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.
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 (RAIRS) and Temperature Programmed Desorption-Quadrupole Mass Spectrometry (TPD-QMS) 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 conclusions. Additional H2CO + D experiments confirm these conclusions.
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.
Simple and complex organic molecules (COMs) are observed along different phases of star and planet formation and have been successfully identified in prestellar environments such as dark and ...translucent clouds. Yet the picture of organic molecule formation at those earliest stages of star formation is not complete and an important reason is the lack of specific laboratory experiments that simulate carbon atom addition reactions on icy surfaces of interstellar grains. Here we present experiments in which CO molecules as well as C- and H-atoms are co-deposited with H\(_2\)O molecules on a 10 K surface mimicking the ongoing formation of an "H\(_2\)O-rich" ice mantle. To simulate the effect of impacting C-atoms and resulting surface reactions with ice components, a specialized C-atom beam source is used, implemented on SURFRESIDE\(^3\), an UHV cryogenic setup. Formation of ketene (CH\(_2\)CO) in the solid state is observed "in situ" by means of reflection absorption IR spectroscopy. C$^1$$^8\(O and D isotope labelled experiments are performed to further validate the formation of ketene. Data analysis supports that CH\)_2\(CO is formed through C-atom addition to a CO-molecule, followed by successive hydrogenation transferring the formed :CCO into ketene. Efficient formation of ketene is in line with the absence of an activation barrier in C+CO reaction reported in the literature. We also discuss and provide experimental evidence for the formation of acetaldehyde (CH\)_3\(CHO) and possible formation of ethanol (CH\)_3\(CH\)_2\(OH), two COM derivatives of CH\)_2$CO hydrogenation. The underlying reaction network is presented and the astrochemical implications of the derived pathways are discussed.
The radicals HCO and CH\(_3\) on carbon monoxide ice surfaces were simulated using density functional theory. Their binding energy on amorphous CO ice shows broad distributions, with approximative ...average values of 500 K for HCO and 200 K for CH\(_3\). If they are located on the surface close to each other (3 to 4 \AA), molecular dynamics calculations based on density functional theory show that they can form acetaldehyde (CH\(_3\)CHO) or CH\(_4\) + CO in barrier-less reactions, depending on the initial orientation of the molecules with respect to each other. In some orientations, no spontaneous reactions were found, the products remained bound to the surface. Sufficient configurational sampling, inclusion of the vibrational zero point energy, and a thorough benchmark of the applied electronic structure method are important to predict reliable binding energies for such weakly interacting systems. From these results it is clear that complex organic molecules, like acetaldehyde, can be formed by recombination reactions of radicals on CO surfaces.
The hydrogen abstraction reaction between H and H\(_2\)S, yielding HS and H\(_2\) as products, has been studied within the framework of interstellar surface chemistry. High-temperature rate constants ...up to 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. Strictly speaking, 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 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 only affected to a small extent by the presence of H\(_2\)O or H\(_2\)S molecular clusters that simulate an ice surface, calculated at the MPWB1K/def2-TZVP level of theory.
Methoxymethanol (CH3OCH2OH, MM) has been identified through gas-phase signatures in both high- and low-mass star-forming regions. This molecule is expected to form upon hydrogen addition and ...abstraction reactions in CO-rich ice through radical recombination of CO hydrogenation products. The goal of this work is to investigate experimentally and theoretically the most likely solid-state MM reaction channel -- the recombination of CH2OH and CH3O radicals -- for dark interstellar cloud conditions and to compare the formation efficiency with that of other species that were shown to form along the CO-hydrogenation line. Hydrogen atoms and CO or H2CO molecules are co-deposited on top of the predeposited H2O ice to mimic the conditions associated with the beginning of 'rapid' CO freeze-out. Quadrupole mass spectrometry is used to analyze the gas-phase COM composition following a temperature programmed desorption. Monte Carlo simulations are used for an astrochemical model comparing the MM formation efficiency with that of other COMs. Unambiguous detection of newly formed MM has been possible both in CO+H and H2CO+H experiments. The resulting abundance of MM with respect to CH3OH is about 0.05, which is about 6 times less than the value observed toward NGC 6334I and about 3 times less than the value reported for IRAS 16293B. The results of astrochemical simulations predict a similar value for the MM abundance with respect to CH3OH factors ranging between 0.06 to 0.03. We find that MM is formed by co-deposition of CO and H2CO with H atoms through the recombination of CH2OH and CH3O radicals. In both the experimental and modeling studies, the efficiency of this channel alone is not sufficient to explain the observed abundance of MM. These results indicate an incomplete knowledge of the reaction network or the presence of alternative solid-state or gas-phase formation mechanisms.
Methane is typically thought to be formed in the solid state on the surface of cold interstellar icy grain mantles via the successive atomic hydrogenation of a carbon atom. In the current work we ...investigate the potential role of molecular hydrogen in the CH\(_4\) reaction network. We make use of an ultra-high vacuum cryogenic setup combining an atomic carbon atom beam and both atomic and/or molecular beams of hydrogen and deuterium on a H\(_2\)O ice. These experiments lead to the formation of methane isotopologues detected in situ through reflection absorption infrared spectroscopy. Most notably, CH\(_4\) is formed in an experiment combining C atoms with H\(_2\) on amorphous solid water, albeit slower than in experiments with H atoms present. Furthermore, CH\(_2\)D\(_2\) is detected in an experiment of 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 chemical insights. This leads to the following conclusions: a) the reaction C + H\(_2\) -> CH\(_2\) can take place, although not barrierless in the presence of 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.
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.
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 GOTHAM deep spectral ...survey toward TMC-1
with the 100 m Green Bank Telescope. We performed 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$\sigma$ level. We derive a total
column density of $3.8^{+1.0}_{-0.9}\times 10^{10}$ 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 $2.34 \times 10^{10}\ \mathrm{cm}^{-2}$ 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 1--4 orders of magnitude.
Meanwhile, the abundances of the five-membered ring molecules increase
proportionally with 1,3-butadiene by two orders of magnitudes. We discuss
implications for bottom-up formation routes to aromatic and polycyclic aromatic
molecules.
