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.

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.

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.

A Photoionization Study on the Detection of 1-Sila Glycolaldehyde (HSiOCH2 OH), 2-Sila Acetic Acid (H3 SiCOOH), and 1,2-Disila Acetaldehyde (HSiOSiH3 ).

The identification of silicon-substituted, complex organics carrying multiple functional groups by classical infrared spectroscopy is challenging because the group frequencies of functional groups often overlap. Photoionization (PI) reflectron time-of-fight mass spectrometry (ReTOF-MS) in combination with temperature-programmed desorption (TPD) holds certain advantages because molecules are identified after sublimation from the matrix into in the gas phase based on distinct ionization energies and sublimation temperatures. In this study, we reveal the detection of 1-silaglycolaldehyde (HSiOCH2 OH), 2-sila-acetic acid (H3 SiCOOH), and 1,2-disila-acetaldehyde (H3 SiSiHO)-the silicon analogues of the well-known glycolaldehyde (HCOCH2 OH), acetic acid (H3 CCOOH), and acetaldehyde (H3 CCHO), in the gas phase after preparation in silane (SiH4 )-carbon dioxide ices exposed to energetic electrons and subliming the neutral reaction products formed within the ices into the gas phase.

VIZSLA-Versatile Ice Zigzag Sublimation Setup for Laboratory Astrochemistry.

In this article, a new multi-functional high-vacuum astrophysical ice setup, VIZSLA (Versatile Ice Zigzag Sublimation Setup for Laboratory Astrochemistry), is introduced. The instrument allows for the investigation of astrophysical processes both in a low-temperature para-H2 matrix and in astrophysical analog ices. In the para-H2 matrix, the reaction of astrochemical molecules with H atoms and H+ ions can be studied effectively. For the investigation of astrophysical analog ices, the setup is equipped with various irradiation and particle sources: an electron gun for modeling cosmic rays, an H atom beam source, a microwave H atom lamp for generating H Lyman-α radiation, and a tunable (213-2800 nm) laser source. For analysis, an FT-IR (and a UV-visible) spectrometer and a quadrupole mass analyzer are available. The setup has two cryostats, offering novel features for analysis. Upon the so-called temperature-programmed desorption (TPD), the molecules, desorbing from the substrate of the first cryogenic head, can be mixed with Ar and can be deposited onto the substrate of the other cryogenic head. The efficiency of the redeposition was measured to be between 8% and 20% depending on the sample and the redeposition conditions. The well-resolved spectrum of the molecules isolated in an Ar matrix serves a unique opportunity to identify the desorbing products of a processed ice. Some examples are provided to show how the para-H2 matrix experiments and the TPD-matrix-isolation recondensation experiments can help understand astrophysically important chemical processes at low temperatures. It is also discussed how these experiments can complement the studies carried out by using similar astrophysical ice setups.

Odd-even alternations in helical propensity of a homologous series of hydrocarbons.

Odd and even homologues of some n-alkane-based systems are known to exhibit notably different trends in solid-state properties; a well-known illustration is the zigzag plot of their melting point versus chain length. Odd-even effects in the solid state often arise from intermolecular interactions that involve fully extended molecules. These effects have also been observed in less condensed phases, such as self-assembled monolayers; however, the origins of these effects in such systems can be difficult to determine. Here we combined NMR and computational analysis to show that all-syn contiguously methyl-substituted hydrocarbons, with chain lengths from C6 to C11, exhibit a dramatic odd-even effect in helical propensity. The even- and odd-numbered hydrocarbons populate regular and less-controlled helical conformations, respectively. This knowledge will guide the design of helical hydrocarbons as rigid scaffolds or as hydrophobic components in soft materials.

Revealing Long-Range Substituent Effects in the Laser-Induced Fluorescence and Dispersed Fluorescence Spectra of Jet-Cooled CHxF3-xCH2O (x = 1, 2, 3) Radicals.

The B̃-X̃ laser-induced fluorescence (LIF) and dispersed fluorescence (DF) spectra of the atmospherically important ß-monofluoro ethoxy (MFEO), ß,ß-difluoro ethoxy (DFEO), and ß,ß,ß-trifluoro ethoxy (TFEO) radicals were recorded with vibronic resolution under jet-cooled conditions. To simulate the spectra, Franck-Condon factors were obtained from quantum chemical computations carried out at the CAM-B3LYP/6-311++G(d,p) level of theory. The simulations reproduce well both the LIF and DF spectra. Both conformers (G and T) of MFEO and one (G) of the two conformers of DFEO contribute to the LIF spectrum. A comparison between the experimental and calculated spectra confirms the expected long-range field effects of the CHxF3-x group on electronic transition energies and bond strengths, especially in the excited electronic (B̃) state. Although TFEO has only one conformer, its LIF spectrum is highly congested, which is attributed to the interaction between CO stretch and the -CF3 internal rotation.

Selective conformational control by excitation of NH imino vibrational antennas.

An imino group was used for the first time as a vibrational antenna to manipulate molecular conformations. Imino-thiol isomers of thioacetamide were generated upon UV-irradiation of its amino-thione tautomer isolated in argon matrices at 11 K. Selective and reversible conformational isomerizations were induced by narrowband near-IR irradiation tuned at the frequencies of the 2ν(NH) first stretching overtone of each imino-thiol isomer. The conformational isomerization concerns the change in the orientation of a remote -SH group, while the orientation of the imino (C[double bond, length as m-dash]NH) group remains the same. Supported by quantum chemical anharmonic computations, this allowed for a reliable, isomer-selective vibrational assignment of the four imino-thiol isomers extending now over the full mid-IR and near-IR ranges. It was found that the experimental IR intensities of the 2ν(NH) first stretching overtones (computed 4-5 km mol-1) of the imino-thiol forms are comparable to those of the ν(NH) stretching fundamentals (computed 2-4 km mol-1). This is the first time such a phenomenon is reported for an imine molecule. The kinetics of conformational isomerization was monitored in situ, indicating that the irradiation-induced processes are significantly faster than the tunneling-driven spontaneous cis-trans rotamerization of the -SH group. Quantum yields for the rotamerizations of the -SH group resulting from the vibrational excitation of a remote -NH group were estimated and found to be comparable to those observed for matrix-isolated carboxylic acids and amino acids, where conformational changes of the -OH group were induced by the direct vibrational excitation of 2ν(OH) first stretching overtones.

S-H rotamerization via tunneling in a thiol form of thioacetamide.

Rotamerization of a hydroxyl (O-H) group by tunneling is well-known and has been extensively studied. On the other hand, similar tunneling processes for the thiol (S-H) group have not been reported yet. In this work, the imino-thiol forms of thioacetamide were studied in cryogenic matrices (Ar, Xe) after UV-irradiation of the common amino-thione form of the compound. Four different imino-thiol forms were generated, corresponding to the cis or trans thiol (C/T) conformers of the two imino isomers (syn and anti; s/a). Noteworthy, the syn-cis (sC) imino-thiol form was found to convert spontaneously to the syn-trans (sT) form (with a half-life of 80 min), in a process whose reaction rate is independent of the temperature (i.e., at 11 or 20 K). Such conformational transformation represents the first experimental observation of an S-H rotamerization occurring by tunneling. Computations based on the Wentzel-Kramers-Brillouin formalism predict a tunneling half-life for the S-H rotamerization of syn-imino sC to sT on the time scale of minutes, in agreement with the experimental observations.

Hydrogen Abstraction/Addition Tunneling Reactions Elucidate the Interstellar H2NCHO/HNCO Ratio and H2 Formation.

Formamide (H2NCHO) is the smallest molecule possessing the biologically important amide bond. Recent interstellar observations have shown a strong correlation between the abundance of formamide and isocyanic acid (HNCO), indicating that they are likely to be chemically related, but no experiment or theory explains this correlation satisfactorily. We performed H + H2NCHO reactions in a para-hydrogen quantum-solid matrix host and identified production of H2NCO and HNCO from hydrogen-abstraction reactions. We identified also D2NCO, DNCO, HDNCO, and HDNCHO from the reaction H + D2NCHO, indicating the presence of hydrogen-addition reactions of DNCO and HDNCO. From the observed temporal profiles of H2NCHO, H2NCO, HNCO, and their deuterium isotopologues, we showed that a dual-cycle consisting of hydrogen abstraction and hydrogen addition can satisfactorily explain the quasi-equilibrium between H2NCHO and HNCO and explain other previous experimental results. Furthermore, this mechanism also indicates that the catalytic formation of H2 from H atoms might occur in interstellar ice grains.

Directed Gas-Phase Formation of the Germaniumsilylene Butterfly Molecule (Ge(µ-H2)Si).

The hitherto elusive dibridged germaniumsilylene molecule (Ge(µ-H2)Si) has been formed for the first time via the bimolecular gas-phase reaction of ground-state germanium atoms (Ge) with silane (SiH4) under single-collision conditions. Merged with state-of-the-art electronic structure calculations, the reaction was found to proceed through initial formation of a van der Waals complex in the entrance channel, insertion of the germanium into a silicon-hydrogen bond, intersystem crossing from the triplet to the singlet surface, hydrogen migrations, and eventually elimination of molecular hydrogen via a tight exit transition state, leading to the germaniumsilylene "butterfly". This investigation provides an extraordinary peek at the largely unknown silicon-germanium chemistry on the molecular level and sheds light on the essential nonadiabatic reaction dynamics of germanium and silicon, which are quite distinct from those of the isovalent carbon system, thus offering crucial insights that reveal exotic chemistry and intriguing chemical bonding in the germanium-silicon system on the most fundamental, microscopic level.

Near-infrared in situ generation of the higher-energy trans conformer of tribromoacetic acid: Observation of a large-scale matrix-site changing mediated by conformational conversion.

The first observation of the higher-energy conformer of tribromoacetic acid (trans-TBAA) is reported. The conformer was produced in cryogenic matrices (Ar, Kr, and N2) by in situ selective narrowband near-infrared excitation of the lower-energy cis-TBAA conformer and characterized both structurally and vibrationally. The novel trans-TBAA conformer is shown to spontaneously decay to the most stable cis-TBAA form in all studied matrix media, by tunneling, and the measured decay rates in the different matrices were compared with those of the trans conformers of other carboxylic acids in similar experimental conditions. In the N2 matrix, where trans-TBAA establishes a specific stabilizing intermolecular interaction with the host N2 molecules via its OH group and is about 11 times more stable than in rare gas matrices, the effect of changing the irradiation wavenumber within the 2νOH absorption profile was investigated in detail. An interesting phenomenon of matrix-site changing mediated by conformational conversion was observed in the N2 matrix: vibrational excitation of cis-TBAA in the 2νOH wavenumber range predominantly converts the molecules located in a specific "matrix site" into trans-TBAA; then, relaxation (by tunneling) of the produced higher-energy conformer back to the cis form populates almost exclusively another "matrix site." The experimental studies received support from quantum chemistry calculations, which allowed a detailed characterization of the relevant regions of the potential energy surface of the molecule and the detailed assignment of the infrared spectra of the two conformers in the various matrices.

Photochemical Formation of Diazenecarbaldehyde (HNNCHO) and Diazenecarbothialdehyde (HNNCHS) in Low-Temperature Matrices.

The photochemical decomposition of 1,2,4-oxadiazole-3,5-diamine and 1,2,4-thiadiazole-3,5-diamine was investigated in low-temperature Ar and Kr matrixes at different wavelengths. The analysis of matrix-isolation infrared (MI-IR) spectra aided by high-level quantum chemical computations showed not only that these photochemical reactions yield [NH2, C, N, X] (X = O, S) isomers but also that the bands of a novel, formerly unobserved species were observed. The comparison of computed IR spectra of potential products with the observed spectra suggests that these species are the diazenecarbaldehyde (HNNCHO) and diazenecarbothialdehyde (HNNCHS). Neither of the reactive HNNCHO and HNNCHS molecules was observed experimentally before. Both molecules are identified in the matrix as a complex with the other photoproduct, NH2CN. Comparison of the present experiments with former photochemical experiments on 1,2,5-oxadiazole-3,4-diamine and 1,2,5-thiadiazole-3,4-diamine and the analysis of the rate of formation of the different photoproducts indicate that HNNCHO and HNNCHS are formed in a different reaction path than H2NNCX and H2NC(NX) (X = O, S), and not by photoisomerization from these latter products.

Experimental Evidence of Long-Range Intramolecular Vibrational Energy Redistribution through Eight Covalent Bonds: NIR Irradiation Induced Conformational Transformation of E-Glutaconic Acid.

Long-range intramolecular vibrational energy redistribution (IVR) driven conformational changes were investigated in a matrix-isolated open-chain, asymmetrical dicarboxylic acid, E-glutaconic acid. Although the analysis was challenging due to the presence of multiple backbone conformers and short lifetimes of the prepared higher energy cis conformers, it was shown that the selective excitation of the O-H stretching overtone of one of the carboxylic groups can induce the conformational change (trans to cis) of the other carboxylic group, located at the other end of the E-glutaconic acid molecule. This is a direct proof that the IVR process can act through eight covalent bonds in a flexible molecule before the excess energy completely dissipates into the matrix. The lifetime of the prepared higher energy conformers (averaged over the different backbones) was measured to be 12 s.

Photochemical generation of H2NCNX, H2NNCX, H2NC(NX) (X = O, S) in low-temperature matrices.

The [NH2, C, N, O] and the [NH2, C, N, S] molecular systems were investigated by computational and matrix-isolation spectroscopic methods. The determination of the equilibrium structures and relative energies by CCSD(T) method was followed by the computation of the harmonic and anharmonic vibrational wavenumbers, infrared intensities, relative Raman activities, and UV excitation energies. These computed data were used to assist the identification of products obtained by UV laser photolysis of 3,4-diaminofurazan and 3,4-diaminothiadiazole in low-temperature Ar and Kr matrices. It is shown that two open-chain H2NNCX and H2NCNX and one cyclic H2NC(NX) (X = O, S) isomers are generated in the case of both systems. Except for H2NNCO and H2NCNS, the present study reports the first generation and spectroscopic identification of these compounds.

Synthesis of the Smallest Member of the Silylketene Family: H3 SiC(H)=C=O.

Exploiting photoionization reflectron time-of-flight mass spectrometry (PI-ReTOF-MS) combined with electronic structure calculations, it is shown that the hitherto elusive silylketene molecule (H3 SiC(H)=C=O)-the isovalent counterpart of the well-known methylketene molecule-is forming via interaction of energetic electrons with low-temperature silane-carbon monoxide ices. In combination with the infrared spectroscopically detected triplet dicarbon monoxide reactant, electronic structure calculations suggest that dicarbon monoxide reacts with silane via a de facto insertion of the terminal carbon atom into a silicon-hydrogen single bond. This is followed by non-adiabatic reaction dynamics triggered by the heavy silicon atom intersystem crossing from the triplet to the singlet manifold, eventually leading to the formation of silylketene. The non-equilibrium nature of the elementary reactions within the exposed ices results in an exciting and novel chemistry which cannot be explored via traditional preparative chemistry. Since the replacement of hydrogen in silane can introduce side groups such as silyl or alkyl, the reaction of triplet dicarbon monoxide with silane represents the parent system for a previously disregarded reaction class revealing an elegant path to access the largely reactive group of silylketenes.

Formation of Higher Silanes in Low-Temperature Silane (SiH4) Ices.

A novel approach for the synthesis and identification of higher silanes (SinH2n+2, where n ≤ 19) is presented. Thin films of (d4-)silane deposited onto a cold surface were exposed under ultra-high-vacuum conditions to energetic electrons and sampled on line and in situ via infrared and ultraviolet-visible spectroscopy. Gas phase products released by fractional sublimation in the warm-up phase after the irradiation were probed via a reflectron time-of-flight mass spectrometer coupled with a tunable vacuum ultraviolet photon ionization source. The formation mechanisms of (higher) silanes were investigated by irradiating codeposited 1:1 silane (SiH4)/d4-silane (SiD4) ices, suggesting that both radical-radical recombination and radical insertion pathways contribute to the formation of disilane along with higher silanes up to nonadecasilane (Si19H40).

Identification of Serine Conformers by Matrix-Isolation IR Spectroscopy Aided by Near-Infrared Laser-Induced Conformational Change, 2D Correlation Analysis, and Quantum Mechanical Anharmonic Computations.

The conformers of α-serine were investigated by matrix-isolation IR spectroscopy combined with NIR laser irradiation. This method, aided by 2D correlation analysis, enabled unambiguously grouping the spectral lines to individual conformers. On the basis of comparison of at least nine experimentally observed vibrational transitions of each conformer with empirically scaled (SQM) and anharmonic (GVPT2) computed IR spectra, six conformers were identified. In addition, the presence of at least one more conformer in Ar matrix was proved, and a short-lived conformer with a half-life of (3.7 ± 0.5) × 10(3) s in N2 matrix was generated by NIR irradiation. The analysis of the NIR laser-induced conversions revealed that the excitation of the stretching overtone of both the side chain and the carboxylic OH groups can effectively promote conformational changes, but remarkably different paths were observed for the two kinds of excitations.