# Formation of interstellar propanal and 1-propanol ice: a pathway involving solid-state CO hydrogenation

## TL;DR

This study demonstrates laboratory pathways for forming interstellar 1-propanol and propanal ice via solid-state CO hydrogenation, supporting their potential formation in cold dark interstellar cores and comparing results with astronomical observations.

## Contribution

It introduces a two-step laboratory synthesis pathway for interstellar 1-propanol involving CO hydrogenation and C2H2 addition, expanding understanding of complex organic molecule formation in space.

## Key findings

- Solid-state propanal and 1-propanol can form via radical recombination during CO hydrogenation.
- The 1-propanol:propanal ratio is constrained to less than 0.55, aligning with astronomical data.
- Reaction pathways involve radicals HCO, H2CCH, and H3CCH2, supported by experiments and calculations.

## Abstract

1-propanol (CH3CH2CH2OH) is a three carbon-bearing representative of primary linear alcohols that may have its origin in the cold dark cores in interstellar space. To test this, we investigated in the laboratory whether 1-propanol ice can be formed along pathways possibly relevant to the prestellar core phase. We aim to show in a two-step approach that 1-propanol can be formed through reaction steps that are expected to take place during the heavy CO freeze-out stage by adding C2H2 into the CO + H hydrogenation network via the formation of propanal (CH3CH2CHO) as an intermediate and its subsequent hydrogenation. Temperature programmed desorption-quadrupole mass spectrometry (TPD-QMS) is used to identify the newly formed propanal and 1-propanol. Reflection absorption infrared spectroscopy (RAIRS) is used as a complementary diagnostic tool. The mechanisms that can contribute to the formation of solid-state propanal and 1-propanol, as well as other organic compounds, during the heavy CO freeze-out stage are constrained by both laboratory experiments and theoretical calculations. Here it is shown that recombination of HCO radicals, formed upon CO hydrogenation, with radicals formed upon C2H2 processing - H2CCH and H3CCH2 - offers possible reaction pathways to solid-state propanal and 1-propanol formation. This extends the already important role of the CO hydrogenation chain in the formation of larger COMs (complex organic molecules). The results are used to compare with ALMA observations. The resulting 1-propanol:propanal ratio concludes an upper limit of < 0:35-0:55, which is complemented by computationally-derived activation barriers in addition to the experimental results.