A gas-phase "top-down" chemical link between aldehydes and alcohols

Alcohols and aldehydes represent two key classes of interstellar complex organic molecules (COMs). This work seeks to better understand their possible chemical connections, with a focus on such molecules in the sources of the star-forming region Sgr B2 (N). The gas-phase reaction between ethanol (CH3CH2OH) and the halogens fluorine and chlorine was investigated via DFT calculations, with the goal of determining whether astrochemically viable chemical pathways leading to acetaldehyde (CH3CHO) exist. The studied reactions were then included in an astrochemical model of Sgr B2 (N) to determine their significance under real interstellar conditions. Our DFT calculations revealed that both chlorine and fluorine can react barrierlessly with ethanol to abstract a hydrogen atom. We further found that, following this initial step, the resulting ethanol radicals can undergo further reactions with atomic hydrogen, with some routes leading to acetaldehyde. Incorporation of these novel reactions in astrochemical models of hot cores suggest that they are indeed efficient under those conditions, and can lead to modest increases in the abundance of CH3CHO during model times where gas-phase ethanol is abundant. Of the ethanol radicals included in our chemical network, we found CH3CHOH to have the highest abundances in our simulations comparable to that of ethanol at some model times. Overall, this work reveals a novel gas-phase top-down'' link from alcohols to aldehydes that compliments the better studied bottom-up’’ routes involving grain-surface H-addition reactions yielding alcohols from aldehydes. Moreover, results from our astrochemical models suggest that the ethanol radical CH3CHOH may be detectable in the interstellar medium.

💡 Research Summary

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This paper investigates a gas‑phase “top‑down” chemical link that converts alcohols into aldehydes, focusing on ethanol (CH₃CH₂OH) and its conversion to acetaldehyde (CH₃CHO) in the star‑forming region Sgr B2(N). The authors first explore the elementary reactions of ethanol with atomic fluorine (F) and chlorine (Cl) using high‑level quantum chemistry. Geometry optimizations are performed with the double‑hybrid functional rev‑DSD‑PBEP86(D4) and the cc‑pCVTZ basis set, followed by single‑point energy refinements at the CCSD(T)/cc‑pCVTZ level. Six possible hydrogen‑abstraction channels are examined, three for each halogen: abstraction from the methyl (α‑C) hydrogen, from the methylene (β‑C) hydrogen, and abstraction of the hydroxyl hydrogen leading to CH₃CHOH, CH₃CH₂O, or CH₂CH₂OH radicals plus HF or HCl.

A key finding is that the pathways leading to the ethanol radical CH₃CHOH are completely barrierless; nudged‑elastic‑band (NEB) calculations confirm a downhill energy profile with no transition‑state saddle point. The other channels possess modest barriers that are quantified. Rate constants for all channels are calculated using an ab‑initio transition‑state‑theory master‑equation (AITSTME) approach, incorporating capture theory for the pre‑reactive van‑der‑Waals complexes and RRKM theory with Eckart tunnelling for any submerged barriers. Long‑range interactions are modeled with r⁻⁶ potentials for entrance channels and r⁻⁵ for exit channels. The temperature‑dependent phenomenological rate constants are fitted to a three‑parameter Arrhenius‑Kooij expression (k = α (T/300)ᵝ exp(−γ/T)) over 30–500 K, providing ready‑to‑use parameters for astrochemical networks.

To assess astrophysical relevance, the authors embed the new reactions into the NAUTILUS v1.1 three‑phase chemical model, adopting physical conditions that mimic Sgr B2(N): a collapse phase (10³ → 10⁸ cm⁻³, 10 K), a warm‑up phase (10 → 400 K over ~10⁶ yr), and a static hot‑core phase up to 10⁷ yr. Cosmic‑ray ionization is set to ζ = 1.3 × 10⁻¹⁶ s⁻¹, reflecting the elevated Galactic‑Center environment. Initial elemental abundances include chlorine (2.9 × 10⁻⁷) and fluorine (3.6 × 10⁻⁸) relative to hydrogen, with additional runs using the lower values (10⁻⁷ and 1.8 × 10⁻⁸) to test sensitivity. The base network (Byrne et al. 2024) is expanded with the six abstraction reactions and a suite of destruction pathways for the three ethanol radicals (CH₃CHOH, CH₃CH₂O, CH₂CH₂OH) reacting with H, OH, and NH₂. All destruction reactions are assumed barrierless with a canonical collisional rate of 10⁻¹⁰ cm³ s⁻¹, split equally between aldehyde‑forming and carbon‑attack channels.

Model results show that during the warm‑up stage (≈150–250 K) the halogen‑induced abstraction efficiently populates the ethanol radicals, especially CH₃CHOH, whose gas‑phase abundance can become comparable to that of parent ethanol. Subsequent reactions of CH₃CHOH with atomic hydrogen dominate the formation of acetaldehyde via CH₃CHOH + H → CH₃CHO + H₂, while competing channels produce CH₄ + H₂CO. The inclusion of the new halogen‑driven pathways raises the steady‑state CH₃CHO abundance by a factor of 2–3 relative to models lacking them, bringing the simulated acetaldehyde column densities into better agreement with observations of Sgr B2(N). Sensitivity tests indicate that the results are robust against reasonable variations in the initial Cl and F abundances.

The authors argue that this “top‑down” route complements the well‑studied “bottom‑up” grain‑surface hydrogenation pathways (aldehyde → alcohol) and offers a plausible gas‑phase mechanism for aldehyde production in hot cores where halogen atoms are present. Moreover, the predicted high abundance of the CH₃CHOH radical suggests it could be detectable with current radio‑astronomy facilities, providing an observational test of the proposed chemistry. The paper also outlines possible extensions: experimental determination of the rate coefficients, spectroscopic characterization of the ethanol radicals, and exploration of analogous halogen‑mediated conversions for other interstellar alcohols (e.g., methanol, propanol).

In conclusion, the study demonstrates that (1) atomic fluorine and chlorine can abstract hydrogen from ethanol without an activation barrier, (2) the resulting ethanol radicals readily react with abundant interstellar radicals to form acetaldehyde, and (3) incorporating these reactions into realistic astrochemical models of Sgr B2(N) yields modest but significant enhancements in acetaldehyde abundances, supporting the viability of a gas‑phase “top‑down” alcohol‑to‑aldehyde pathway in star‑forming regions.