Albert Rimola - Academia.edu (original) (raw)

Papers by Albert Rimola

Astronomy & astrophysics, May 17, 2024

Zenodo (CERN European Organization for Nuclear Research), Dec 30, 2022

Zenodo (CERN European Organization for Nuclear Research), Jul 5, 2022

Journal of applied crystallography, Mar 15, 2024

Astronomy and Astrophysics, Nov 30, 2023

Context. Gas-phase chemistry at extreme conditions (low densities and temperatures) is difficult,... more Context. Gas-phase chemistry at extreme conditions (low densities and temperatures) is difficult, so the presence of interstellar grains is especially important for the synthesis of molecules that cannot form in the gas phase. Interstellar grains are advocated to enhance the encounter rate of the reactive species on their surfaces and to dissipate the energy excess of largely exothermic reactions, but less is known of their role as chemical catalysts that provide low activation energy pathways with enhanced reaction rates. Different materials with catalytic properties are present in interstellar environments, like refractory grains containing space-abundant d-block transition metals. Aims. In this work we report for first time mechanistic insights on the Fischer-Tropsch methanol (CH 3 OH) synthesis under astrophysical conditions using single-atom Fe-containing silica surfaces as interstellar heterogeneous catalysts. Methods. Quantum chemical calculations considering extended periodic surfaces were carried out in order to search for the stationary points and transitions states to finally construct the reaction potential energy surfaces. Binding energy and kinetic calculations based on the Rice-Ramsperger-Kassel-Marcus (RRKM) scheme were also performed to evaluate the catalytical capacity of the grain and to allocate those reaction processes within the astrochemical framework. Results. Our mechanistic studies demonstrate that astrocatalysis is feasible in astrophysical environments. Thermodynamically the proposed process is largely exergonic, but kinetically it shows energy barriers that would need from an energy input in order to go through. Kinetic calculations also demonstrate the strong temperature dependency of the reaction process as tunnelling is not relevant in the involved energetic barriers. The present results can explain the presence of CH 3 OH in diverse regions where current models fail to reproduce its observational quantity. Conclusions. The evidence of astrocatalysis opens a completely new spectrum of synthetic routes triggering chemical evolution in space. From the mechanistic point of view the formation of methanol catalysed by a single atom of Fe 0 is feasible; however, its dependency on the temperature makes the energetics a key issue in this scenario.

arXiv (Cornell University), Jan 19, 2022

The evolution of star-forming regions and their thermal balance are strongly influenced by their ... more The evolution of star-forming regions and their thermal balance are strongly influenced by their chemical composition, that, in turn, is determined by the physicochemical processes that govern the transition between the gas phase and the solid state, specifically icy dust grains (e.g., particles adsorption and desorption). Gas-grain and grain-gas transitions as well as formation and sublimation of interstellar ices are thus essential elements of understanding astrophysical observations of cold environments (e.g., pre-stellar cores) where unexpected amounts of a large variety of chemical species have been observed in the gas phase. Adsorbed atoms and molecules also undergo chemical reactions which are not efficient in the gas phase. Therefore the parameterization of the physical properties of atoms and molecules interacting with dust grain particles is clearly a key aspect to interpret astronomical observations and to build realistic and predictive astrochemical models. In this consensus evaluation, we focus on parameters controlling the thermal desorption of ices and how these determine pathways towards molecular complexity and define the location of snowlines, which ultimately influence the planet formation process. We review different crucial aspects of desorption parameters both from a theoretical and experimental point of view. We critically assess the desorption parameters (the binding energies E b and the pre-exponential factor ν) commonly used in the astrochemical community for astrophysically relevant species and provide tables with recommended values. The aim of these tables is to provide a coherent set of critically assessed desorption parameters for common use in future work. In addition, we show that a non-trivial determination of the pre-exponential factor ν using the Transition State Theory can affect the binding energy value. The primary focus is on pure ices, but we also discuss the desorption behavior of mixed, i.e. astronomically more realistic ices. This allows discussion of segregation effects. Finally, we conclude this work by discussing the limitations of theoretical and experimental approaches currently used to determine the desorption properties with suggestions for future improvements.

Astronomy and Astrophysics, May 1, 2020

Context. Low-mass protostars drive powerful molecular outflows that can be observed with millimet... more Context. Low-mass protostars drive powerful molecular outflows that can be observed with millimetre and submillimetre telescopes. Various sulfuretted species are known to be bright in shocks and could be used to infer the physical and chemical conditions throughout the observed outflows. Aims. The evolution of sulfur chemistry is studied along the outflows driven by the NGC 1333-IRAS4A protobinary system located in the Perseus cloud to constrain the physical and chemical processes at work in shocks. Methods. We observed various transitions from OCS, CS, SO, and SO 2 towards NGC 1333-IRAS4A in the 1.3, 2, and 3 mm bands using the IRAM NOrthern Extended Millimeter Array and we interpreted the observations through the use of the Paris-Durham shock model. Results. The targeted species clearly show different spatial emission along the two outflows driven by IRAS4A. OCS is brighter on small and large scales along the south outflow driven by IRAS4A1, whereas SO 2 is detected rather along the outflow driven by IRAS4A2 that is extended along the north east-south west direction. SO is detected at extremely high radial velocity up to +25 km s −1 relative to the source velocity, clearly allowing us to distinguish the two outflows on small scales. Column density ratio maps estimated from a rotational diagram analysis allowed us to confirm a clear gradient of the OCS/SO 2 column density ratio between the IRAS4A1 and IRAS4A2 outflows. Analysis assuming non Local Thermodynamic Equilibrium of four SO 2 transitions towards several SiO emission peaks suggests that the observed gas should be associated with densities higher than 10 5 cm −3 and relatively warm (T > 100 K) temperatures in most cases. Conclusions. The observed chemical differentiation between the two outflows of the IRAS4A system could be explained by a different chemical history. The outflow driven by IRAS4A1 is likely younger and more enriched in species initially formed in interstellar ices, such as OCS, and recently sputtered into the shock gas. In contrast, the longer and likely older outflow triggered by IRAS4A2 is more enriched in species that have a gas phase origin, such as SO 2 .

arXiv (Cornell University), May 25, 2023

Journal of Physical Chemistry C, Jul 23, 2015

A systematic computational study of the gas-phase adsorption of different amino-acid-analogues (A... more A systematic computational study of the gas-phase adsorption of different amino-acid-analogues (AA-an’s) on a (6,0) boron nitride nanotube (BNNT) and on a boron nitride monolayer (BNML) has been performed by means of B3LYP-D2* periodic calculations. The AA-an’s are CH3-R molecules, where R represents functional groups present in amino acid side chains, i.e., OH, COOH, CONH2, NH2, imidazole, guanidine, phenyl, phenol, indole, and CONHCH3. On (6,0) BNNT, AA-an species containing N electron donor groups (i.e., R = NH2, imidazole, and guanidine) are strongly chemisorbed through dative interactions between the N atom of the AA-an and a B atom of the nanotube and present the largest adsorption energies (ΔEads). For AA-an bearing aromatic rings (i.e., R = phenyl, phenol and indole) and R = CONHCH3, adsorption is driven by π-stacking interactions (with lower ΔEads values than the previous group), while for AA-an with O electron donor groups and H-bonding donor groups (i.e., R = OH, COOH, and CONH2) adsorption is ...

European Planetary Science Congress, Sep 1, 2018

Physical Chemistry Chemical Physics

Water formation by reaction of H2 and O on silicate surfaces as a first step towards the generati... more Water formation by reaction of H2 and O on silicate surfaces as a first step towards the generation of interstellar ice mantles is possible thanks to the activation of H2 inferred by Fe2+ ions and quantum tunnelling effects.

L'objectiu de la missió espacial Rosetta és aterrar sobre el cometa 67P per realitzar experim... more L'objectiu de la missió espacial Rosetta és aterrar sobre el cometa 67P per realitzar experiments que permetin conèixer millor aquest tipus de cossos celestes, formats a les primeres etapes del nostre sistema planetari. Es pot estudiar la composició de la matèria dels cometes a partir d'experiments en laboratoris que simulin les reaccions que poden succeir en aquests astres. Investigadors de la UAB han participat en unes d'aquestes simulacions, els resultats de les quals coincideixen amb alguns dels obtinguts per la sonda Philae a la superfície del cometa.El objetivo de la misión espacial Rosetta es aterrizar sobre el cometa 67P para realizar experimentos que permitan conocer mejor este tipo de cuerpos celestes, formados en las primeras etapas de nuestro sistema planetario. Se puede estudiar la composición de la materia de los cometas a partir de experimentos en laboratorios que simulen las reacciones que pueden suceder en estos astros. Investigadores de la UAB han parti...

Ethanol (CH 3 CH 2 OH) is a relatively common molecule, often found in star-forming regions. Rece... more Ethanol (CH 3 CH 2 OH) is a relatively common molecule, often found in star-forming regions. Recent studies suggest that it could be a parent molecule of several so-called interstellar complex organic molecules (iCOMs). However, the formation route of this species remains under debate. In the present work, we study the formation of ethanol through the reaction of CCH with one H 2 O molecule belonging to the ice as a test case to investigate the viability of chemical reactions based on a "radical + ice component" scheme as an alternative mechanism for the synthesis of iCOMs, beyond the usual radical− radical coupling. This has been done by means of DFT calculations adopting two clusters of 18 and 33 water molecules as ice models. Results indicate that CH 3 CH 2 OH can potentially be formed by this proposed reaction mechanism. The reaction of CCH with H 2 O on the water ice clusters can be barrierless (because of the help of boundary icy water molecules acting as proton-transfer assistants), leading to the formation of vinyl alcohol precursors (H 2 CCOH and CHCHOH). Subsequent hydrogenation of vinyl alcohol yielding ethanol is the only step presenting a low activation energy barrier. We finally discuss the astrophysical implications of these findings.

Physical Chemistry Chemical Physics, 2017

Physical Chemistry Chemical Physics

HCN/forsterite interactions are present in many environments of the interstellar medium, leading ... more HCN/forsterite interactions are present in many environments of the interstellar medium, leading to both molecular and dissociative HCN adsorptions, by means of its catalytic surface sites.

The Astrophysical Journal, Nov 30, 2017

Complex organic molecules have been observed for decades in the interstellar medium. Some of them... more Complex organic molecules have been observed for decades in the interstellar medium. Some of them might be considered as small bricks of the macromolecules at the base of terrestrial life. It is hence particularly important to understand organic chemistry in Solar-like star-forming regions. In this article, we present a new observational project: Seeds Of Life In Space (SOLIS). This is a Large Project using the IRAM-NOEMA interferometer, and its scope is to image the emission of several crucial organic molecules in a sample of Solar-like star-forming regions in different evolutionary stages and environments. Here we report the first SOLIS results, obtained from analyzing the spectra of different regions of the Class 0 source NGC 1333-IRAS4A, the protocluster OMC-2 FIR4, and the shock site L1157-B1. The different regions were identified based on the images of formamide (NH 2 CHO) and cyanodiacetylene (HC 5 N) lines. We discuss the observed large diversity in the molecular and organic content, both on large (3000-10,000 au) and relatively small (300-1000 au) scales. Finally, we derive upper limits to the methoxy fractional abundance in the three observed regions of the same order of magnitude of that measured in a few cold prestellar objects, namely 10 12-10 −11 with respect to H 2 molecules.

Frontiers in Astronomy and Space Sciences, Apr 21, 2021

The issue of formation of dust grains in the interstellar medium is still a matter of debate. One... more The issue of formation of dust grains in the interstellar medium is still a matter of debate. One of the most developed proposals suggests that atomic and heteromolecular seeds bind together to initiate a nucleation process leading to the formation of nanostructures resembling very small grain components. In the case of silicates, nucleated systems can result in molecular nanoclusters with diameters ≤ 2 nm. A reasonable path to further increase the size of these proto-silicate nanoclusters is by mutual aggregation. The present work deals with modeling this proto-silicate nanocluster aggregation process by means of quantum chemical density functional theory calculations. We simulate nanocluster aggregation by progressively reducing the size of a periodic array of initially well-separated nanoclusters. The resulting aggregation leads to a set of silicate bulk structures with gradually increasing density which we analyze with respect to structure, energetics and spectroscopic properties. Our results indicate that aggregation is a highly energetically favorable process, in which the infrared spectra of the finally formed amorphous silicates match well with astronomical observations.

Monthly Notices of the Royal Astronomical Society: Letters, Feb 26, 2016

Complex Organic Molecules (COMs) have been detected in the interstellar medium (ISM). However, it... more Complex Organic Molecules (COMs) have been detected in the interstellar medium (ISM). However, it is not clear whether their synthesis occurs on the icy surfaces of interstellar grains or via a series of gas-phase reactions. As a test case of the COMs synthesis in the ISM, we present new quantum chemical calculations on the formation of acetaldehyde (CH 3 CHO) from the coupling of the HCO and CH 3 radicals, both in gas phase and on water ice surfaces. The binding energies of HCO and CH 3 on the amorphous water ice were also computed (2333 and 734 K, respectively). Results indicate that, in gas phase, the products could be either CH 3 CHO, CH 4 + CO, or CH 3 OCH, depending on the relative orientation of the two radicals. However, on the amorphous water ice, only the CH 4 + CO product is possible due to the geometrical constraints imposed by the water ice surface. Therefore, acetaldehyde cannot be synthesized by the CH 3 + HCO coupling on the icy grains. We discuss the implications of these results and other cases, such as ethylene glycol and dimethyl ether, in which similar situations can occur, suggesting that formation of these molecules on the grain surfaces might be unlikely.

Astronomy & astrophysics, May 17, 2024

Zenodo (CERN European Organization for Nuclear Research), Dec 30, 2022

Zenodo (CERN European Organization for Nuclear Research), Jul 5, 2022

Journal of applied crystallography, Mar 15, 2024

Astronomy and Astrophysics, Nov 30, 2023

Context. Gas-phase chemistry at extreme conditions (low densities and temperatures) is difficult,... more Context. Gas-phase chemistry at extreme conditions (low densities and temperatures) is difficult, so the presence of interstellar grains is especially important for the synthesis of molecules that cannot form in the gas phase. Interstellar grains are advocated to enhance the encounter rate of the reactive species on their surfaces and to dissipate the energy excess of largely exothermic reactions, but less is known of their role as chemical catalysts that provide low activation energy pathways with enhanced reaction rates. Different materials with catalytic properties are present in interstellar environments, like refractory grains containing space-abundant d-block transition metals. Aims. In this work we report for first time mechanistic insights on the Fischer-Tropsch methanol (CH 3 OH) synthesis under astrophysical conditions using single-atom Fe-containing silica surfaces as interstellar heterogeneous catalysts. Methods. Quantum chemical calculations considering extended periodic surfaces were carried out in order to search for the stationary points and transitions states to finally construct the reaction potential energy surfaces. Binding energy and kinetic calculations based on the Rice-Ramsperger-Kassel-Marcus (RRKM) scheme were also performed to evaluate the catalytical capacity of the grain and to allocate those reaction processes within the astrochemical framework. Results. Our mechanistic studies demonstrate that astrocatalysis is feasible in astrophysical environments. Thermodynamically the proposed process is largely exergonic, but kinetically it shows energy barriers that would need from an energy input in order to go through. Kinetic calculations also demonstrate the strong temperature dependency of the reaction process as tunnelling is not relevant in the involved energetic barriers. The present results can explain the presence of CH 3 OH in diverse regions where current models fail to reproduce its observational quantity. Conclusions. The evidence of astrocatalysis opens a completely new spectrum of synthetic routes triggering chemical evolution in space. From the mechanistic point of view the formation of methanol catalysed by a single atom of Fe 0 is feasible; however, its dependency on the temperature makes the energetics a key issue in this scenario.

arXiv (Cornell University), Jan 19, 2022

The evolution of star-forming regions and their thermal balance are strongly influenced by their ... more The evolution of star-forming regions and their thermal balance are strongly influenced by their chemical composition, that, in turn, is determined by the physicochemical processes that govern the transition between the gas phase and the solid state, specifically icy dust grains (e.g., particles adsorption and desorption). Gas-grain and grain-gas transitions as well as formation and sublimation of interstellar ices are thus essential elements of understanding astrophysical observations of cold environments (e.g., pre-stellar cores) where unexpected amounts of a large variety of chemical species have been observed in the gas phase. Adsorbed atoms and molecules also undergo chemical reactions which are not efficient in the gas phase. Therefore the parameterization of the physical properties of atoms and molecules interacting with dust grain particles is clearly a key aspect to interpret astronomical observations and to build realistic and predictive astrochemical models. In this consensus evaluation, we focus on parameters controlling the thermal desorption of ices and how these determine pathways towards molecular complexity and define the location of snowlines, which ultimately influence the planet formation process. We review different crucial aspects of desorption parameters both from a theoretical and experimental point of view. We critically assess the desorption parameters (the binding energies E b and the pre-exponential factor ν) commonly used in the astrochemical community for astrophysically relevant species and provide tables with recommended values. The aim of these tables is to provide a coherent set of critically assessed desorption parameters for common use in future work. In addition, we show that a non-trivial determination of the pre-exponential factor ν using the Transition State Theory can affect the binding energy value. The primary focus is on pure ices, but we also discuss the desorption behavior of mixed, i.e. astronomically more realistic ices. This allows discussion of segregation effects. Finally, we conclude this work by discussing the limitations of theoretical and experimental approaches currently used to determine the desorption properties with suggestions for future improvements.

Astronomy and Astrophysics, May 1, 2020

Context. Low-mass protostars drive powerful molecular outflows that can be observed with millimet... more Context. Low-mass protostars drive powerful molecular outflows that can be observed with millimetre and submillimetre telescopes. Various sulfuretted species are known to be bright in shocks and could be used to infer the physical and chemical conditions throughout the observed outflows. Aims. The evolution of sulfur chemistry is studied along the outflows driven by the NGC 1333-IRAS4A protobinary system located in the Perseus cloud to constrain the physical and chemical processes at work in shocks. Methods. We observed various transitions from OCS, CS, SO, and SO 2 towards NGC 1333-IRAS4A in the 1.3, 2, and 3 mm bands using the IRAM NOrthern Extended Millimeter Array and we interpreted the observations through the use of the Paris-Durham shock model. Results. The targeted species clearly show different spatial emission along the two outflows driven by IRAS4A. OCS is brighter on small and large scales along the south outflow driven by IRAS4A1, whereas SO 2 is detected rather along the outflow driven by IRAS4A2 that is extended along the north east-south west direction. SO is detected at extremely high radial velocity up to +25 km s −1 relative to the source velocity, clearly allowing us to distinguish the two outflows on small scales. Column density ratio maps estimated from a rotational diagram analysis allowed us to confirm a clear gradient of the OCS/SO 2 column density ratio between the IRAS4A1 and IRAS4A2 outflows. Analysis assuming non Local Thermodynamic Equilibrium of four SO 2 transitions towards several SiO emission peaks suggests that the observed gas should be associated with densities higher than 10 5 cm −3 and relatively warm (T > 100 K) temperatures in most cases. Conclusions. The observed chemical differentiation between the two outflows of the IRAS4A system could be explained by a different chemical history. The outflow driven by IRAS4A1 is likely younger and more enriched in species initially formed in interstellar ices, such as OCS, and recently sputtered into the shock gas. In contrast, the longer and likely older outflow triggered by IRAS4A2 is more enriched in species that have a gas phase origin, such as SO 2 .

arXiv (Cornell University), May 25, 2023

Journal of Physical Chemistry C, Jul 23, 2015

A systematic computational study of the gas-phase adsorption of different amino-acid-analogues (A... more A systematic computational study of the gas-phase adsorption of different amino-acid-analogues (AA-an’s) on a (6,0) boron nitride nanotube (BNNT) and on a boron nitride monolayer (BNML) has been performed by means of B3LYP-D2* periodic calculations. The AA-an’s are CH3-R molecules, where R represents functional groups present in amino acid side chains, i.e., OH, COOH, CONH2, NH2, imidazole, guanidine, phenyl, phenol, indole, and CONHCH3. On (6,0) BNNT, AA-an species containing N electron donor groups (i.e., R = NH2, imidazole, and guanidine) are strongly chemisorbed through dative interactions between the N atom of the AA-an and a B atom of the nanotube and present the largest adsorption energies (ΔEads). For AA-an bearing aromatic rings (i.e., R = phenyl, phenol and indole) and R = CONHCH3, adsorption is driven by π-stacking interactions (with lower ΔEads values than the previous group), while for AA-an with O electron donor groups and H-bonding donor groups (i.e., R = OH, COOH, and CONH2) adsorption is ...

European Planetary Science Congress, Sep 1, 2018

Physical Chemistry Chemical Physics

Water formation by reaction of H2 and O on silicate surfaces as a first step towards the generati... more Water formation by reaction of H2 and O on silicate surfaces as a first step towards the generation of interstellar ice mantles is possible thanks to the activation of H2 inferred by Fe2+ ions and quantum tunnelling effects.

L'objectiu de la missió espacial Rosetta és aterrar sobre el cometa 67P per realitzar experim... more L'objectiu de la missió espacial Rosetta és aterrar sobre el cometa 67P per realitzar experiments que permetin conèixer millor aquest tipus de cossos celestes, formats a les primeres etapes del nostre sistema planetari. Es pot estudiar la composició de la matèria dels cometes a partir d'experiments en laboratoris que simulin les reaccions que poden succeir en aquests astres. Investigadors de la UAB han participat en unes d'aquestes simulacions, els resultats de les quals coincideixen amb alguns dels obtinguts per la sonda Philae a la superfície del cometa.El objetivo de la misión espacial Rosetta es aterrizar sobre el cometa 67P para realizar experimentos que permitan conocer mejor este tipo de cuerpos celestes, formados en las primeras etapas de nuestro sistema planetario. Se puede estudiar la composición de la materia de los cometas a partir de experimentos en laboratorios que simulen las reacciones que pueden suceder en estos astros. Investigadores de la UAB han parti...

Ethanol (CH 3 CH 2 OH) is a relatively common molecule, often found in star-forming regions. Rece... more Ethanol (CH 3 CH 2 OH) is a relatively common molecule, often found in star-forming regions. Recent studies suggest that it could be a parent molecule of several so-called interstellar complex organic molecules (iCOMs). However, the formation route of this species remains under debate. In the present work, we study the formation of ethanol through the reaction of CCH with one H 2 O molecule belonging to the ice as a test case to investigate the viability of chemical reactions based on a "radical + ice component" scheme as an alternative mechanism for the synthesis of iCOMs, beyond the usual radical− radical coupling. This has been done by means of DFT calculations adopting two clusters of 18 and 33 water molecules as ice models. Results indicate that CH 3 CH 2 OH can potentially be formed by this proposed reaction mechanism. The reaction of CCH with H 2 O on the water ice clusters can be barrierless (because of the help of boundary icy water molecules acting as proton-transfer assistants), leading to the formation of vinyl alcohol precursors (H 2 CCOH and CHCHOH). Subsequent hydrogenation of vinyl alcohol yielding ethanol is the only step presenting a low activation energy barrier. We finally discuss the astrophysical implications of these findings.

Physical Chemistry Chemical Physics, 2017

Physical Chemistry Chemical Physics

HCN/forsterite interactions are present in many environments of the interstellar medium, leading ... more HCN/forsterite interactions are present in many environments of the interstellar medium, leading to both molecular and dissociative HCN adsorptions, by means of its catalytic surface sites.

The Astrophysical Journal, Nov 30, 2017

Complex organic molecules have been observed for decades in the interstellar medium. Some of them... more Complex organic molecules have been observed for decades in the interstellar medium. Some of them might be considered as small bricks of the macromolecules at the base of terrestrial life. It is hence particularly important to understand organic chemistry in Solar-like star-forming regions. In this article, we present a new observational project: Seeds Of Life In Space (SOLIS). This is a Large Project using the IRAM-NOEMA interferometer, and its scope is to image the emission of several crucial organic molecules in a sample of Solar-like star-forming regions in different evolutionary stages and environments. Here we report the first SOLIS results, obtained from analyzing the spectra of different regions of the Class 0 source NGC 1333-IRAS4A, the protocluster OMC-2 FIR4, and the shock site L1157-B1. The different regions were identified based on the images of formamide (NH 2 CHO) and cyanodiacetylene (HC 5 N) lines. We discuss the observed large diversity in the molecular and organic content, both on large (3000-10,000 au) and relatively small (300-1000 au) scales. Finally, we derive upper limits to the methoxy fractional abundance in the three observed regions of the same order of magnitude of that measured in a few cold prestellar objects, namely 10 12-10 −11 with respect to H 2 molecules.

Frontiers in Astronomy and Space Sciences, Apr 21, 2021

The issue of formation of dust grains in the interstellar medium is still a matter of debate. One... more The issue of formation of dust grains in the interstellar medium is still a matter of debate. One of the most developed proposals suggests that atomic and heteromolecular seeds bind together to initiate a nucleation process leading to the formation of nanostructures resembling very small grain components. In the case of silicates, nucleated systems can result in molecular nanoclusters with diameters ≤ 2 nm. A reasonable path to further increase the size of these proto-silicate nanoclusters is by mutual aggregation. The present work deals with modeling this proto-silicate nanocluster aggregation process by means of quantum chemical density functional theory calculations. We simulate nanocluster aggregation by progressively reducing the size of a periodic array of initially well-separated nanoclusters. The resulting aggregation leads to a set of silicate bulk structures with gradually increasing density which we analyze with respect to structure, energetics and spectroscopic properties. Our results indicate that aggregation is a highly energetically favorable process, in which the infrared spectra of the finally formed amorphous silicates match well with astronomical observations.

Monthly Notices of the Royal Astronomical Society: Letters, Feb 26, 2016

Complex Organic Molecules (COMs) have been detected in the interstellar medium (ISM). However, it... more Complex Organic Molecules (COMs) have been detected in the interstellar medium (ISM). However, it is not clear whether their synthesis occurs on the icy surfaces of interstellar grains or via a series of gas-phase reactions. As a test case of the COMs synthesis in the ISM, we present new quantum chemical calculations on the formation of acetaldehyde (CH 3 CHO) from the coupling of the HCO and CH 3 radicals, both in gas phase and on water ice surfaces. The binding energies of HCO and CH 3 on the amorphous water ice were also computed (2333 and 734 K, respectively). Results indicate that, in gas phase, the products could be either CH 3 CHO, CH 4 + CO, or CH 3 OCH, depending on the relative orientation of the two radicals. However, on the amorphous water ice, only the CH 4 + CO product is possible due to the geometrical constraints imposed by the water ice surface. Therefore, acetaldehyde cannot be synthesized by the CH 3 + HCO coupling on the icy grains. We discuss the implications of these results and other cases, such as ethylene glycol and dimethyl ether, in which similar situations can occur, suggesting that formation of these molecules on the grain surfaces might be unlikely.