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

TL;DR

This study investigates a gas-phase, top-down link between alcohols and aldehydes in the interstellar medium by examining halogen-initiated hydrogen abstractions from ethanol with F and Cl. Combining high-level quantum chemical calculations with astrochemical modeling for Sgr B2(N), the work shows barrierless pathways preferentially favoring CH3CHOH formation, which can subsequently yield acetaldehyde CH3CHO under realistic hot-core conditions. The results indicate modest yet detectable enhancements in CH3CHO and highlight CH3CHOH as a potential interstellar radical observable in appropriate environments. Overall, the findings complement grain-surface bottom-up routes and expand the repertoire of gas-phase mechanisms connecting alcohols and aldehydes in astrochemistry.

Abstract

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.

Figures (8)

• Figure 1: Capture rigid scans leading to PRCs or VDW complexes, see text. In the legend we show the value of the capture coefficient $C_{6}$. The scans are not ZPE corrected.
• Figure 2: Gas and grain temperature evolution profiles.
• Figure 3: Density evolution profile.
• Figure 4: Potential energy profiles for the three abstraction channels in the CH3CH2OH + F reaction. All energies are ZPVE corrected.
• Figure 5: Reaction rate constants for the H-abstraction reactions of CH3CH2OH with F (Top panel) and Chlorine (bottom panel)