UV photolysis of thiourea and its N-methylated derivative in cryogenic matrices.

Exploration of the photolytic dynamics of sulfurous compounds is essential, eventually contributing not only to our comprehension of their fundamental organic chemistry but also shedding light on astrophysical implications. This study aims to investigate two astrochemically relevant sulfur-containing molecules, namely, thiourea (TU) and its N-methylated counterpart, N-methyl thiourea (NMTU), in cryogenic matrices. These molecules were deposited both in solid Ar and in a quantum host, specifically in solid para-H2 matrices, with the latter exhibiting unique properties. The deposited matrices were exposed to a series of UV laser irradiation at various wavelengths to investigate the decomposition paths of TU and NMTU. As a result of the UV photolysis, a plethora of degradation products could be observed in every case. Based on the presence of these product molecules, some considerations can be made regarding the decomposition mechanism of the parent molecules. The use of different matrices allowed for assessing their influence on the decay mechanism, while applying tunable laser light provided insights into the wavelength dependency of the processes.

Energetic processing of thioacetamide in cryogenic matrices.

There is an ongoing debate on the apparent depletion of sulfur in the interstellar medium (ISM) compared to its universal abundance; therefore, the investigation of sulfurous compounds at low temperatures is of utmost importance. This work aims to study thioacetamide, H3C-C(=S)-NH2, in low-temperature inert Ar and para-H2 matrices by IR spectroscopy. The samples have been exposed to various sources of irradiation, such as Lyman-α or laser UV photons as well as energetic electrons. Using different host materials enabled assessing the matrix's impact on precursor decomposition. The response of the molecule to different types of irradiation has also been evaluated. The existence of three main decomposition channels were deduced: formation of (i) CH3, CH4, and HNCS; (ii) H2S and H2C=C=NH; and (iii) NH3 and H2C=C=S. The H3C-CN and H3C-NC isomers of H2C=C=NH could also be identified. Secondary products such as HNC and HCN were also detected in the quantum solid para-H2 in contrast to the more rigid Ar matrix. The listed decomposition products have been observed in the ISM, with the exception of H2C=C=NH and H3C-NC. The results point to the potential sensitivity of the precursor molecule to energetic radiation in space environments. Finally, the findings of this work will serve as a foundation for future irradiation experiments using the astrochemically more relevant pure thioacetamide ice.

Potential Catalytic Role of Small Heterocycles in Interstellar H2 Formation: A Laboratory Astrochemistry Study on Furan and Its Hydrogenated Forms.

It is now well-accepted in astrochemistry that the formation of interstellar H2 is taking place on the surface of interstellar grains. It has also been suggested a long time ago that polyaromatic hydrocarbons (PAHs) can catalyze this process by subsequent H atom addition and H abstraction reactions. Recent quantum chemical computations suggested that small heterocycles can be better catalysts than PAHs. In this study, the reaction of H atoms with furan, 2,3- and 2,5-dihydrofurans, and tetrahydrofuran were studied in solid para-H2 at 3.1 K. The reactions were followed by Fourier transform infrared (FTIR) spectroscopy. By the analysis of spectra, 2-hydrofuran-3-yl, 3-hydrofuran-2-yl, 2,3,4-trihydrofuran-5-yl, and 2,3,5-trihydrofuran-4-yl radicals were identified among the products. The experiments revealed that all the possible H atom addition and H abstraction cycles connecting furan and tetrahydrofuran proceed effectively in both directions at a low temperature. This indicates the possible important role of small heterocycles in interstellar H2 formation. Furthermore, it also indicates that, in the case of H atom excess, a quasi-equilibrium exists between the c-C4HxO (x = 4-8) species, and the ratios of these species in an astrophysical object are determined by the rate of the different H atom addition and H abstraction reaction steps.

Non-energetic, Low-Temperature Formation of Cα-Glycyl Radical, a Potential Interstellar Precursor of Natural Amino Acids.

The reaction of H atoms with glycine was investigated at 3.1 K in para-H2, a quantum-solid host. The reaction was followed by IR spectroscopy, with the spectral analysis aided by quantum chemical computations. Comparison of the experimental IR spectrum with computed anharmonic frequencies and intensities proved that, regardless of the reactant glycine conformation, Cα-glycyl radical is formed in an H-atom-abstraction process with great selectivity. The product of the second H-atom abstraction, iminoacetic acid, was also observed in a smaller amount. The Cα-glycyl radical is sensitive to UV light and decomposes to iminoacetic acid and H atom upon 280 nm radiation. Since the reactive radical center is located on the Cα-atom, it is suggested that natural α-amino acids can be formed from glycine via the Cα-glycyl radical by non-energetic mechanisms in the solid phase of the interstellar medium.