Formation of interstellar complex organic molecules on water-rich ices triggered by atomic carbon freezing (2310.14831v1)

Abstract: The reactivity of interstellar carbon atoms (C) on the water-dominated ices is one of the possible ways to form interstellar complex organic molecules (iCOMs). In this work, we report a quantum chemical study of the coupling reaction of C ($^3$P) with an icy water molecule, alongside possible subsequent reactions with the most abundant closed shell frozen species (NH$_3$, CO, CO$_2$ and H$_2$), atoms (H, N and O), and molecular radicals (OH, NH$_2$ and CH$_3$). We found that C spontaneously reacts with the water molecule, resulting in the formation of $^3$C-OH$_2$, a highly reactive species due to its triplet electronic state. While reactions with the closed-shell species do not show any reactivity, reactions with N and O form CN and CO, respectively, the latter ending up into methanol upon subsequent hydrogenation. The reactions with OH, CH$_3$ and NH$_2$ form methanediol, ethanol and methanimine, respectively, upon subsequent hydrogenation. We also propose an explanation for methane formation, observed in experiments through H additions to C in the presence of ices. The astrochemical implications of this work are: i) atomic C on water ice is locked into $^3$C-OH$_2$, making difficult the reactivity of bare C atoms on the icy surfaces, contrary to what is assumed in astrochemical current models; and ii) the extraordinary reactivity of $^3$C-OH$_2$ provides new routes towards the formation of iCOMs in a non-energetic way, in particular ethanol, mother of other iCOMs once in the gas-phase.

• The paper demonstrates that atomic carbon spontaneously reacts with water ice to form a reactive triplet intermediate without an energy barrier.
• The paper employs quantum mechanical calculations to elucidate reaction mechanisms leading to iCOMs such as methanol, methanediol, and ethanol.
• The paper reveals that while open-shell species readily react with the intermediate, closed-shell molecules remain inert, refining astrochemical models.

Formation of Interstellar Complex Organic Molecules on Water-Rich Ices

The paper by Ferrero et al. investigates the formation of interstellar complex organic molecules (iCOMs) facilitated by the presence of atomic carbon on water-rich ices, a phenomenon that substantially contributes to the chemical complexity of the interstellar medium (ISM). By employing quantum mechanical calculations, the researchers present a detailed analysis of carbon atom (C) reactivity on the surfaces of interstellar icy grains, culminating in the formation of various chemical compounds.

Key Findings

The fundamental assumption underlying this paper is the reactivity of atomic carbon within interstellar ice environments. The authors focus on the initial step of carbon reacting with a water molecule to form a highly reactive $^3$C-OH$_2$ species. This species, existing in a triplet state, serves as an activated complex and exhibits an affinity for further reactions with diverse interstellar species, including atoms, molecular radicals, and closed-shell molecules.

1. Spontaneous Formation of $^3$C-OH$_2$:
   • Atomic carbon spontaneously reacts with water molecules in the ice, resulting in the $^3$C-OH$_2$ formation. This reaction occurs without an energy barrier, rendering it efficient even at the low temperatures typical of interstellar environments.
2. Reactivity with Closed-Shell Molecules:
   • Contrary to the expectation in many astrochemical models, the $^3$C-OH$_2$ species does not react with closed-shell molecules such as NH$_3$, CO, CO$_2$, and H$_2$. This finding suggests that common closed-shell species will not directly alter this reactive center once formed.
3. Enhanced Reactivity with Atoms and Radicals:
   • The paper found substantive interactions of $^3$C-OH$_2$ with open-shell species like H, N, O, OH, CH$_3$, and NH$_2$. Reactions with N and O yield CN and CO, respectively, both important components in the chemical network of interstellar environments.
   • Successive hydrogenation of $^3$C-OH$_2$ results in compounds such as methanol (CH$_3$OH), methanediol (HOCH$_2$OH), and ethanol (CH$_3$CH$_2$OH).
4. Mechanistic Insights and Reaction Pathways:
   • The paper highlights a water-assisted proton transfer mechanism critical to several reaction pathways that culminate in the formation of more complex organic molecules. These reactions often occur without any significant energy barriers, facilitated by the presence and orientation of water molecules in the ice.

Implications and Theoretical Considerations

The research underscores the critical role of atomic carbon and its derivatives on icy grains in forming complex organic molecules in the ISM. The $^3$C-OH$_2$ species acts as an effective mediator and reactive center, unlocking pathways to the synthesis of iCOMs, thus contributing to the overall molecular complexity seen in space.

1. Theoretical Impact:
   • The paper challenges prevailing models by demonstrating that atomic carbon becomes chemically inactive without spontaneously reacting with water to form $^3$C-OH$_2$. This insight necessitates a recalibration of models predicting carbon atom behavior on icy grains.
2. Practical Implications for Astrochemistry:
   • The spontaneous formation of $^3$C-OH$_2$ indicates that certain iCOMs can form in a non-energetic manner, even in environments dominated by extremely low temperatures, such as dense molecular clouds and PDRs.
   • The paper proposes a mechanistic reconciliation of experimental results showing methane (CH$_4$) formation in the presence of icy C and H, attributed to the pathways involving $^3$C-OH$_2$.

Speculation and Future Directions

While this research elucidates the role of atomic carbon in icy environments, further exploration into the kinetics of these reactions, particularly accounting for quantum tunneling effects, can provide deeper insights. Additionally, exploring the interactions and reactivity of $^3$C-OH$_2$ with other potential abundant interstellar radicals and atoms under varying conditions can broaden understanding and improve models' predictive power.

By expanding the understanding of carbon's role in these environments, this work paves the path for more complex simulations, potentially influencing future observational strategies with telescopes such as the James Webb Space Telescope in detecting and identifying these compounds in space.