7 research outputs found
IR Spectral Fingerprint of Carbon Monoxide in Interstellar Water Ice Models
Carbon monoxide (CO) is the second most abundant molecule in the gas-phase of
the interstellar medium. In dense molecular clouds, it is also present in the
solid-phase as a constituent of the mixed water-dominated ices covering dust
grains. Its presence in the solid-phase is inferred from its infrared (IR)
signals. In experimental observations of solid CO/water mixed samples, its IR
frequency splits into two components, giving rise to a blue- and a redshifted
band. However, in astronomical observations, the former has never been
observed. Several attempts have been carried out to explain this peculiar
behaviour, but the question still remains open. In this work, we resorted to
pure quantum mechanical simulations in order to shed some light on this
problem. We adopted different periodic models simulating the CO/HO ice
system, such as single and multiple CO adsorption on water ice surfaces, CO
entrapped into water cages and proper CO:HO mixed ices. We also simulated
pure solid CO. The detailed analysis of our data revealed how the quadrupolar
character of CO and the dispersive forces with water ice determine the
energetic of the CO/HO ice interaction, as well as the CO spectroscopic
behaviour. Our data suggest that the blueshifted peak can be assigned to CO
interacting {\it via} the C atom with dangling H atoms of the water ice, while
the redshifted one can actually be the result of CO involved in different
reciprocal interactions with the water matrix. We also provide a possible
explanation for the lack of the blueshifted peak in astronomical spectra. Our
aim is not to provide a full account of the various interstellar ices, but
rather to elucidate the sensitivity of the CO spectral features to different
water ice environments.Comment: MNRAS, accepte
Binding energies of interstellar molecules on crystalline and amorphous models of water ice by ab-initio calculations
In the denser and colder (20 K) regions of the interstellar medium
(ISM), near-infrared observations have revealed the presence of sub-micron
sized dust grains covered by several layers of H\textsubscript{2}O-dominated
ices and dirtied by the presence of other volatile species. Whether a molecule
is in the gas or solid-phase depends on its binding energy (BE) on ice
surfaces. Thus, BEs are crucial parameters for the astrochemical models that
aim to reproduce the observed evolution of the ISM chemistry. In general, BEs
can be inferred either from experimental techniques or by theoretical
computations. In this work, we present a reliable computational methodology to
evaluate the BEs of a large set (21) of astrochemical relevant species. We
considered different periodic surface models of both crystalline and amorphous
nature to mimic the interstellar water ice mantles. Both models ensure that
hydrogen bond cooperativity is fully taken into account at variance with the
small ice cluster models. Density functional theory adopting both B3LYP-D3 and
M06-2X functionals was used to predict the species/ice structure and their BE.
As expected from the complexity of the ice surfaces, we found that each
molecule can experience multiple BE values, which depend on its structure and
position at the ice surface. A comparison of our computed data with literature
data shows agreement in some cases and (large) differences in others. We
discuss some astrophysical implications that show the importance of calculating
BEs using more realistic interstellar ice surfaces to have reliable values for
inclusion in the astrochemical models.Comment: 30 pages (including Appendix), 16 figures (including Appendix). To be
published in Astrophysical Journa
Formamide adsorption at the amorphous silica surface : A combined experimental and computational approach
Mineral surfaces have been demonstrated to play a central role in prebiotic reactions, which are understood to be at the basis of the origin of life. Among the various molecules proposed as precursors for these reactions, one of the most interesting is formamide. Formamide has been shown to be a pluripotent molecule, generating a wide distribution of relevant prebiotic products. In particular, the outcomes of its reactivity are strongly related to the presence of mineral phases acting as catalysts toward specific reaction pathways. While the mineral-products relationship has been deeply studied for a large pool of materials, the fundamental description of formamide reactivity over mineral surfaces at a microscopic level is missing in the literature. In particular, a key step of formamide chemistry at surfaces is adsorption on available interaction sites. This report aims to investigate the adsorption of formamide over a well-defined amorphous silica, chosen as a model mineral surface. An experimental IR investigation of formamide adsorption was carried out and its outcomes were interpreted on the basis of first principles simulation of the process, adopting a realistic model of amorphous silica
Forsterite Surfaces as Models of Interstellar Core Dust Grains: Computational Study of Carbon Monoxide Adsorption
Carbon
monoxide (CO) is the second most abundant gas-phase molecule
after molecular hydrogen (H<sub>2</sub>) of the interstellar medium
(ISM). In molecular clouds, an important component of the ISM, it
adsorbs at the surface of core grains, usually made of Mg/Fe silicates,
and originates complex organic molecules through the catalytic power
of active sites at the grain surfaces. To understand the atomistic,
energetic, and spectroscopic details of the CO adsorption on core
grains, we resorted to density functional theory based on the hybrid
B3LYP-D* functional inclusive of dispersion contribution. We modeled
the complexity of interstellar silicate grains by studying adsorption
events on a large set of infinite extended surfaces cut out from the
bulk Mg<sub>2</sub>SiO<sub>4</sub> forsterite, the Mg end-member of
olivines (Mg<sub>2<i>x</i></sub>Fe<sub>2–2<i>x</i></sub>SiO<sub>4</sub>), also a very common mineral on the
Earth’s crust. Energetic and structural features indicate that
CO is exclusively physisorbed with binding energy values in the 23–68
kJ mol<sup>–1</sup> range. Detailed analysis of data revealed
that dispersive interactions are relevant together with the important
electrostatic contribution due to the quadrupolar nature of the CO
molecule. We performed a full thermodynamic treatment of the CO adsorption
at the very low temperature typical of the ISM as well as a full spectroscopic
characterization of the CO stretching frequency, which we prove to
be extremely sensitive to the local nature of the surface-active site
of adsorption. We also performed a detailed kinetic analysis of CO
desorption from the surface models at different temperatures characterizing
the colder regions of the ISM. Our computed data could be incorporated
in the various astrochemical models of interstellar grains developed
so far and thus contribute to improve the description of the complex
chemical network occurring at their surfaces
Formamide adsorption at the amorphous silica surface : A combined experimental and computational approach
Mineral surfaces have been demonstrated to play a central role in prebiotic reactions, which are understood to be at the basis of the origin of life. Among the various molecules proposed as precursors for these reactions, one of the most interesting is formamide. Formamide has been shown to be a pluripotent molecule, generating a wide distribution of relevant prebiotic products. In particular, the outcomes of its reactivity are strongly related to the presence of mineral phases acting as catalysts toward specific reaction pathways. While the mineral-products relationship has been deeply studied for a large pool of materials, the fundamental description of formamide reactivity over mineral surfaces at a microscopic level is missing in the literature. In particular, a key step of formamide chemistry at surfaces is adsorption on available interaction sites. This report aims to investigate the adsorption of formamide over a well-defined amorphous silica, chosen as a model mineral surface. An experimental IR investigation of formamide adsorption was carried out and its outcomes were interpreted on the basis of first principles simulation of the process, adopting a realistic model of amorphous silica