On the surface of icy dust grains in the dense regions of the interstellar medium, a rich chemistry can take place. Due to the low temperature, reactions that proceed via a barrier can only take ...place through tunneling. The reaction H + H 2 O 2 ⟶ H 2 O + OH is such a case with a gas-phase barrier of ∼26.5 kJ mol−1. Still, the reaction is known to be involved in water formation on interstellar grains. Here, we investigate the influence of a water ice surface and of bulk ice on the reaction rate constant. Rate constants are calculated using instanton theory down to 74 K. The ice is taken into account via multiscale modeling, describing the reactants and the direct surrounding at the quantum mechanical level with density functional theory (DFT), while the rest of the ice is modeled on the molecular mechanical level with a force field. We find that H2O2 binding energies cannot be captured by a single value, but rather they depend on the number of hydrogen bonds with surface molecules. In highly amorphous surroundings, the binding site can block the routes of attack and impede the reaction. Furthermore, the activation energies do not correlate with the binding energies of the same sites. The unimolecular rate constants related to the Langmuir-Hinshelwood mechanism increase as the activation energy decreases. Thus, we provide a lower limit for the rate constant and argue that rate constants can have values up to two order of magnitude larger than this limit.
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, nondiffusive 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 H2S and HS, and a significant increase in the abundances of OCS, SO2, as well as pure allotropes of sulfur, especially S8. 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 interstellar medium. 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.
The hydrogen abstraction reaction between H and H2S, yielding HS and H2 as products, has been studied within the framework of interstellar surface chemistry. High-temperature rate constants below ...2000 K are calculated in the gas phase and are in agreement with previously reported values. Subsequently, low-temperature rate constants down to 55 K are presented for the first time that are of interest to astrochemistry, i.e., covering both bimolecular and unimolecular reaction mechanisms. For this, a so-called implicit surface model is used. In strict terms, this is a structural gas-phase model in which the restriction of the rotation in the solid state is taken into account. The calculated kinetic isotope effects are explained in terms of the difference in activation and delocalization. All rate constants are calculated at the UCCSD(T)-F12/cc-VTZ-F12 level of theory. Finally, we show that the energetics of the reaction is affected to an only small extent by the presence of H2O or H2S molecular clusters that simulate an ice surface, calculated at the MPWB1K/def2-TZVP level of theory.
The successive addition of H atoms to CO in the solid phase has been hitherto regarded as the primary route to form methanol in dark molecular clouds. However, recent Monte Carlo simulations of ...interstellar ices alternatively suggested the radical-molecule H-atom abstraction reaction CH3O + H2CO → CH3OH + HCO, in addition to CH3O + H → CH3OH, as a very promising and possibly dominating (70%–90%) final step to form CH3OH in those environments. Here, we compare the contributions of these two steps leading to methanol by experimentally investigating hydrogenation reactions on H2CO and D2CO ices, which ensures comparable starting points between the two scenarios. The experiments are performed under ultrahigh vacuum conditions and astronomically relevant temperatures, with H:H2CO (or D2CO) flux ratios of 10:1 and 30:1. The radical-molecule route in the partially deuterated scenario, CHD2O + D2CO → CHD2OD + DCO, is significantly hampered by the isotope effect in the D-abstraction process, and can thus be used as an artifice to probe the efficiency of this step. We observe a significantly smaller yield of D2CO + H products in comparison to H2CO + H, implying that the CH3O-induced abstraction route must play an important role in the formation of methanol in interstellar ices. Reflection-absorption infrared spectroscopy and temperature-programmed desorption-quadrupole mass spectrometry analyses are used to quantify the species in the ice. Both analytical techniques indicate constant contributions of ∼80% for the abstraction route in the 10–16 K interval, which agrees well with the Monte Carlo calculations. Additional H2CO + D experiments confirm these conclusions.
Water is the most abundant molecule found in interstellar icy mantles. In space it is thought to be efficiently formed on the surfaces of dust grains through successive hydrogenation of O, O2 and O3. ...The underlying physico-chemical mechanisms have been studied experimentally in the past decade and in this paper we extend this work theoretically, using Continuous-Time Random-Walk Monte Carlo simulations to disentangle the different processes at play during hydrogenation of molecular oxygen. CTRW-MC offers a kinetic approach to compare simulated surface abundances of different species to the experimental values. For this purpose, the results of four key experiments-sequential hydrogenation as well as co-deposition experiments at 15 and 25 K-are selected that serve as a reference throughout the modeling stage. The aim is to reproduce all four experiments with a single set of parameters. Input for the simulations consists of binding energies as well as reaction barriers (activation energies). In order to understand the influence of the parameters separately, we vary a single process rate at a time. Our main findings are: (i) The key reactions for the hydrogenation route starting from O2 are H + O2, H + HO2, OH + OH, H + H2O2, H + OH. (ii) The relatively high experimental abundance of H2O2 is due to its slow destruction. (iii) The large consumption of O2 at a temperature of 25 K is due to a high hydrogen diffusion rate. (iv) The diffusion of radicals plays an important role in the full reaction network. The resulting set of 'best fit' parameters is presented and discussed for use in future astrochemical modeling.
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 H2 molecules is among the final steps in the ...astrochemical reaction network starting from O, O2, and O3. Experimentally, it was shown that, even at 10 K, this reaction occurs on ice surfaces. Because 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 Ih surface. To our knowledge, low-temperature rate constants on a surface are not available in the literature. All surface calculations were performed using a quantum mechanics/molecular mechanics 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 the instanton theory, which takes atom tunneling into account inherently, with reaction rate 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 rate 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 Ih surface are discussed, and the corresponding rate constants are compared to the gas-phase case.
Abstract
Methane is typically thought to be formed in the solid state on top of cold interstellar icy grain mantles via the successive atomic hydrogenation of a carbon atom. In the current work we ...investigate the role of molecular hydrogen in the CH
4
reaction network. We make use of an ultrahigh vacuum cryogenic setup combining an atomic carbon atom beam with atomic and/or molecular beams of hydrogen and deuterium on a water ice. These experiments lead to the formation of methane isotopologues detected in situ through reflection absorption infrared spectroscopy. Most notably, CH
4
is experimentally formed by combining C atoms with only H
2
on amorphous solid water, albeit more slowly than in experiments where H atoms are also present. Furthermore, CH
2
D
2
is detected in an experiment involving C atoms with H
2
and D
2
on H
2
O ice. CD
4
, however, is only formed when D atoms are present in the experiment. These findings have been rationalized by means of computational and theoretical chemical insights. This leads to the following conclusions: (a) the reaction C + H
2
→ CH
2
takes place, although it is not barrierless for all binding sites on water, (b) the reaction CH + H
2
→ CH
3
is barrierless, but has not yet been included in astrochemical models, (c) the reactions CH
2
+ H
2
→ CH
3
+ H and CH
3
+ H
2
→ CH
4
+ H can take place only via a tunneling mechanism, and (d) molecular hydrogen possibly plays a more important role in the solid-state formation of methane than assumed so far.
Abstract
We report the detection of the lowest-energy conformer of
E
-1-cyano-1,3-butadiene (
E
-1-
C
4
H
5
CN
), a linear isomer of pyridine, using the fourth data reduction of the GBT Observations ...of TMC-1: Hunting for Aromatic Molecules (GOTHAM) deep spectral survey toward TMC-1 with the 100 m Green Bank Telescope. We perform velocity stacking and matched-filter analyses using Markov chain Monte Carlo simulations and find evidence for the presence of this molecule at the 5.1
σ
level. We derive a total column density of
3.8
−
0.9
+
1.0
×
10
10
cm
−2
, which is predominantly found toward two of the four velocity components we observe toward TMC-1. We use this molecule as a proxy for constraining the gas-phase abundance of the apolar hydrocarbon 1,3-butadiene. Based on the three-phase astrochemical modeling code
NAUTILUS
and an expanded chemical network, our model underestimates the abundance of cyano-1,3-butadiene by a factor of 19, with a peak column density of 2.34 × 10
10
cm
−2
for 1,3-butadiene. Compared to the modeling results obtained in previous GOTHAM analyses, the abundance of 1,3-butadiene is increased by about two orders of magnitude. Despite this increase, the modeled abundances of aromatic species do not appear to change and remain underestimated by one to four orders of magnitude. Meanwhile, the abundances of the five-membered ring molecules increase proportionally with 1,3-butadiene by two orders of magnitude. We discuss the implications for bottom-up formation routes to aromatic and polycyclic aromatic molecules.
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