Infrared spectral signatures of nucleobases in interstellar ices II: Pyrimidines

The identification of pyrimidine nucleobases in interstellar environments such as dense clouds and pre-stellar cores represents a much sought-after result in astrochemical and astrobiological research. Indeed, such an identification would allow for insights into the interstellar formation of these complex organic molecules and, possibly, a better understanding of their putative transfer to the early Earth where they may have contributed to the emergence of life. In this present work, we report the results of an extensive mid-infrared spectroscopic study of the solid pyrimidine nucleobases cytosine, thymine, and uracil under conditions representative of the dense interstellar medium to potentially facilitate their identification in such environments. Spectra of the neat crystalline nucleobases, as well as a mixture of all three, were acquired at temperatures of 300 and 20 K. Furthermore, the spectra of these nucleobases in the presence of interstellar ice analogues containing H₂O:NH₃:CH₄:CO:CH₃OH in a 10:1:1:1:1 ratio were also acquired to determine whether any nucleobase absorption features were evident in the presence of ubiquitous volatile interstellar molecular ices, and thus could be used as possible spectroscopic fingerprints for the identification of interstellar pyrimidines. Our present work has identified three mid-infrared absorption bands at 1512, 1237, and 758 cm^-1 (6.61, 8.08, and 13.19 μm) that could be useful in this regard. The potential utility of these absorption features in observational surveys of the dense interstellar medium, together with the limitations of our study and recommendations for future work, are discussed in detail.