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.

Infrared spectroscopy of the α-hydroxyethyl radical isolated in cryogenic solid media.

The α-hydroxyethyl radical (CH3·CHOH, 2A) is a key intermediate in ethanol biochemistry, combustion, atmospheric chemistry, radiation chemistry, and astrochemistry. Experimental data on the vibrational spectrum of this radical are crucially important for reliable detection and understanding of the chemical dynamics of this species. This study represents the first detailed experimental report on the infrared absorption bands of the α-hydroxyethyl radical complemented by ab initio computations. The radical was generated in solid para-H2 and Xe matrices via the reactions of hydrogen atoms with matrix-isolated ethanol molecules and radiolysis of isolated ethanol molecules with x rays. The absorption bands with maxima at 3654.6, 3052.1, 1425.7, 1247.9, 1195.6 (1177.4), and 1048.4 cm-1, observed in para-H2 matrices appearing upon the H· atom reaction, were attributed to the OHstr, α-CHstr, CCstr, COstr + CCObend, COstr, and CCstr + CCObend vibrational modes of the CH3·CHOH radical, respectively. The absorption bands with the positions slightly red-shifted from those observed in para-H2 were detected in both the irradiated and post-irradiation annealed Xe matrices containing C2H5OH. The results of the experiments with the isotopically substituted ethanol molecules (CH3CD2OH and CD3CD2OH) and the quantum-chemical computations at the UCCSD(T)/L2a_3 level support the assignment. The photolysis with ultraviolet light (240-300 nm) results in the decay of the α-hydroxyethyl radical, yielding acetaldehyde and its isomer, vinyl alcohol. A comparison of the experimental and theoretical results suggests that the radical adopts the thermodynamically more stable anti-conformation in both matrices.