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 toward young stellar objects and quiescent molecular clouds. This is mainly because of the lack of dedicated laboratory studies. The aim of the present 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.
We present a combined experimental and theoretical study focussing on the quantum tunneling of atoms in the reaction between CH4 and OH. The importance of this reaction pathway is derived by ...investigating isotope substituted analogs. Quantitative reaction rates needed for astrochemical models at low temperature are currently unavailable both in the solid state and in the gas phase. Here, we study tunneling effects upon hydrogen abstraction in CH4 + OH by focusing on two reactions: CH4 + OD -> CH3 + HDO and CD4 + OH -> CD3 + HDO. The experimental study shows that the solid-state reaction rate R(CH4 + OD) is higher than R(CD4 + OH) at 15 K. Experimental results are accompanied by calculations of the corresponding unimolecular and bimolecular reaction rate constants using instanton theory taking into account surface effects. From the work presented here, the unimolecular reactions are particularly interesting as these provide insight into reactions following a Langmuir-Hinshelwood process. The resulting ratio of the rate constants shows that the H abstraction (k(CH4 + OD)) is approximately ten times faster than D-abstraction (k(CD4 + OH)) at 65 K. We conclude that tunneling is involved at low temperatures in the abstraction reactions studied here. The unimolecular rate constants can be used by the modeling community as a first approach to describe OH-mediated abstraction reactions in the solid phase. For this reason we provide fits of our calculated rate constants that allow the inclusion of these reactions in models in a straightforward fashion.
The final step of the water formation network on interstellar grain surfaces
starting from the H + O$_2$ route is the reaction between H and H$_2$O$_2$.
This reaction is known to have a high ...activation energy and therefore at low
temperatures it can only proceed via tunneling. To date, however, no rate
constants are available at temperatures below 200 K. In this work, we use
instanton theory to compute rate constants for the title reaction with and
without isotopic substitutions down to temperatures of 50 K. The calculations
are based on density functional theory, with additional benchmarks for the
activation energy using unrestricted single-reference and multireference
coupled-cluster single-point energies. Gas-phase bimolecular rate constants are
calculated and compared with available experimental data not only for H +
H$_2$O$_2$ $\rightarrow$ H$_2$O + OH, but also for H + H$_2$O$_2$ $\rightarrow$
H$_2$ + HO$_2$. We find a branching ratio where the title reaction is favored
by at least two orders of magnitude at 114 K. In the interstellar medium this
reaction predominantly occurs on water surfaces, which increases the
probability that the two reactants meet. To mimic this one, two, or three
spectator H2O molecules are added to the system. Eley-Rideal bimolecular and
Langmuir-Hinshelwood unimolecular rate constants are presented here. The
kinetic isotope effects for the various cases are compared to experimental data
as well as to expressions commonly used in astrochemical models. Both the
rectangular barrier and the Eckart approximations lead to errors of about an
order of magnitude. Finally, fits of the rate constants are provided as input
for astrochemical models.
We report new computational and experimental evidence of an efficient and astrochemically relevant formation route to formaldehyde (H\(_2\)CO). This simplest carbonylic compound is central to the ...formation of complex organics in cold interstellar clouds, and is generally regarded to be formed by the hydrogenation of solid-state carbon monoxide. We demonstrate H\(_2\)CO formation via the reaction of carbon atoms with amorphous solid water. Crucial to our proposed mechanism is a concerted proton transfer catalyzed by the water hydrogen bonding network. Consequently, the reactions \(^3\)C + H\(_2\)O -> \(^3\)HCOH and \(^1\)HCOH -> \(^1\)H\(_2\)CO can take place with low or without barriers, contrary to the high-barrier traditional internal hydrogen migration. These low barriers or absence thereof explain the very small kinetic isotope effect in our experiments when comparing the formation of H\(_2\)CO to D\(_2\)CO. Our results reconcile the disagreement found in the literature on the reaction route: C + H\(_2\)O -> H\(_2\)CO.
Hydroxylamine (NH2OH) is one of the potential precursors of complex pre-biotic species in space. Here we present a detailed experimental study of hydroxylamine formation through nitric oxide (NO) ...surface hydrogenation for astronomically relevant conditions. The aim of this work is to investigate hydroxylamine formation efficiencies in polar (water-rich) and non-polar (carbon monoxide-rich) interstellar ice analogues. A complex reaction network involving both final (N2O, NH2OH) and intermediate (HNO, NH2O, etc.) products is discussed. The main conclusion is that hydroxylamine formation takes place via a fast and barrierless mechanism and it is found to be even more abundantly formed in a water-rich environment at lower temperatures. In parallel, we experimentally verify the non-formation of hydroxylamine upon UV photolysis of NO ice at cryogenic temperatures as well as the non-detection of NC- and NCO-bond bearing species after UV processing of NO in carbon monoxide-rich ices. Our results are implemented into an astrochemical reaction model, which shows that NH2OH is abundant in the solid phase under dark molecular cloud conditions. Once NH2OH desorbs from the ice grains, it becomes available to form more complex species (e.g., glycine and beta-alanine) in gas phase reaction schemes.
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.
OH radicals play a key role as an intermediate in the water formation chemistry of the interstellar medium. For example the reaction of OH radicals with H\(_2\) molecules is among the final steps in ...the astrochemical reaction network starting from O, O\(_2\), and O\(_3\). Experimentally it was shown that even at 10 K this reaction occurs on ice surfaces. As the reaction has a high activation energy only atom tunneling can explain such experimental findings. In this study we calculated reaction rate constants for the title reaction on a water-ice I\(_h\) surface. To our knowledge, low-temperature rate constants on a surface are not available in the literature. All surface calculations were done using a QM/MM framework (BHLYP/TIP3P) after a thorough benchmark of different density functionals and basis sets to highly accurate correlation methods. Reaction rate constants are obtained using instanton theory which takes atom tunneling into account inherently, with constants down to 110 K for the Eley-Rideal mechanism and down to 60 K for the Langmuir-Hinshelwood mechanism. We found that the reaction is nearly temperature independent below 80 K. We give kinetic isotope effects for all possible deuteration patterns for both reaction mechanisms. For the implementation in astrochemical networks, we also give fit parameters to a modified Arrhenius equation. Finally, several different binding sites and binding energies of OH radicals on the I\(_h\) surface are discussed and the corresponding rate constants are compared to the gas-phase case.
We have investigated the thermally induced proton/deuteron exchange in mixed amorphous H\(_2\)O:D\(_2\)O ices by monitoring the change in intensity of characteristic vibrational bending modes of ...H\(_2\)O, HDO, and D\(_2\)O with time and as function of temperature. The experiments have been performed using an ultra-high vacuum setup equipped with an infrared spectrometer that is used to investigate the spectral evolution of homogeneously mixed ice upon co-deposition in thin films, for temperatures in the 90 to 140 K domain. With this non-energetic detection method we find a significantly lower activation energy for H/D exchange -- \(3840 \pm 125\) K -- than previously reported. Very likely this is due to the amorphous nature of the interstellar ice analogues involved. This provides reactive timescales (\(\tau<10^4\) years at \(T\) \(>70\) K) fast enough for the process to be important in interstellar environments. Consequently, an astronomical detection of D\(_2\)O will be even more challenging because of its potential to react with H\(_2\)O to form HDO. Furthermore, additional experiments, along with previous studies, show that proton/deuteron swapping also occurs in ice mixtures of water with other hydrogen bonded molecules, in particular on the OH and NH moieties. We conclude that H/D exchange in ices is a more general process that should be incorporated into ice models that are applied to protoplanetary disks or to simulate the warming up of cometary ices in their passage of the perihelion, to examine the extent of its influence on the final deuteron over hydrogen ratio.
In this work, we reexamine sulfur chemistry occurring on and in the ice mantles of interstellar dust grains, and report the effects of two new modifications to standard astrochemical models; namely, ...(a) the incorporation of cosmic ray-driven radiation chemistry and (b) the assumption of fast, non-diffusive reactions for key radicals in the bulk. Results from our models of dense molecular clouds show that these changes can have a profound influence on the abundances of sulfur-bearing species in ice mantles, including a reduction in the abundance of solid-phase H\(_2\)S and HS, and a significant increase in the abundances of OCS, SO\(_2\), as well as pure allotropes of sulfur, especially S\(_8\). These pure-sulfur species - though nearly impossible to observe directly - have long been speculated to be potential sulfur reservoirs and our results represent possibly the most accurate estimates yet of their abundances in the dense ISM. Moreover, the results of these updated models are found to be in good agreement with available observational data. Finally, we examine the implications of our findings with regard to the as-yet-unknown sulfur reservoir thought to exist in dense interstellar environments.
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