Identification of Glycolaldehyde Enol (HOHC═CHOH) in Interstellar Analogue Ices

Formation of methylglyoxal (CH3C(O)CHO) in interstellar analog ices - a key intermediate in cellular metabolism

Ketoaldehydes are key intermediates in biochemical processes including carbohydrate, lipid, and amino acid metabolism. Despite their crucial role in the interstellar synthesis of essential biomolecules necessary for the Origins of Life, their formation mechanisms have largely remained elusive. Here, we report the first bottom-up formation of methylglyoxal (CH3C(O)CHO)-the simplest ketoaldehyde-through the barrierless recombination of the formyl (HĊO) radical with the acetyl (CH3ĊO) radical in low-temperature interstellar ice analogs upon exposure to energetic irradiation as proxies of galactic cosmic rays. Utilizing vacuum ultraviolet photoionization reflectron time-of-flight mass spectrometry and isotopic substitution studies, methylglyoxal and its enol tautomer 2-hydroxypropenone (CH3C(OH)CO) were identified in the gas phase during the temperature-programmed desorption of irradiated carbon monoxide-acetaldehyde (CO-CH3CHO) ices, suggesting their potential as promising candidates for future astronomical searches. Once synthesized in cold molecular clouds, methylglyoxal can serve as a key precursor to sugars, sugar acids, and amino acids. Furthermore, this work provides the first experimental evidence for tautomerization of a ketoaldehyde in interstellar ice analogs, advancing our fundamental knowledge of how ketoaldehydes and their enol tautomers can be synthesized in deep space.

Tracing Prebiotic Molecules: Rotational Spectroscopy of Deuterated Glycolaldehyde and (Z)-1,2-Ethenediol

Glycolaldehyde, an important prebiotic molecule, along with its monodeuterated species and its higher energy tautomer, (Z)-1,2-ethenediol, has been detected in the interstellar medium. Although the elemental D/H ratio in the universe is only ∼1.6 × 10-5, the deuterium relative abundance in interstellar molecules might be by far larger than this. As such, it provides a remarkable and almost unique diagnostic tool. In particular, it might help elucidate the reaction mechanisms that lead to the formation of the so-called complex organic molecules. It is therefore crucial to extend the census of the interstellar deuterated molecules. To this aim, in this work, we present for the first time a spectroscopic investigation of the rotational spectra of the CHDOD-CHO bideuterated variant of glycolaldehyde and of mono- and bideuterated species of (Z)-1,2-ethenediol (CHOD═CHOD, CHOD═CHOH, and CHOH═CHOD rotamers). For each species, more than a hundred transitions have been assigned. Their analysis led to the accurate determination of all rotational constants as well as quartic and sextic centrifugal distortion terms, thus providing spectroscopic line catalogs suitable for supporting astronomical searches. In addition, the rotational constants of the bideuterated glycolaldehyde isotopologue studied in this work allowed us to improve the semiexperimental equilibrium structure determination for this molecule.

Interstellar formation of glyceric acid [HOCH2CH(OH)COOH]-The simplest sugar acid

Glyceric acid [HOCH2CH(OH)COOH]-the simplest sugar acid-represents a key molecule in biochemical processes vital for metabolism in living organisms such as glycolysis. Although critically linked to the origins of life and identified in carbonaceous meteorites with abundances comparable to amino acids, the underlying mechanisms of its formation have remained elusive. Here, we report the very first abiotic synthesis of racemic glyceric acid via the barrierless radical-radical reaction of the hydroxycarbonyl radical (HOĊO) with 1,2-dihydroxyethyl (HOĊHCH2OH) radical in low-temperature carbon dioxide (CO2) and ethylene glycol (HOCH2CH2OH) ices. Using isomer-selective vacuum ultraviolet photoionization reflectron time-of-flight mass spectrometry, glyceric acid was identified in the gas phase based on the adiabatic ionization energies and isotopic substitution studies. This work reveals the key reaction pathways for glyceric acid synthesis through nonequilibrium reactions from ubiquitous precursor molecules, advancing our fundamental knowledge of the formation pathways of key biorelevant organics-sugar acids-in deep space.

Identification and Photochemistry of the Mercaptomethyl Radical

The mercaptomethyl radical (·CH2SH) is a higher-energy isomer of the methylthio radical (CH3S·) that has been proposed as an important intermediate in atmospheric and interstellar sulfur chemistry. Herein, we report the spectroscopic identification of ·CH2SH during the UV (365 nm) photolysis of CH3S· in a solid Ar-matrix at 10 K. Upon subsequent irradiation at 266 nm, the dehydrogenation of ·CH2SH to yield CS via the intermediacy of the elusive thioformyl radical (HCS·) has also been observed. The characterization of ·CH2SH and HCS· with matrix-isolation IR and UV-vis spectroscopy is supported by 13C-isotope labeling and quantum chemical calculations at the CCSD(T)-F12a/cc-pVTZ-F12 level using configuration-selective vibrational configuration interaction theory (VCI). The disclosed photochemistry of ·CH2SH provides new insight into understanding the chemical evolution of organosulfur molecules in the interstellar medium (ISM).

Efficacious degradation of ethylene glycol by ultraviolet activated persulphate: reaction kinetics, transformation mechanisms, energy demand, and toxicity assessment

Ethylene glycol or 1,2-ethanediol (EG) is a persistent and toxic substance in the environment and extensively applied in petrochemical, surfactants, antifreeze, asphalt emulsion paints, cosmetics, plastics, and polyester fiber industries. Degradation of EG by using ultraviolet (UV) activated hydrogen peroxide (H2O2) and persulfate (PS) or persulfate anion (S2O82-) based advanced oxidation processes (AOPs) were explored. The result obtained demonstrate that UV/PS (85.7 ± 2.5%) has exhibited improved degradation efficiency of EG as compared to UV/H2O2 (40.4 ± 3.2%) at optimal operating conditions of 24 mM of EG concentration, 5 mM of H2O2, 5 mM of PS, 1.02 mW cm-2 of UV fluence, and pH of 7.0. Impacts of operating factors, including initial EG concentration, oxidant dosage, reaction duration, and the impact of different water quality parameters, were also explored in this present investigation. The degradation of EG in Milli-Q® water followed pseudo - first order reaction kinetics in both methods having a rate constant of about 0.070 min-1 and 0.243 min-1 for UV/H2O2 and UV/PS, respectively, at optimum operating conditions. Additionally, an economic assessment was also conducted under optimal experimental conditions, and the electrical energy per order and total operational cost for treating per m3 of EG-laden wastewater was observed to be about 0.042 kWh m-3 order-1 and 0.221 $ m-3 order-1, respectively, for UV/PS, which was slightly lower than UV/H2O2 (0.146 kWh m-3 order-1; 0.233 $ m-3 order-1). The potential degradation mechanisms were proposed based on intermediate by-products detected by Fourier transform infrared (FTIR) spectroscopy and gas chromatography-mass spectroscopy (GC-MS). Moreover, real petrochemical effluent containing EG was also treated by UV/PS, demonstrating 74.7 ± 3.8% of EG and 40.7 ± 2.6% of total organic carbon removal at 5 mM of PS and 1.02 mW cm-2 of UV fluence. A toxicity tests on Escherichia coli (E. coli) and Vigna radiata (green gram) confirmed non-toxic nature of UV/PS treated water.

Prebiotic Synthesis and Isomerization in Interstellar Analog Ice: Glycinal, Acetamide, and Their Enol Tautomers

Glycinal (HCOCH₂ NH₂ ) and acetamide (CH₃ CONH₂ ) are simple molecular building blocks of biomolecules in prebiotic chemistry, though their origin on early Earth and formation in interstellar media remain a mystery. These molecules are formed with their tautomers in low temperature interstellar model ices upon interaction with simulated galactic cosmic rays. Glycinal and acetamide are accessed via barrierless radical-radical reactions of vinoxy (⋅CH₂ CHO) and acetyl (⋅C(O)CH₃ ), and then undergo keto-enol tautomerization. Exploiting tunable photoionization reflectron time-of-flight mass spectroscopy and photoionization efficiency (PIE) curves, these results demonstrate fundamental reaction pathways for the formation of complex organics through non-equilibrium ice reactions in cold molecular cloud environments. These molecules demonstrate an unconventional starting point for abiotic synthesis of organics relevant to contemporary biomolecules like polypeptides and cell membranes in deep space.

Gas-phase detection of oxirene

Oxirenes-highly strained 4π Hückel antiaromatic organics-have been recognized as key reactive intermediates in the Wolff rearrangement and in interstellar environments. Predicting short lifetimes and tendency toward ring opening, oxirenes are one of the most mysterious classes of organic transients, with the isolation of oxirene (c-C₂H₂O) having remained elusive. Here, we report on the preparation of oxirene in low-temperature methanol-acetaldehyde matrices upon energetic processing through isomerization of ketene (H₂CCO) followed by resonant energy transfer of the internal energy of oxirene to the vibrational modes (hydroxyl stretching and bending, methyl deformation) of methanol. Oxirene was detected upon sublimation in the gas phase exploiting soft photoionization coupled with a reflectron time-of-flight mass spectrometry. These findings advance our fundamental understanding of the chemical bonding and stability of cyclic, strained molecules and afford a versatile strategy for the synthesis of highly ring-strained transients in extreme environments.

Matrix isolation and photorearrangement of cis- and trans-1,2-ethenediol to glycolaldehyde

1,2-Ethenediols are deemed key intermediates in prebiotic and interstellar syntheses of carbohydrates. Here we present the gas-phase synthesis of these enediols, the high-energy tautomers of glycolaldehyde, trapped in cryogenic argon matrices. Importantly, upon photolysis at λ = 180-254 nm, the enols rearrange to the simplest sugar glycolaldehyde.

Mechanistical study on the formation of hydroxyacetone (CH3COCH2OH), methyl acetate (CH3COOCH3), and 3-hydroxypropanal (HCOCH2CH2OH) along with their enol tautomers (prop-1-ene-1,2-diol (CH3C(OH)CHOH), prop-2-ene-1,2-diol (CH2C(OH)CH2OH), 1-methoxyethen-1-ol (CH3OC(OH)CH2) and prop-1-ene-1,3-diol (HOCH2CHCHOH)) in interstellar ice analogs

We unravel, for the very first time, the formation pathways of hydroxyacetone (CH₃COCH₂OH), methyl acetate (CH₃COOCH₃), and 3-hydroxypropanal (HCOCH₂CH₂OH), as well as their enol tautomers within mixed ices of methanol (CH₃OH) and acetaldehyde (CH₃CHO) analogous to interstellar ices in the ISM exposed to ionizing radiation at ultralow temperatures of 5 K. Exploiting photoionization reflectron time-of-flight mass spectrometry (PI-ReToF-MS) and isotopically labeled ices, the reaction products were selectively photoionized allowing for isomer discrimination during the temperature-programmed desorption phase. Based on the distinct mass-to-charge ratios and ionization energies of the identified species, we reveal the formation pathways of hydroxyacetone (CH₃COCH₂OH), methyl acetate (CH₃COOCH₃), and 3-hydroxypropanal (HCOCH₂CH₂OH) via radical-radical recombination reactions and of their enol tautomers (prop-1-ene-1,2-diol (CH₃C(OH)CHOH), prop-2-ene-1,2-diol (CH₂C(OH)CH₂OH), 1-methoxyethen-1-ol (CH₃OC(OH)CH₂) and prop-1-ene-1,3-diol (HOCH₂CHCHOH)) via keto-enol tautomerization. To the best of our knowledge, 1-methoxyethen-1-ol (CH₃OC(OH)CH₂) and prop-1-ene-1,3-diol (HOCH₂CHCHOH) are experimentally identified for the first time. Our findings help to constrain the formation mechanism of hydroxyacetone and methyl acetate detected within star-forming regions and suggest that the hitherto astronomically unobserved isomer 3-hydroxypropanal and its enol tautomers represent promising candidates for future astronomical searches. These enol tautomers may contribute to the molecular synthesis of biologically relevant molecules in deep space due to their nucleophilic character and high reactivity.

Exploitation of Synchrotron Radiation Photoionization Mass Spectrometry in the Analysis of Complex Organics in Interstellar Model Ices

Unravelling the generation of complex organic molecules (COMs) on interstellar nanoparticles (grains) is essential in establishing predictive astrochemical reaction networks and recognizing evolution stages of molecular clouds and star-forming regions. The formation of COMs has been associated with the irradiation of interstellar ices by ultraviolet photons and galactic cosmic rays. Herein, we pioneer the first incorporation of synchrotron vacuum ultraviolet photoionization reflectron time-of-flight mass spectrometry (SVUV-PI-ReTOF-MS) in laboratory astrophysics simulation experiments to afford an isomer-selective identification of key COMs (ketene (H₂C═CO); acetaldehyde (CH₃CHO); vinyl alcohol (H₂C═CHOH)) based on photoionization efficiency (PIE) curves of molecules desorbing from exposed carbon monoxide-methane (CO-CH₄) ices. Our results demonstrate that the SVUV-PI-ReTOF-MS approach represents a versatile, rapid methodology for a comprehensive identification and explicit understanding of the complex organics produced in space simulation experiments. This methodology is expected to significantly improve the predictive nature of astrochemical models of complex organic molecules formed abiotically in deep space, including biorelated species linked to the origins-of-life topic.

Photolytic insertion of carbon monoxide into nitrosyl chloride: formation of nitrosoformyl chloride

Nitrosocarbonyls are exotic intermediates that remain scarcely characterized. By UV photolysis (365 nm) of nitrosyl chloride (ClNO) embedded in solid CO ice at 20 K, the elusive nitrosoformyl chloride (ClC(O)NO) has been synthesized via CO-insertion into the Cl-N bond in ClNO. The characterization of ClC(O)NO with matrix-isolation IR spectroscopy is supported by ¹³C and ¹⁵N isotope labeling and quantum chemical calculations at the B3LYP/6-311+G(3df) level of theory. Upon subsequent laser irradiation at 266 nm, CO-elimination in ClC(O)NO occurs by reformation of ClNO. In line with the calculated potential energy surface for ClC(O)NO at the CCSD(T)-F12a/aug-cc-pVTZ//B3LYP/6-311+G(3df) level, the observed IR frequencies and the corresponding isotopic shifts coincide with the calculated values for the lowest-energy planar conformer, in which the CO and NO moities adopt trans configuration with respect to the C-N bond. Furthermore, the CO-insertion in ClNO involves a stepwise pathway by first homolytic cleavage of the Cl-N bond in ClNO (→ Cl˙ + ˙NO), followed by successive CO-trapping (CO + Cl˙ → ClCO˙) and radical combination (ClCO˙ + ˙NO → ClC(O)NO) inside the solid CO-matrix cages.

Gas-phase identification of (Z)-1,2-ethenediol, a key prebiotic intermediate in the formose reaction

Prebiotic sugars are thought to be formed on primitive Earth by the formose reaction. However, their formation is not fully understood and it is plausible that key intermediates could have formed in extraterrestrial environments and subsequently delivered on early Earth by cometary bodies. 1,2-Ethenediol, the enol form of glycolaldehyde, represents a highly reactive intermediate of the formose reaction and is likely detectable in the interstellar medium. Here, we report the identification and first characterization of (Z)-1,2-ethenediol by means of rotational spectroscopy. The title compound has been produced in the gas phase by flash vacuum pyrolysis of bis-exo-5-norbornene-2,3-diol at 750 °C, through a retro-Diels-Alder reaction. The spectral analysis was guided by high-level quantum-chemical calculations, which predicted spectroscopic parameters in very good agreement with the experiment. Our study provides accurate spectral data to be used for searches of (Z)-1,2-ethenediol in the interstellar space.

Bottom-Up Synthesis of 1,1-Ethenediol (H2CC(OH)2)─The Simplest Unsaturated Geminal Diol─In Interstellar Analogue Ices

Because of their nucleophilic character and high reactivity, enols─reaction intermediates carrying a hydroxyl group connected to a carbon-carbon double bond─play a key role in the formation of complex organic molecules in astrobiology and biochemistry. Here, we report the first bottom-up preparation of 1,1-ethenediol (H₂CC(OH)₂)─the simplest unsaturated geminal enol of acetic acid (CH₃COOH) and potential precursor for the formation of glycine─in interstellar analogue ices of carbon dioxide and methane processed by proxies of galactic cosmic rays. These enols can easily form via nonequilibrium chemistry in low temperature (10 K) interstellar ices at abundances orders of magnitude higher than thermodynamically predicted. These energetically less favorable tautomers remain stable in ice-coated interstellar nanoparticles in molecular clouds and also upon sublimation into the gas phase in star forming regions thus providing the raw material to a complex and exotic organic chemistry under extreme conditions in deep space.