Binding Energies of Interstellar Molecules on Crystalline and Amorphous Models of Water Ice by Ab Initio Calculations (original) (raw)
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Springer eBooks, 2022
The interstellar medium is extremely heterogeneous in terms of physical environments and chemical composition. Spectroscopic observations in the recent decades have revealed the presence of gaseous material and dust grains covered in ices predominantly of water in interstellar clouds, the interplay of which may elucidate the existence of more than 250 molecular species. Of these species of varied complexity, several terrestrial carbon-containing compounds have been discovered, known as interstellar complex organic molecules (iCOMs) in the astrochemical argot. In order to investigate the formation of iCOMs, it is crucial to explore gas-grain chemistry and in this regard, one of the fundamental parameters is the binding energy (BE), which is an essential input in astrochemical models. In this work, the BEs of 13 iCOMs on a crystalline H 2 O-ice surface have been computed by means of quantum chemical periodic calculations. The hybrid B3LYP-D3 DFT method was used for the geometry optimizations of the adsorbate/ice systems and for computing the BEs. Furthermore, to refine the BE values, an ONIOM2-like approximation has been employed to obtain them at CCSD(T), which correlate well with those obtained at B3LYP-D3. Additionally, aiming to lower the computational cost, structural optimizations were carried out using the HF-3c level of theory, followed by single point energy calculations at B3LYP-D3 in order to obtain BE values comparable to the full DFT treatment.
Computational Science and Its Applications – ICCSA 2021, 2021
There are different environments in the interstellar medium (ISM), depending on the density, temperature and chemical composition. Among them, molecular clouds, often referred to as the cradle of stars, are paradigmatic environments relative to the chemical diversity and complexity in space. Indeed, there, radio to far-infrared observations revealed the presence of several molecules in the gas phase, while nearinfrared spectroscopy detected the existence of submicron sized dust grains covered by H 2 O-dominated ice mantles. The interaction between gas-phase species and the surfaces of water ices is measured by the binding energy (BE), a crucial parameter in astrochemical modelling. In this work, the BEs of a set of sulphur-containing species on water ice mantles have been computed by adopting a periodic ab initio approach using a crystalline surface model. The Density Functional Theory (DFT)-based B3LYP-D3(BJ) functional was used for the prediction of the structures and energetics. DFT BEs were refined by adopting an ONIOM-like procedure to estimate them at CCSD(T) level toward complete basis set extrapolation, in which a very good correlation between values has been found. Moreover, we show that geometry optimization with the computationally cheaper HF-3c method followed by single point energy calculations at DFT to compute the BEs is a suitable cost-effective recipe to arrive at BE values of the same quality as those computed at full DFT level. Finally, computed data were compared with the available literature data.
The Astrophysical Journal
Binding energies (BEs) are one of the most important parameters for astrochemical modeling determining, because they govern whether a species stays in the gas phase or is frozen on the grain surfaces. It is currently known that, in the denser and colder regions of the interstellar medium, sulfur is severely depleted in the gas phase. It has been suggested that it may be locked into the grain icy mantles. However, which are the main sulfur carriers is still a matter of debate. This work aims to establish accurate BEs of 17 sulfur-containing species on two validated water ice structural models, the proton-ordered crystalline (010) surface and an amorphous water ice surface. We adopted density functional theory-based methods (the hybrid B3LYP-D3(BJ) and the hybrid meta-GGA M06-2X functionals) to predict structures and energetics of the adsorption complexes. London’s dispersion interactions are shown to be crucial for an accurate estimate of the BEs due to the presence of the high polar...
Minerals
The universe is molecularly rich, comprising from the simplest molecule (H2) to complex organic molecules (e.g., CH3CHO and NH2CHO), some of which of biological relevance (e.g., amino acids). This chemical richness is intimately linked to the different physical phases forming Solar-like planetary systems, in which at each phase, molecules of increasing complexity form. Interestingly, synthesis of some of these compounds only takes place in the presence of interstellar (IS) grains, i.e., solid-state sub-micron sized particles consisting of naked dust of silicates or carbonaceous materials that can be covered by water-dominated ice mantles. Surfaces of IS grains exhibit particular characteristics that allow the occurrence of pivotal chemical reactions, such as the presence of binding/catalytic sites and the capability to dissipate energy excesses through the grain phonons. The present know-how on the physicochemical features of IS grains has been obtained by the fruitful synergy of as...
Astronomy and Astrophysics, 1998
Ethane, acetylene, ethanol, hydrazine and hydrogen peroxyde are either predicted by theoretical models to be abundant in the icy mantles on grains in dense clouds, or were found to be produced by comets. We present measurements of the spectra of these species embedded in astrophysically relevant ice matrices. Additionally, we obtained the spectrum of the hydrozonium ion (N 2 H + 5) which could be produced by activationless acid-base reactions. The laboratory results are compared to the ISO and ground-based spectra of NGC 7538:IRS 9. Strict upper-limits compared to the solid water could be found for ethane, ethanol and hydrogen peroxyde of < 0.4%, < 1.2%, and < 6.1%, respectively. These results give some important information on the relationship between cometary and interstellar ices and on the nature of grain surface reactions.
Molecular Simulation, 2018
In this paper, we present a brief review of what has been learned about the adsorption characteristics of various organic molecules at the surface of ice, from more than 15 years of computer simulation studies at the molecular scale. In particular, grand canonical Monte Carlo and molecular dynamics calculations were performed to determine the adsorption isotherms, the saturation coverage of the first molecular layer at the ice surface, the preferred orientations of the molecules in their adsorption sites, and the corresponding adsorption energies. The results of the simulations indicated that the main driving force for trace gas adsorption on ice is hydrogen bonding not only between the adsorbate and the water molecules of the ice surface, but also within the adsorbate. When possible, the comparison with available experimental data showed a close agreement, supporting thus the methodology used in the modelling. Finally, the present review demonstrates how computer simulation can nicely complement experimental approaches for studying interactions between trace gases and ice under tropospheric and interstellar condition
Spectroscopy and processing of interstellar ice analogs
AIP Conference Proceedings, 2006
Recent results from the Raymond and Beverly Sackler Laboratory for Astrophysics on spectroscopy and processing of interstellar ice analogues are summarized. This includes thermal desorption studies of pure, layered and mixed CO, N2 and O2 ices, and infrared spectroscopy and heating of CO-CO2, CO-H2O, CO-HCOOH, CO-CH4 and CO-CH3OH layered and mixed ices. Laboratory data of CO-surface adsorbates show good agreement with the unidentified 2175 cm-1 interstellar feature. Complementary ab initio quantum chemical calculations and molecular dynamics simulations have been performed to provide insight into the gas-grain interactions and interstellar ice processing. This includes the first molecular dynamics study of the photodissociation of water ice and the corresponding photodesorption efficiencies. The relevance of these data in the analysis of astronomical data is emphasized throughout.
An Approach to Estimate the Binding Energy of Interstellar Species
The Astrophysical Journal Supplement Series, 2018
One of the major obstacles to accurately model the interstellar chemistry is an inadequate knowledge about the binding energy (BE) of interstellar species with dust grains. In denser region of molecular cloud, where very complex chemistry is active, interstellar dust is predominantly covered by H 2 O molecules and thus it is essential to know the interaction of gas phase species with water ice to trace realistic physical and chemical processes. To this effect, we consider water (cluster) ice to calculate the BE of several atoms, molecules, and radicals of astrochemical interest. Systematic studies have been carried out to come up with a relatively more accurate BE of astrophysically relevant species on water ice. We increase the size of the water cluster methodically to capture the realistic situation. Sequentially one, three, four, five and six water molecules are considered to represent water ice analogue in increasing order of complexity. We notice that for most of the species considered here, as we increase the cluster size, our calculated BE value starts to converge towards the experimentally obtained value. More specifically, our computed results with water c-pentamer (average deviation from experiment ∼ ±15.8%) and c-hexamer (chair) (average deviation from experiment ∼ ±16.7%) configuration are found to be more nearer as the experimentally obtained value than other water clusters considered.
2013
Interstellar ices are layers of molecules deposited on ne dust grains in dark and dense molecular cloud cores. Subsurface ice has been considered in a few astrochemical models, which have shown that it can be of great importance. The aim of this work is to introduce an established subsurface ice description into the state-of-the-art astrochemical model ALCHEMIC. The model has been developed by the Heidelberg astrochemistry group. The result is an up-to-date model for interstellar molecular cloud research with possible application for protoplanetary disks.
Binding Energy of Molecules on Water Ice: Laboratory Measurements and Modeling
The Astrophysical Journal, 2016
We measured the binding energy of N 2 , CO, O 2 , CH 4 , and CO 2 on non-porous (compact) amorphous solid water (np-ASW), of N 2 and CO on porous amorphous solid water (p-ASW), and of NH 3 on crystalline water ice. We were able to measure binding energies down to a fraction of 1% of a layer, thus making these measurements more appropriate for astrochemistry than the existing values. We found that CO 2 forms clusters on np-ASW surface even at very low coverages. The binding energies of N 2 , CO, O 2 , and CH 4 decrease with coverage in the submonolayer regime. Their values at the low coverage limit are much higher than what is commonly used in gas-grain models. An empirical formula was used to describe the coverage dependence of the binding energies. We used the newly determined binding energy distributions in a simulation of gas-grain chemistry for cold cloud and hot core models. We found that owing to the higher value of desorption energy in the sub-monlayer regime a fraction of all these ices stays much longer and up to higher temperature on the grain surface compared to the single value energies currently used in the astrochemical models.