2022ApJ...941..102S

The formation of molecules in the interstellar medium (ISM) remains a complex and unresolved question in astrochemistry. A group of molecules of particular interest involves the linkage between a carboxyl and amine group, similar to that of a peptide bond. The detection of molecules containing these peptide-like bonds in the ISM can help elucidate possible formation mechanisms, as well as indicate the level of molecular complexity available within certain regions of the ISM. Two of the simplest molecules containing a peptide-like bond, formamide (NH₂CHO) and acetamide (CH₃CONH₂), have previously been detected toward the star-forming region Sagittarius B2 (Sgr B2). Recently, the interstellar detection of propionamide (C₂H₅CONH₂) was reported toward Sgr B2(N) with Atacama Large Millimeter/submillimeter Array (ALMA) observations at millimeter wavelengths. Yet, this detection has been questioned by others from the same set of ALMA observations as no statistically significant line emission was identified from any uncontaminated transitions. Using the Prebiotic Interstellar Molecule Survey (PRIMOS) observations, we report an additional search for C₂H₅CONH₂ at centimeter wavelengths conducted with the Green Bank Telescope. No spectral signatures of C₂H₅CONH₂ were detected. An upper limit for C₂H₅CONH₂ at centimeter wavelengths was determined to be N_T < 1.8 × 10¹⁴ cm^–2 and an upper limit to the C₂H₅CONH₂/CH₃CONH₂ ratio is found to be <2.34. This work again questions the initial detection of C₂H₅CONH₂ and indicates that more complex peptide-like structures may have difficulty forming in the ISM or are below the detection limits of current astronomical facilities. Additional structurally related species are provided to aid in future laboratory and astronomical searches.