partitioning, and suggesting an equally broader distribution of organics in these objects.

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arXiv2025

Discovery of interstellar phenalene ($c$-C$_{13}$H$_{10}$): A new piece for the chemical puzzle of PAHs in space

We present the discovery of the unsubstituted polycyclic aromatic hydrocarbon (PAH) phenalene ($c$-C$_{13}$H$_{10}$) in TMC-1 as part of the QUIJOTE line survey. In spite of the low dipole moment of this three-ring PAH we have found a total of 267 rotational transitions with quantum numbers $J$ and $K_a$ up to 34 and 14, respectively, corresponding to 100 independent frequencies. The identification of this new PAH from our survey was based on the agreement between the rotational parameters derived from the analysis of the lines and those obtained by quantum chemical calculations. Subsequent chemical synthesis of this PAH and the investigation of its laboratory microwave spectrum unequivocally support our identification. The column density of phenalene in TMC-1 is (2.8$\pm$1.6)$\times$10$^{13}$ cm$^{-2}$.

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arXiv2025

Conformational isomerism of methyl formate: new detections of the higher-energy trans conformer and theoretical insights

In recent astrochemical studies it has become crucial to study all the complete conformational panorama of the molecule, some of which are potentially detectable in the interstellar medium (ISM). In this context, the isomeric ratio can be used as a powerful tool to distinguish between different formation routes of molecules with increasing levels of complexity. While the most stable cis conformer of methyl formate (CH3OCHO, MF) is ubiquitous in the ISM, there is just one tentative detection of the higher-energy trans form toward the envelope of the star-forming region Sgr B2(N). Here, we present the detections of trans-methyl formate toward the Galactic Center molecular cloud G+0.693-0.027 and the protostellar shock L1157-B1, providing definite observational evidence of its presence in the ISM. Numerous unblended or slightly blended $a$-type $K_a$ = 0, 1 transitions belonging to the $A$-substate of trans-MF have been identified in both sources. We derive a molecular column density for trans-methyl formate of N = (8.2 $\pm$ 0.4) $\times$10$^{12}$ cm$^{-2}$ and N = (1.6 $\pm$ 0.3) $\times$10$^{12}$ cm$^{-2}$, respectively, yielding a molecular abundance with respect to H$_2$ of $\sim$6 $\times$ 10$^{-11}$ and $\sim$8 $\times$ 10$^{-10}$. Therefore, we obtain cis/trans isomeric ratios of $\sim$72 and $\sim$34 toward G+0.693 and L1157-B1, which are $\sim$7 and 3 times larger than that found in the Sgr B2(N) region. These results are compared with new grain-surface theoretical computations, which suggest that a stereoespecific formation of trans-MF via the CH3O + HCO route on grain surfaces can qualitatively explain the observed cis/trans abundance ratio. Nevertheless, we show that additional stereoespecific gas-phase routes could also play a crucial role in maintaining the intricate balance between formation and destruction of trans-MF, ultimately leading to its detection.

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arXiv2025

Diffusive vs. non-diffusive paths to interstellar hydrogen peroxide. A machine learning-based molecular dynamics study

Context. Radical chemical reactions on cosmic dust grains play a crucial role in forming various chemical species. Among different radicals, the hydroxyl (OH) is one of the most important ones, with a rather specific chemistry. Aims. The goal of this work is to simulate the recombination dynamics of hydroxyl radicals and the subsequent formation of hydrogen peroxide (H$_{2}$O$_{2}$). Mthods. We employed neural network potentials trained on ONIOM(QM/QM) data, combining multi-reference (CASPT2) and density functional theory (DFT) calculations. This approach allows us to model the recombination of hydroxyl radicals on ice surfaces with high computational efficiency and accuracy. Results. Our simulations reveal that the initial position of the radicals plays a decisive role in determining recombination probability. We found that the formation of hydrogen-bond between radicals, competes with the formation of hydrogen peroxide, reducing the recombination efficiency, contrary to expectancy. This competition reduces the recombination probability for radicals that are initially formed approximately 3 Å apart. Recombination probabilities also depend on the kinetic energy of the added radicals, with values around 0.33 for thermal radicals and a wide range of values between 0.33 and 1.00 for suprathermal OH radicals. Conclusions. Based on our calculations we provide recommendations for introducing OH radical recombination into kinetic astrochemical models, differentiating between thermal and suprathermal radicals. The recombination behavior varies significantly between these two cases: while thermal radicals are sometimes trapped in hydrogen-bonded minima, the case of suprathermal radicals varies with the added energy. Our most important conclusion is that OH radical recombination probability cannot be assumed 1.0 for a wide variety of cases.

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arXiv2025

Hydrogenation of HOCO and formation of interstellar CO$_2$: A not so straightforward relation

Carbon dioxide (CO$_2$) is one of the most important interstellar molecules. While it is considered that it forms on the surface of interstellar dust grains, the exact contribution of different chemical mechanisms is still poorly constrained. Traditionally it is deemed that the CO + OH reaction occurring on top of ices is the main reaction path for its formation. Recent investigations showed that in reality the reaction presents a more complex mechanism, requiring an additional H-abstraction step. Building on our previous works, we carried out a detailed investigation of such H abstraction reactions with the hydrogen atom as a reactant for the abstraction reaction. We found an unconventional chemistry for this reaction, markedly depending on the isomeric form of the HOCO radical prior to reaction. The favored reactions are t-HOCO + H -> CO + H$_2$O, c-HOCO + H -> CO$_2$ + H$_2$ and t/c-HOCO + H -> c/t-HCOOH. We estimate bounds for the rate constants of the less favored reaction channels, t-HOCO + H -> CO$_2$ + H and c-HOCO + H -> CO + H$_2$O, to be approximately 10$^{4-6}$ s$^{-1}$. However, these estimates should be interpreted cautiously due to the significant role of quantum tunneling in these reactions and the complex electronic structure of the involved molecules, which complicates their study. Our findings underscore the need for detailed investigation into the chemistry of interstellar CO$_2$ and pave the way for a reevaluation of its primary formation mechanisms in the interstellar medium.

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arXiv2025

Hydroxylamine in Astrophysical Ices: IR Spectra and Cosmic Ray-Induced Radiolytic Chemistry

Gas-phase hydroxylamine (NH$_2$OH) has recently been detected within dense clouds in the interstellar medium. However, it is also likely present within interstellar ices, as well as on the icy surfaces of outer Solar System bodies, where it may react to form more complex prebiotic molecules such as amino acids. In this work, we aimed to provide IR spectra of NH$_2$OH in astrophysical ice analogues that will help in the search for this molecule in various astrophysical environments. Furthermore, we aimed to provide quantitative information on the stability of NH$_2$OH upon exposure to ionizing radiation analogous to cosmic rays, as well as on the ensuing chemistry and potential formation of complex prebiotic molecules. Ices composed of NH$_2$OH, H$_2$O, and CO were prepared by vapor deposition and IR spectra were acquired between $4000-500$~cm$^{-1}$ ($2.5-20 μ$m) prior and during irradiation using 15 keV protons. In interstellar ice analogues, the most prominent IR absorption band of NH$_2$OH is that at about 1188~cm$^{-1}$, which may be a good candidate to use in searches of this species in icy space environments. Calculated effective destruction cross-sections and G-values for the NH$_2$OH-rich ices studied show that NH$_2$OH is rapidly destroyed upon exposure to ionizing radiation (more rapidly than a number of previously studied organic molecules), and that this destruction is slightly enhanced when it is mixed with other icy species. The irradiation of a NH$_2$OH:H$_2$O:CO ternary ice mixture leads to a rich chemistry that includes the formation of simple inorganic molecules such as NH$_3$, CO$_2$, OCN$^-$, and H$_2$O$_2$, as well as ammonium salts and, possibly, complex organic molecules relevant to life such as formamide, formic acid, urea, and glycine.

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arXiv2024

Hydrogenation of acetaldehyde on interstellar ice analogs reveals limited destruction

We sought to determine which are the main hydrogenation paths of acetaldehyde (CH3CHO). As a partially unsaturated molecule, CH3CHO can have links with more hydrogenated species, like ethanol (C2H5OH) or with more unsaturated ones, like ketene (H2CCO). We used highly accurate quantum chemical calculations to determine the reaction rate constants for the CH3CHO + H/D reaction. Our theoretical results are confronted against our experiments on the hydrogenation and deuteration of CH3CHO ice. We find that acetaldehyde resists hydrogenation, with only a 10\% of conversion to products different than CH3CHO. This is due to a predominance of H-abstraction at the HCO moiety, with reaction rate constants up to four orders of magnitude higher than the next possible reaction channel, that is hydrogenation at the aldehydic carbon. The formed CH3CO radical experiences barrierless or nearly barrierless reactions in all possible reaction positions, reforming CH3CHO and creating a closed loop that protects the molecule against hydrogenation. We constrain the branching ratios for the second reaction from experiments. Our experiments agree with the calculations and from the combination of both we can explain the presence of H2CCO, CO, CH4, C2H5OH, H2CO or CH3OH as minor products at the end of the reaction. We provide recommendations for future modeling efforts. Our results show limited destruction of acetaldehyde, reinforcing the vision of this molecule as an abundant and resilient COM. From the experiments, we are not able to observe the reactive desorption of this molecule. Our results align with other modeling works, showing that the link between CH3CHO and C2H5OH is not direct. Finally, our results can explain the excess of CH3CDO found in prestellar cores.

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arXiv2024

Proposed importance of HOCO chemistry: Inefficient formation of CO$_2$ from CO and OH reactions on ice dust

With the advent of JWST ice observations, dedicated studies on the formation reactions of detected molecules are becoming increasingly important. One of the most interesting molecules in interstellar ice is CO$_2$. Despite its simplicity, the main formation reaction considered, CO + OH -> CO$_2$ + H through the energetic HOCO* intermediate on ice dust, is subject to uncertainty because it directly competes with the stabilization of HOCO as a final product which is formed through energy dissipation of HOCO* to the water ice. When energy dissipation to the surface is effective during reaction, HOCO can be a dominant product. In this study, we experimentally demonstrate that the major product of the reaction is indeed not CO$_2$, but rather the highly reactive radical HOCO. The HOCO radical can later evolve into CO$_2$ through H-abstraction reactions, but these reactions compete with addition reactions, leading to the formation of carboxylic acids (R-COOH). Our results highlight the importance of HOCO chemistry and encourage further exploration of the chemistry of this radical.

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arXiv2024

Methane Formation Efficiency on Icy Grains: Role of Adsorption States

Methane (CH4) is one of the major components of the icy mantle of cosmic dust prevalent in cold, dense regions of interstellar media, playing an important role in the synthesis of complex organic molecules and prebiotic molecules. Solid CH4 is considered to be formed via the successive hydrogenation of C atoms accreting onto dust: C + 4H -> CH4. However, most astrochemical models assume this reaction on the ice mantles of dust to be barrierless and efficient, without considering the states of adsorption. Recently, we found that C atoms exist in either the physisorbed or chemisorbed state on compact amorphous solid water, which is analogous to an interstellar ice mantle. These distinct adsorption states considerably affect the hydrogenation reactivity of the C atom. Herein, we elucidate the reactivities of physisorbed and chemisorbed C atoms with H atoms via sequential deposition and co-deposition processes. The results indicate that only physisorbed C atoms can produce CH4 on ice. Combining this finding with a previous estimate for the fraction of physisorbed C atoms on ice, we determined the upper limit for the conversion of C atoms into CH4 to be 30%.

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