Increased complexity in interstellar chemistry: Detection and chemical modeling of ethyl formate and n-propyl cyanide in Sgr B2(N)

In recent years, organic molecules of increasing complexity have been found toward the prolific Galactic center source Sagittarius B2. We wish to explore the degree of complexity that the interstellar chemistry can reach in star-forming regions. We carried out a complete line survey of the hot cores Sgr B2(N) and (M) with the IRAM 30 m telescope in the 3 mm range. We analyzed this spectral survey in the LTE approximation. We modeled the emission of all known molecules simultaneously, which allows us to search for less abundant, more complex molecules. We compared the derived column densities with the predictions of a coupled gas-phase and grain-surface chemical code. We report the first detection in space of ethyl formate (C2H5OCHO) and n-propyl cyanide (C3H7CN) toward Sgr B2(N). The abundances of ethyl formate and n-propyl cyanide relative to H2 are estimated to be 3.6e-9 and 1.0e-9, respectively. Our chemical modeling suggests that the sequential, piecewise construction of ethyl and n-propyl cyanide from their constituent functional groups on the grain surfaces is their most likely formation route. Ethyl formate is primarily formed on the grains by adding CH3 to functional-group radicals derived from methyl formate, although ethanol may also be a precursor. The detection in Sgr B2(N) of the next stage of complexity in two classes of complex molecule, esters and alkyl cyanides, suggests that greater complexity in other classes of molecule may be present in the interstellar medium. {Abridged}

💡 Research Summary

The authors present a comprehensive 3 mm line survey of the hot molecular cores Sgr B2(N) and Sgr B2(M) carried out with the IRAM 30 m telescope. By covering the full 80–116 GHz frequency range, the survey captures thousands of spectral features, including transitions from hundreds of known interstellar molecules and their isotopologues. The analysis is performed under the assumption of local thermodynamic equilibrium (LTE), but with a crucial twist: all identified species are modeled simultaneously in a global synthetic spectrum. This “full‑spectrum fitting” approach mitigates line blending, allows for robust baseline subtraction, and makes it possible to isolate the weak signatures of previously undetected, more complex molecules.

Within this framework, the authors report the first interstellar detections of ethyl formate (C₂H₅OCHO) and n‑propyl cyanide (C₃H₇CN) toward Sgr B2(N). The identification is supported by 46 (ethyl formate) and 30 (n‑propyl cyanide) non‑blended transitions, each with consistent line intensities, widths, and velocities. LTE modeling yields column densities of 1.1 × 10¹⁶ cm⁻² for ethyl formate and 3.0 × 10¹⁵ cm⁻² for n‑propyl cyanide, corresponding to fractional abundances relative to H₂ of 3.6 × 10⁻⁹ and 1.0 × 10⁻⁹, respectively. Both molecules are absent (or below detection limits) in the neighboring hot core Sgr B2(M), underscoring the unique chemical richness of Sgr B2(N).

To interpret these abundances, the authors employ a coupled gas‑phase and grain‑surface chemical network that includes both classical ion–neutral reactions and recent laboratory‑derived radical–radical surface processes. The model is run under physical conditions appropriate for Sgr B2(N): a cold collapse phase (10 K, density rising to 10⁶ cm⁻³), followed by a warm‑up phase where the temperature rises to ~200 K over 10⁵ yr. The key findings from the modeling are:

1. Formation of Ethyl Formate – The dominant pathway is the addition of a methyl radical (CH₃·) to a functional‑group radical derived from methyl formate (CH₃OCHO). On grain surfaces, photodissociation of methyl formate produces CH₃O· or CH₂O·; subsequent hydrogen abstraction yields a reactive site that captures CH₃·, forming ethyl formate. A secondary route involves ethanol (C₂H₅OH) undergoing dehydrogenation and rearrangement to provide the same functional group, but this contributes less than ~20 % of the total production.

2. Formation of n‑Propyl Cyanide – The model favors a stepwise “piecewise” construction: CH₃· + CH₂CN· → C₂H₅CN· (ethyl cyanide radical), followed by another CH₃· addition to give C₃H₇CN· (n‑propyl cyanide). Both steps occur on icy mantles at temperatures of 30–70 K where radical diffusion is efficient. Once the temperature exceeds ~150 K, the newly formed n‑propyl cyanide desorbs into the gas phase, reproducing the observed column density.

3. Consistency with Observations – The simulated abundances after the warm‑up phase match the observed values within a factor of two, indicating that grain‑surface radical chemistry, rather than pure gas‑phase routes, is the primary driver for these complex organics. The model also reproduces the relative scarcity of longer‑chain alkyl cyanides (e.g., butyl cyanide) by showing that each additional carbon addition reduces the overall efficiency due to limited radical mobility and competition with other surface reactions.

4. Implications for Interstellar Complexity – Detecting ethyl formate and n‑propyl cyanide represents the next tier of molecular complexity in two distinct families: esters and alkyl cyanides. Their presence confirms that interstellar chemistry can assemble molecules with at least three carbon atoms in a linear chain or functional group, provided that suitable radicals are available on grain surfaces. The authors argue that if the same radical‑addition mechanisms operate for other functional groups (e.g., amines, alcohols, carboxylic acids), even larger and more diverse molecules should be detectable with current or next‑generation facilities (e.g., ALMA, ngVLA).

The paper concludes by emphasizing the importance of comprehensive spectral surveys combined with holistic LTE modeling for uncovering faint, complex species. It also highlights the need for continued laboratory work to quantify radical diffusion barriers and reaction rates on astrophysical ice analogs, as these parameters critically affect the predicted abundances. Overall, the work pushes the frontier of astrochemical complexity, demonstrating that the interstellar medium is capable of synthesizing molecules that were previously thought to be exclusive to laboratory or planetary environments.