Disproportionation of Nitric Oxide at a Surface-Bound Nickel Porphyrinoid

Uncommon metal oxidation states in porphyrinoid cofactors are responsible for the activity of many enzymes. The F430 and P450nor co-factors, with their reduced NiI - and FeIII -containing tetrapyrrolic cores, are prototypical examples of biological systems involved in methane formation and in the reduction of nitric oxide, respectively. Herein, using a comprehensive range of experimental and theoretical methods, we raise evidence that nickel tetraphenyl porphyrins deposited in vacuo on a copper surface are reactive towards nitric oxide disproportionation at room temperature. The interpretation of the measurements is far from being straightforward due to the high reactivity of the different nitrogen oxides species (eventually present in the residual gas background) and of the possible reaction intermediates. The picture is detailed in order to disentangle the challenging complexity of the system, where even a small fraction of contamination can change the scenario.

Disproportionation of Nitric Oxide at a Surface-Bound Nickel Porphyrinoid

Uncommon metal oxidation states in porphyrinoid cofactors are responsible for the activity of many enzymes. The F430 and P450nor co-factors, with their reduced NiI- and FeIII-containing tetrapyrrolic cores, are prototypical examples of biological systems involved in methane formation and in the reduction of nitric oxide, respectively. Herein, using a comprehensive range of experimental and theoretical methods, we raise evidence that nickel tetraphenyl porphyrins deposited in vacuo on a copper surface are reactive towards nitric oxide disproportionation at room temperature. The interpretation of the measurements is far from being straightforward due to the high reactivity of the different nitrogen oxides species (eventually present in the residual gas background) and of the possible reaction intermediates. The picture is detailed in order to disentangle the challenging complexity of the system, where even a small fraction of contamination can change the scenario.

Infrared Spectroscopic Study on Swift-Ion Irradiation of Solid N2O-H2O Samples: Synthesis of N-O Bearing Species in Astrophysical Ices

As of early 2022, only six species bearing an N-O bond have been detected toward cold molecular clouds and regions of star formation. It is not clear yet if the small number of N-O bond species found in the interstellar medium so far stems from physical and technological limitations of astronomical detection techniques, or whether in fact molecules that bear an N-O bond are for some reason rare in these objects of the interstellar medium. Astronomical N-O bearing molecules are important because they are part of astrochemical models which propose that they are precursors of hydroxylamine (NH₂OH), a species linked to the formation of prebiotic amino acids in space. The aim of this study is the better understanding of the open question of the interstellar synthesis of N-O bearing species. We have analyzed by infrared spectroscopy an astrophysically relevant polar ice mixture of N₂O:H₂O processed by 90 MeV ¹³⁶Xe²³⁺ ions, which can mimic the physicochemical processes triggered by cosmic rays in water-covered interstellar ice grains. The results show the formation of N₂O₃ and of H₂O₂, but no HN-O species of any kind were detected. Such findings are discussed in light of recent studies from our group and from the literature.

An Experimental and Theoretical Investigation of the Gas-Phase C(3P) + N2O Reaction. Low Temperature Rate Constants and Astrochemical Implications

The reaction between atomic carbon in its ground electronic state, C(³P), and nitrous oxide, N₂O, has been studied below room temperature due to its potential importance for astrochemistry, with both species considered to be present at high abundance levels in a range of interstellar environments. On the experimental side, we measured rate constants for this reaction over the 50-296 K range using a continuous supersonic flow reactor. C(³P) atoms were generated by the pulsed photolysis of carbon tetrabromide at 266 nm and were detected by pulsed laser-induced fluorescence at 115.8 nm. Additional measurements allowing the major product channels to be elucidated were also performed. On the theoretical side, statistical rate theory was used to calculate low temperature rate constants. These calculations employed the results of new electronic structure calculations of the ³A″ potential energy surface of CNNO and provided a basis to extrapolate the measured rate constants to lower temperatures and pressures. The rate constant was found to increase monotonically as the temperature falls (k_C(³P)+N2O (296 K) = (3.4 ± 0.3) × 10^-11 cm³ s^-1), reaching a value of k_C(³P)+N2O (50 K) = (7.9 ± 0.8) × 10^-11 cm³ s^-1 at 50 K. As current astrochemical models do not include the C + N₂O reaction, we tested the influence of this process on interstellar N₂O and other related species using a gas-grain model of dense interstellar clouds. These simulations predict that N₂O abundances decrease significantly at intermediate times (10³ - 10⁵ years) when gas-phase C(³P) abundances are high.

Energy Redistribution Following CO2 Formation on Cold Amorphous Solid Water

The formation of molecules in and on amorphous solid water (ASW) as it occurs in interstellar space releases appreciable amounts of energy that need to be dissipated to the environment. Here, energy transfer between CO₂ formed within and on the surface of amorphous solid water (ASW) and the surrounding water is studied. Following CO(¹Σ⁺) + O(¹D) recombination the average translational and internal energy of the water molecules increases on the ∼10 ps time scale by 15–25% depending on whether the reaction takes place on the surface or in an internal cavity of ASW. Due to tight coupling between CO₂ and the surrounding water molecules the internal energy exhibits a peak at early times which is present for recombination on the surface but absent for the process inside ASW. Energy transfer to the water molecules is characterized by a rapid ∼10 ps and a considerably slower ∼1 ns component. Within 50 ps a mostly uniform temperature increase of the ASW across the entire surface is found. The results suggest that energy transfer between a molecule formed on and within ASW is efficient and helps to stabilize the reaction products generated.

The Ice Chamber for Astrophysics-Astrochemistry (ICA): A new experimental facility for ion impact studies of astrophysical ice analogs

The Ice Chamber for Astrophysics-Astrochemistry (ICA) is a new laboratory end station located at the Institute for Nuclear Research (Atomki) in Debrecen, Hungary. The ICA has been specifically designed for the study of the physico-chemical properties of astrophysical ice analogs and their chemical evolution when subjected to ionizing radiation and thermal processing. The ICA is an ultra-high-vacuum compatible chamber containing a series of IR-transparent substrates mounted on a copper holder connected to a closed-cycle cryostat capable of being cooled down to 20 K, itself mounted on a 360° rotation stage and a z-linear manipulator. Ices are deposited onto the substrates via background deposition of dosed gases. The ice structure and chemical composition are monitored by means of FTIR absorbance spectroscopy in transmission mode, although the use of reflectance mode is possible by using metallic substrates. Pre-prepared ices may be processed in a variety of ways. A 2 MV Tandetron accelerator is capable of delivering a wide variety of high-energy ions into the ICA, which simulates ice processing by cosmic rays, solar wind, or magnetospheric ions. The ICA is also equipped with an electron gun that may be used for electron impact radiolysis of ices. Thermal processing of both deposited and processed ices may be monitored by means of both FTIR spectroscopy and quadrupole mass spectrometry. In this paper, we provide a detailed description of the ICA setup as well as an overview of the preliminary results obtained and future plans.

A new multi-beam apparatus for the study of surface chemistry routes to formation of complex organic molecules in space

A multi-beam ultra-high vacuum apparatus is presented. In this article, we describe the design and construction of a new laboratory astrophysics experiment-VErs de NoUvelles Synthèses (VENUS)-that recreates the solid-state non-energetic formation conditions of complex organic molecules in dark clouds and circumstellar environments. The novel implementation of four operational differentially pumped beam lines will be used to determine the feasibility and the rates for the various reactions that contribute to formation of molecules containing more than six atoms. Data are collected by means of Fourier transform infrared spectroscopy and quadrupole mass spectrometry. The gold-coated sample holder reaches temperatures between 7 K and 400 K. The apparatus was carefully calibrated and the acquisition system was developed to ensure that experimental parameters are recorded as accurately as possible. A great effort has been made to have the beam lines converge toward the sample. Experiments have been developed to check the beam alignment using reacting systems of neutral species (NH₃ and H₂CO). Preliminary original results were obtained for the {NO + H} system, which shows that chemistry occurs only in the very first outer layer of the deposited species, that is, the chemical layer and the physical layer coincide. This article illustrates the characteristics, performance, and future potential of the new apparatus in view of the forthcoming launch of the James Webb Space Telescope. We show that VENUS will have a major impact through its contributions to surface science and astrochemistry.

Formation and Stabilization of Ground and Excited-State Singlet O2 upon Recombination of 3P Oxygen on Amorphous Solid Water

The recombination dynamics of ³P oxygen atoms on cold amorphous solid water to form triplet and singlet molecular oxygen (O₂) is investigated under conditions representative of cold clouds. Reactive molecular dynamics simulations including Landau-Zener-based hopping to account for nonadiabatic transitions find that both ground-state (X³Σ_g^-) O₂ and molecular oxygen in the two lowest singlet states (a¹Δ_g and b¹Σ_g⁺) can be formed and the molecular species stabilize through vibrational relaxation. The relative populations of the species are approximately 1:1:1. These results also agree qualitatively with a kinetic model based on simplified wavepacket simulations. The presence and stabilization of higher electronic states of O₂ are expected to modify the chemical evolution of cold interstellar (T ∼ 10-50 K) and warmer noctilucent (T ∼ 100 K) clouds.

Spectroscopy of prospective interstellar ions and radicals isolated in para-hydrogen matrices

The p-H₂ matrix-isolation technique coupled with photolysis in situ or electron bombardment produces protonated or hydrogenated species important in astrochemistry.

Reaction of H + HONO in solid para-hydrogen: infrared spectrum of ˙ONH(OH)

Hydrogenation reactions in the N/O chemical network are important for an understanding of the mechanism of formation of organic molecules in dark interstellar clouds, but many reactions remain unknown.

Solid state chemistry of nitrogen oxides – Part II: surface consumption of NO2

Efficient surface destruction mechanisms (NO₂ + H/O/N), leading to solid H₂O, NH₂OH, and N₂O, can explain the non-detection of NO₂ in space.

THz and mid-IR spectroscopy of interstellar ice analogs: methyl and carboxylic acid groups

A fundamental problem in astrochemistry concerns the synthesis and survival of complex organic molecules (COMs) throughout the process of star and planet formation. While it is generally accepted that most complex molecules and prebiotic species form in the solid phase on icy grain particles, a complete understanding of the formation pathways is still largely lacking. To take full advantage of the enormous number of available THz observations (e.g.,Herschel Space Observatory, SOFIA, and ALMA), laboratory analogs must be studied systematically. Here, we present the THz (0.3–7.5 THz; 10–250 cm⁻¹) and mid–IR (400–4000 cm⁻¹) spectra of astrophysically-relevant species that share the same functional groups, including formic acid (HCOOH) and acetic acid (CH₃COOH), and acetaldehyde (CH₃CHO) and acetone ((CH₃)₂CO), compared to more abundant interstellar molecules such as water (H₂O), methanol (CH₃OH), and carbon monoxide (CO). A suite of pure and mixed binary ices are discussed. The effects on the spectra due to the composition and the structure of the ice at different temperatures are shown. Our results demonstrate that THz spectra are sensitive to reversible and irreversible transformations within the ice caused by thermal processing, suggesting that THz spectra can be used to study the composition, structure, and thermal history of interstellar ices. Moreover, the THz spectrum of an individual species depends on the functional group(s) within that molecule. Thus, future THz studies of different functional groups will help in characterizing the chemistry and physics of the interstellar medium (ISM).