The 29 reactions linking carbon monosulfide (CS) to methyl mercaptan (CH3SH) via ten intermediate radicals and molecules have been characterized with relevance to surface chemistry in cold ...interstellar ices. More intermediate species than previously considered are found likely to be present in these ices, such as trans- and cis-HCSH. Both activation and reaction energies have been calculated, along with low-temperature (T > 45 K) rate constants for the radical-neutral reactions. For barrierless radical-radical reactions on the other hand, branching ratios have been determined. The combination of these two sets of information provides, for the first time, quantitative information on the full H + CS reaction network. Early on in this network, that is, early on in the lifetime of an interstellar cloud, HCS is the main radical, while later on this becomes first CH2SH and finally CH3S.
Context.
Glycoaldehyde, ethylene glycol, and methyl formate are complex organic molecules that have been observed in dark molecular clouds. Because there is no efficient gas-phase route to produce ...these species, it is expected that a low-temperature surface route existst that does not require energetic processing. CO hydrogenation experiments at low temperatures showed that this is indeed the case. Glyoxal can form through recombination of two HCO radicals and is then further hydrogenated.
Aims.
Here we aim to constrain the methyl formate, glycolaldehyde, and ethylene glycol formation on the surface of interstellar dust grains through this cold and dark formation route. We also probe the dependence of the grain mantle composition on the initial gas-phase composition and the dust temperature.
Methods.
A full CO hydrogenation reaction network was built based on quantum chemical calculations for the rate constants and branching ratios. This network was used in combination with a microscopic kinetic Monte Carlo simulation to simulate ice chemistry, taking into account all positional information. After benchmarking the model against CO-hydrogenation experiments, simulations under molecular cloud conditions were performed.
Results.
Glycoaldehyde, ethylene glycol, and methyl formate are formed in all interstellar conditions we studied, even at temperatures as low as 8 K. This is because the HCO + HCO reaction can occur when HCO radicals are formed close to each other and do not require to diffuse. Relatively low abundances of methyl formate are formed. The final COM abundances depend more on the H-to-CO ratio and less on temperature. Only above 16 K, where CO build-up is less efficient, does temperature start to play a role. Molecular hydrogen is predominantly formed through abstraction reactions on the surface. The most important reaction leading to methanol is H
2
CO + CH
3
O → HCO + CH
3
OH. Our simulations are in agreement with observed COM ratios for mantles that have been formed at low temperatures.
The cross-disciplinary field of astrochemistry exists to understand the formation, destruction, and survival of molecules in astrophysical environments. Molecules in space are synthesized via a large ...variety of gas-phase reactions, and reactions on dust-grain surfaces, where the surface acts as a catalyst. A broad consensus has been reached in the astrochemistry community on how to suitably treat gas-phase processes in models, and also on how to present the necessary reaction data in databases; however, no such consensus has yet been reached for grain-surface processes. A team of
∼
25
experts covering observational, laboratory and theoretical (astro)chemistry met in summer of 2014 at the Lorentz Center in Leiden with the aim to provide solutions for this problem and to review the current state-of-the-art of grain surface models, both in terms of technical implementation into models as well as the most up-to-date information available from experiments and chemical computations. This review builds on the results of this workshop and gives an outlook for future directions.
Interstellar dust grains are known to facilitate chemical reactions by acting as a meeting place and adsorbing energy. This process strongly depends on the ability of the reactive species to ...effectively diffuse over the surface. The cold temperatures around 10 K strongly hamper this for species other than H and H2. However, complex organic molecules have been observed in the gas phase at these cold conditions, indicating that their formation, as well as their return to the gas phase, should be effective. Here, we show how the energy released following surface reactions can be employed to solve both problems by inducing desorption or diffusion. To this purpose, we have performed thousands of Molecular Dynamics simulations to quantify the outcome of an energy dissipation process. Admolecules on top of a crystalline water surface have been given translational energy between 0.5 and 5 eV. Three different surface species are considered (CO2, H2O, and CH4), spanning a range in binding energies, number of internal degrees of freedom, and molecular weights. The admolecules are found to be able to travel up to several hundreds of angstroms before coming to a stand still, allowing for follow-up reactions en route. The probability of travel beyond any particular radius, as determined by our simulations, shows the same r dependence for all three admolecule species. Furthermore, we have been able to quantify the desorption probability, which depends on the binding energy of the species and the translational excitation. We provide expressions that can be incorporated in astrochemical models to predict grain surface formation and return into the gas phase of these products.
Solid-state astrochemical reaction pathways have the potential to link the formation of small nitrogen-bearing species, like NH3 and HNCO, and prebiotic molecules, specifically amino acids. To date, ...the chemical origin of such small nitrogen-containing species is still not well understood, despite the fact that ammonia is an abundant constituent of interstellar ices towards young stellar objects and quiescent molecular clouds. This is mainly because of the lack of dedicated laboratory studies. The aim of this work is to experimentally investigate the formation routes of NH3 and HNCO through non-energetic surface reactions in interstellar ice analogues under fully controlled laboratory conditions and at astrochemically relevant temperatures. This study focuses on the formation of NH3 and HNCO in CO-rich (non-polar) interstellar ices that simulate the CO freeze-out stage in dark interstellar cloud regions, well before thermal and energetic processing start to become relevant. We demonstrate and discuss the surface formation of solid HNCO through the interaction of CO molecules with NH radicals – one of the intermediates in the formation of solid NH3 upon sequential hydrogenation of N atoms. The importance of HNCO for astrobiology is discussed.
This study focuses on the formation of two molecules of astrobiological importance – glycolaldehyde (HC(O)CH2OH) and ethylene glycol (H2C(OH)CH2OH) – by surface hydrogenation of CO molecules. Our ...experiments aim at simulating the CO freeze-out stage in interstellar dark cloud regions, well before thermal and energetic processing become dominant. It is shown that along with the formation of H2CO and CH3OH – two well-established products of CO hydrogenation – also molecules with more than one carbon atom form. The key step in this process is believed to be the recombination of two HCO radicals followed by the formation of a C–C bond. The experimentally established reaction pathways are implemented into a continuous-time random-walk Monte Carlo model, previously used to model the formation of CH3OH on astrochemical time-scales, to study their impact on the solid-state abundances in dense interstellar clouds of glycolaldehyde and ethylene glycol.
ABSTRACT
The formation of interstellar complex organic molecules is currently thought to be dominated by the barrierless coupling between radicals on the interstellar icy grain surfaces. Previous ...standard density functional theory (DFT) results on the reactivity between CH3 and HCO on amorphous water surfaces showed that the formation of CH4 + CO by H transfer from HCO to CH3 assisted by water molecules of the ice was the dominant channel. However, the adopted description of the electronic structure of the biradical (i.e. CH3/HCO) system was inadequate without the broken-symmetry (BS) approach. In this work, we revisit the original results by means of BS-DFT both in gas phase and with one water molecule simulating the role of the ice. Results indicate that the adoption of BS-DFT is mandatory to describe properly biradical systems. In the presence of the single water molecule, the water-assisted H transfer exhibits a high energy barrier. In contrast, CH3CHO formation is found to be barrierless. However, direct H transfer from HCO to CH3 to give CO and CH4 presents a very low energy barrier, hence being a potential competitive channel to the radical coupling and indicating, moreover, that the physical insights of the original work remain valid.
Abstract Parkinson's disease (PD) is mainly attributed to degeneration of dopamine neurons in the substantia nigra, but its etiopathogenesis also includes impaired protein clearance and axonal ...transport dysfunction, among others. The spread of α-synuclein (α-syn) aggregates from one neuron to another, in a prion-like manner, is hypothesized to contribute to PD progression. Axonal transport is likely to play a crucial role in this movement of α-syn aggregates between brain regions. At the same time, deficits in axonal transport are suggested to contribute to neuronal failure in PD. In this review, we discuss the apparent contradiction that axonal transport might be essential for disease progression, while dysfunction of axonal transport could simultaneously be a cornerstone of PD pathogenesis. We speculate around models that reconcile how axonal transport can play such a paradoxical role.
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