Context.
Methoxymethanol (CH
3
OCH
2
OH) has been identified through gas-phase signatures in both high- and low-mass star-forming regions. Like several other C-, O-, and H-containing complex organic ...molecules (COMs), this molecule is expected to form upon hydrogen addition and abstraction reactions in CO-rich ice through radical recombination of CO hydrogenation products.
Aims.
The goal of this work is to experimentally and theoretically investigate the most likely solid-state methoxymethanol reaction channel – the recombination of CH
2
OH and CH
3
O 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. We also investigate an alternative hydrogenation channel starting from methyl formate.
Methods.
Hydrogen atoms and CO or H
2
CO molecules were co-deposited on top of predeposited H
2
O ice to mimic the conditions associated with the beginning of “rapid” CO freeze-out. The formation of simple species was monitored in situ using infrared spectroscopy. Quadrupole mass spectrometry was used to analyze the gas-phase COM composition following a temperature-programmed desorption. Monte Carlo simulations were used for an astrochemical model comparing the methoxymethanol formation efficiency with that of other COMs.
Results.
The laboratory identification of methoxymethanol is found to be challenging, in part because of diagnostic limitations, but possibly also because of low formation efficiencies. Nevertheless, unambiguous detection of newly formed methoxymethanol has been possible in both CO+H and H
2
CO+H experiments. The resulting abundance of methoxymethanol with respect to CH
3
OH is about 0.05, which is about six times lower than the value observed toward NGC 6334I and about three times lower than the value reported for IRAS 16293B. Astrochemical simulations predict a similar value for the methoxymethanol abundance with respect to CH
3
OH, with values ranging between 0.03 and 0.06.
Conclusions.
We find that methoxymethanol is formed by co-deposition of CO and H
2
CO with H atoms through the recombination of CH
2
OH and CH
3
O radicals. In both the experimental and modeling studies, it is found that the efficiency of this channel alone is not sufficient to explain the observed abundance of methoxymethanol with respect to methanol. The rate of a proposed alternative channel, the direct hydrogenation of methyl formate, is found to be even less efficient. These results suggest that our knowledge of the reaction network is incomplete or involving alternative solid-state or gas-phase formation mechanisms.
Density functional theory (DFT) has provided deep atomic-level insights into the adsorption behavior of aromatic molecules on solid surfaces. However, modeling the surface phenomena of large ...molecules on mineral surfaces with accurate plane wave methods (PW) can be orders of magnitude more computationally expensive than localized atomic orbitals (LCAO) methods. In the present work, we propose a less costly approach based on the DFT-D4 method (PBE-D4), using LCAO, to study the interactions of aromatic molecules with the {010} forsterite (Mg2SiO4) surface for their relevance in astrochemistry. We studied the interaction of benzene with the pristine {010} forsterite surface and with transition-metal cations (Fe2+ and Ni2+) using PBE-D4 and a vdW-inclusive density functional (Dion, Rydberg, Schröder, Langreth, and Lundqvist (DRSLL)) with LCAO methods. PBE-D4 shows good agreement with coupled-cluster methods (CCSD(T)) for the binding energy trend of cation complexes and with PW methods for the binding energy of benzene on the forsterite surface with a difference of about 0.03 eV. The basis set superposition error (BSSE) correction is shown to be essential to ensure a correct estimation of the binding energies even when large basis sets are employed for single-point calculations of the optimized structures with smaller basis sets. We also studied the interaction of naphthalene and benzocoronene on pristine and transition-metal-doped {010} forsterite surfaces as a test case for PBE-D4. Yielding results that are in good agreement with the plane wave methods with a difference of about 0.02–0.17 eV, the PBE-D4 method is demonstrated to be effective in unraveling the binding structures and the energetic trends of aromatic molecules on pristine and transition-metal-doped forsterite mineral surfaces. Furthermore, PBE-D4 results are in good agreement with its predecessor PBE-D3(BJM) and with the vdW-inclusive density functionals, as long as transition metals are not involved. Hence, PBE-D4/CP-DZP has been proven to be a robust theory level to study the interaction of aromatic molecules on mineral surfaces.
Formation of Acetaldehyde on CO-Rich Ices Lamberts, Thanja; Markmeyer, Max N; Kolb, Florian J ...
ACS earth and space chemistry,
06/2019, Volume:
3, Issue:
6
Journal Article
Peer reviewed
Open access
The radicals HCO and CH3 on carbon monoxide ice surfaces were simulated using the 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 CH3. If they are located on the surface close to each other (3–4 Å), molecular dynamics calculations based on the density functional theory show that they can form acetaldehyde (CH3CHO) or CH4 + CO in barrier-less reactions, depending upon the initial orientation of the molecules with respect to each other. In some orientations, no spontaneous reactions were found and 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, such as acetaldehyde, can be formed by recombination reactions of radicals on CO surfaces.
We quantitatively investigated the hydrogen addition reactions of acetylene (C{sub 2}H{sub 2}) and ethylene (C{sub 2}H{sub 4}) on amorphous solid water (ASW) at 10 and 20 K relevant to the formation ...of ethane (C{sub 2}H{sub 6}) on interstellar icy grains. We found that the ASW surface enhances the reaction rates for C{sub 2}H{sub 2} and C{sub 2}H{sub 4} by approximately a factor of 2 compared to those on the pure-solid C{sub 2}H{sub 2} and C{sub 2}H{sub 4} at 10 K, probably due to an increase in the sticking coefficient and adsorption energy of the H atoms on ASW. In contrast to the previous proposal that the hydrogenation rate of C{sub 2}H{sub 4} is orders of magnitude larger than that of C{sub 2}H{sub 2}, the present results show that the difference in hydrogenation rates of C{sub 2}H{sub 2} and C{sub 2}H{sub 4} is only within a factor of 3 on both the surfaces of pure solids and ASW. In addition, we found the small kinetic isotope effect for hydrogenation/deuteration of C{sub 2}H{sub 2} and C{sub 2}H{sub 4} at 10 K, despite the requirement of quantum tunneling. At 20 K, the reaction rate of deuteration becomes even larger than that of hydrogenation. These unusual isotope effects might originate from a slightly larger number density of D atoms than H atoms on ASW at 20 K. The hydrogenation of C{sub 2}H{sub 2} is four times faster than CO hydrogenation and can produce C{sub 2}H{sub 6} efficiently through C{sub 2}H{sub 4} even in the environment of a dark molecular cloud.
Abstract
We quantitatively investigated the hydrogen addition reactions of acetylene (C
2
H
2
) and ethylene (C
2
H
4
) on amorphous solid water (ASW) at 10 and 20 K relevant to the formation of ...ethane (C
2
H
6
) on interstellar icy grains. We found that the ASW surface enhances the reaction rates for C
2
H
2
and C
2
H
4
by approximately a factor of 2 compared to those on the pure-solid C
2
H
2
and C
2
H
4
at 10 K, probably due to an increase in the sticking coefficient and adsorption energy of the H atoms on ASW. In contrast to the previous proposal that the hydrogenation rate of C
2
H
4
is orders of magnitude larger than that of C
2
H
2
, the present results show that the difference in hydrogenation rates of C
2
H
2
and C
2
H
4
is only within a factor of 3 on both the surfaces of pure solids and ASW. In addition, we found the small kinetic isotope effect for hydrogenation/deuteration of C
2
H
2
and C
2
H
4
at 10 K, despite the requirement of quantum tunneling. At 20 K, the reaction rate of deuteration becomes even larger than that of hydrogenation. These unusual isotope effects might originate from a slightly larger number density of D atoms than H atoms on ASW at 20 K. The hydrogenation of C
2
H
2
is four times faster than CO hydrogenation and can produce C
2
H
6
efficiently through C
2
H
4
even in the environment of a dark molecular cloud.
Understanding how to catalytically break the C–H bond of aromatic molecules, such as polycyclic aromatic hydrocarbons (PAHs), is currently a big challenge and a subject of study in catalysis, ...astrochemistry, and planetary science. In the latter, the study of the breakdown reaction of PAHs on mineral surfaces is important to understand if PAHs are linked to prebiotic molecules in regions of star and planet formation. In this work, we employed a periodic density functional theory along with Grimme’s D4 (DFT-D4) approach for studying the adsorption of a sample of PAHs (naphthalene, anthracene, fluoranthene, pyrene, coronene, and benzocoronene) and fullerene on the 010 forsterite surface and its defective surfaces (Fe-doped and Ni-doped surfaces and a MgO-Schottky vacancy) for their implications in catalysis and astrochemistry. On the basis of structural and binding energy analysis, large PAHs and fullerene present stronger adsorption on the pristine, Fe-doped, and Ni-doped forsterite surfaces than small PAHs. On a MgO-Schottky vacancy, parallel adsorption of the PAH leads to the chemisorption process (C–Si and/or C–O bonds), whereas perpendicular orientation of the PAH leads to the catalytic breaking of the aromatic C–H bond via a barrierless reaction. Spin density and charge analysis show that C–H dissociation is promoted by electron donation from the vacancy to the PAH. As a result of the undercoordinated Si and O atoms, the vacancy acts as a Frustrated Lewis Pair (FLP) catalyst. Therefore, a MgO-Schottky vacancy 010 forsterite surface proved to have potential catalytic activity for the activation of C–H bond in aromatic molecules.
In the interstellar medium, six molecules have been conclusively detected in the solid state in interstellar ices, and a few dozen have been hypothesized and modeled to be present in the solid state ...as well. The icy mantles covering micrometer-sized dust grains are, in fact, thought to be at the core of complex molecule formation as a consequence of the local high density of molecules that are simultaneously adsorbed. From a structural perspective, the icy mantle is considered to be layered, with an amorphous water-rich inner layer surrounding the dust grain, covered by an amorphous CO-rich outer layer. Moreover, recent studies have suggested that the CO-rich layer might be crystalline and possibly even be segregated as a single crystal atop the ice mantle. If so, there are far-reaching consequences for the formation of more complex organic molecules, such as methanol and sugars, that use CO as a backbone. Validation of these claims requires further investigation, in particular on acquiring atomistic insight into surface processes, such as adsorption, diffusion, and reactivity on CO ices. Here, we present the first detailed computational study toward treating the weak interaction of (pure) CO ices. We provide a benchmark of the performance of various density functional theory methods in treating the binding of pure CO ices. Furthermore, we perform an atomistic and in-depth study of the binding energy of CO on amorphous and crystalline CO ices using a pair-potential-based force field. We find that CO adsorption is represented by a large distribution of binding energies (200–1600 K) on amorphous CO, including a significant amount of weak binding sites (<350 K). Increasing both the cluster size and the number of neighbors increases the mean of the observed binding energy distribution. Finally, we find that CO binding energies are dominated by dispersion and, as such, exchange-correlation functionals need to include a treatment of dispersion to accurately simulate surface processes on CO ices. In particular, we find the ωB97M-V functional to be a strong candidate for such simulations.
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