Formation of S-Bearing Complex Organic Molecules in Interstellar Clouds via Ice Reactions with C2H2, HS, and Atomic H

Abstract

The chemical network governing interstellar sulfur has been the topic of unrelenting discussion for the past few decades due to the conspicuous discrepancy between its expected and observed abundances in different interstellar environments. More recently, the astronomical detections of CH₃CH₂SH and CH₂CS highlighted the importance of interstellar formation routes for sulfur-bearing organic molecules with two carbon atoms. In this work, we perform a laboratory investigation of the solid-state chemistry resulting from the interaction between C₂H₂ molecules and SH radicals-both thought to be present in interstellar icy mantles-at 10 K. Reflection absorption infrared spectroscopy and quadrupole mass spectrometry combined with temperature-programmed desorption experiments are employed as analytical techniques. We confirm that SH radicals can kick-start a sulfur reaction network under interstellar cloud conditions and identify at least six sulfurated products: CH₃CH₂SH, CH₂CHSH, HSCH₂CH₂SH, H₂S₂, and tentatively CH₃CHS and CH₂CS. Complementarily, we utilize computational calculations to pinpoint the reaction routes that play a role in the chemical network behind our experimental results. The main sulfur-bearing organic molecule formed under our experimental conditions is CH₃CH₂SH, and its formation yield increases with the ratios of H to other reactants. It serves as a sink to the sulfur budget within the network, being formed at the expense of the other unsaturated products. The astrophysical implications of the chemical network proposed here are discussed.

Figure 1

Relevant QMS signals recorded during a TPD experiment after codepositon of C₂H₂:H₂S:H (1:1:20) at 10 K for 6 h. The dotted lines mark the desorption peak of each band, and their assignments are denoted in gray. The asterisks indicate tentative identification.

Figure 2

Representative structures of the six S-bearing products identified in the experiments with C₂H₂ + H₂S + H. These are provided for visualization purposes and may not depict the accurate minimum-energy geometries of the species formed in the experiments.

Figure 3

Assignment of the TPD-QMS peaks at ∼117 and ∼121 K as vinyl mercaptan (CH₂CHSH) and ethanethiol (CH₃CH₂SH), respectively. The deposition is performed with a flux ratio of C₂H₂:H₂S:H = 1:5:10. (a) QMS signals showing two desorption peaks highlighted by the dotted gray lines. (b) Mass fragmentation pattern of the selected signals for the peak at ∼117 K corrected for the sensitivity of the QMS. The standard for CH₂CHSH is shown for comparison. (c) Same as (b) but for the peak at ∼121 K in comparison with CH₃CH₂SH (standard measured in this work).

Figure 4

Infrared spectra utilized to assign CH₃CH₂SH and CH₂CHSH. (a) Spectrum recorded after codeposition of a C₂H₂ + H₂S + H ice (1:1:20; red), together with a C₂H₂ + H (1:10; gray) blank experiment. (b) Difference spectrum acquired during TPD between 100 and 140 K (red) for the C₂H₂ + H₂S + H codeposition, together with a CH₃CH₂SH standard spectrum (green). All depositions were performed at 10 K, and the spectra are offset for clarity.

Figure 5

Assignment of the TPD-QMS peak at ∼131 K as disulfane (H₂S₂) after a codeposition experiment with a flux ratio of C₂H₂:H₂S:H = 1:1:20 (experiment 1). (a) QMS signals of the relevant fragments. (b) Mass fragmentation pattern corrected for the sensitivity of the QMS in comparison with the standard for H₂S₂ from Santos et al. (c) Infrared spectrum at 10 K (black) and difference spectrum between 120 and 150 K (gray).

Figure 6

Assignment of the TPD-QMS peak at ∼158 K as 1,2-ethanedithiol (HSCH₂CH₂SH) after a codeposition experiment with a flux ratio of C₂H₂:H₂S:H = 1:1:20 (experiment 1). (a) QMS signals of the relevant fragments with m/z ≤ 61. (b) Signal for m/z = 94 measured during the same experiment. The dotted gray line indicates the peak desorption temperature. (c) Mass fragmentation pattern of the detected signals for the peak at ∼158 K corrected for the sensitivity of the QMS in comparison with the standard for HSCH₂CH₂SH from the NIST. The larger error bar of m/z = 94 is a consequence of the significantly lower sensitivity of the QMS at this mass-to-charge ratio.

Figure 7

Tentative assignments of the TPD-QMS peaks at ∼74 and ∼84 K as, respectively, thioketene (CH₂CS) and thioacetaldehyde (CH₃CHS). (a) QMS signals of the relevant fragments after a codeposition experiment with a flux ratio of C₂H₂:H₂S:H = 1:1:20 (experiment 1). The dotted gray lines indicates the peak desorption temperatures. (b) Mass fragmentation pattern of the detected signals for the peak at ∼84 K corrected for the sensitivity of the QMS (blue), assigned to CH₃CHS, in comparison to the standard fragmentation pattern of c-(CH₂)₂S from the NIST (gray). (c) QMS signals of the relevant fragments after a codeposition experiment with a flux ratio of C₂H₂:H₂S:H = 1:1:5 (experiment 3), in which the CH₃CHS production is minimized. The dotted line highlights the contribution from the desorption peak at ∼74 K to the signals of m/z = 58 and m/z = 45.

Figure 8

Chemical network explored in this work. Boxes denote closed-shell species with solid and dashed lines indicating confirmed and tentative detections, respectively. Open-shell species in boldface are formed barrierlessly. Reactions with SH radicals constrained in this work are indicated by orange arrows, gray arrows display potential reactions not investigated further in this work, and black arrows represent reactions with H atoms.The purple arrow highlights the speculated intermolecular isomerization process.

Figure 9

Relative abundances of the sulfur-bearing COMs formed during deposition as a function of the C₂H₂:H₂S:H ratio. The abundances are shown with respect to the total yield of sulfurated COMs positively identified at the end of each experiment.