Route For Formation Research Articles

HyGAL: Characterizing the Galactic ISM with observations of hydrides and other small molecules

As a complement to the HyGAL Stratospheric Observatory for Infrared Astronomy Legacy Program, we report the results of a groundbased absorption line survey of simple molecules in diffuse and translucent Galactic clouds. Using the Institut de Radioastronomie Millimétrique (IRAM) 30 m telescope, we surveyed molecular lines in the 2 mm and 3 mm wavelength ranges toward 15 millimeter continuum sources. These sources, which are all massive star-forming regions located mainly in the first and second quadrants of the Milky Way, form the subset of the HyGAL sample that can be observed by the IRAM 30 m telescope. We detected HCO+ absorption lines toward 14 sightlines, toward which we identified 78 foreground cloud components, as well as lines from HCN, HNC, C2H, and c-C3H2 toward most sightlines. In addition, CS and H2S absorption lines are found toward at least half of the continuum sources. The spectral line data obtained were analyzed to characterize the chemical and physical properties of the absorbing interstellar medium statistically. The column density ratios of the seven molecular species observed are very similar to values found in previous absorption line studies carried out toward diffuse clouds at high latitudes. As expected, the C2H and c-C3H2 column densities show a tight correlation with that of N(HCO+), because of these all these molecules are considered to be proxies for the H2 column density toward diffuse and translucent clouds. The HCN and HNC column densities, by contrast, exhibit nonlinear correlations with those of C2H, c-C3H2, and HCO+, increasing rapidly at Av ≈ 1 in translucent clouds. Static Meudon photodissociation region (PDR) isobaric models that consider ultraviolet-dominated chemistry were unable to reproduce the column densities of all seven molecular species by just a factor of a few, except for H2S. The inclusion of other formation routes driven by turbulent dissipation could possibly explain the observed high column densities of these species in diffuse clouds. There is a tentative trend for H2S and CS abundances relative to H2 to be larger in diffuse clouds (X(H2S) and X(CS) ~ 10−8−10−7) than in translucent clouds (X(H2S) and X(CS) ~ 10−9−10−8) toward a small sample; however, a larger sample is required in order to confirm this trend. The derived H2S column densities are higher than the values predicted from the isobaric PDR models, suggesting that chemical desorption of H2S from sulfur-containing ice mantles may play a role in increasing the H2S abundance.

Weathering of gneiss saprolites and formation of Planosols under semiarid climate (NE Brazil)

The Brazilian semiarid is one of the most populous regions in the world. The main income for populations in this region is agricultural production, which in turn is limited by climatic and soil conditions. This region has environmental and economic correlations with underdeveloped regions in northwestern Africa, displaying soil surface with monotonous landscapes of smooth undulating relief with large extensions of Planosols. Climatic conditions that lead to the formation of poorly developed soils and to losses of surface layers make these soils susceptible to degradation. In the Brazilian semiarid, lithology shows that felsic rocks are the main factor in pedogenesis of Planosols, conferring the main physical, chemical, and mineralogical attributes to the soil. In this study, we evaluated the pedogenesis by weathering of gneisses (felsic rocks) in two profiles of Planosols in the Cabrobó Desertification Nucleus, Pernambuco State, northeast Brazil, highlighting the role of the parent material in the soil intrinsic characteristics in the pedogenetic processes. The primary assemblage and pedological features were determined by optical microscopy in thin sections of the A, B, and C horizons from the two soil profiles. The secondary assemblage was identified by X-ray diffractometry using the technique: SEM-scanning electron microscopy. Clay formation in situ, colluvial contribution, and elutriation process contributed to the formation of the gradient textural in both Planosols, with no evidence of argilluviation. For total regolith profile, acidic metamorphic rocks present high structural variability directly influencing the evolution of saprolites, the alteration of primary minerals, and the formation routes of secondary minerals. The secondary assemblage was rather similar and comprised illite, smectite, and kaolinite in both regoliths. The saprolite inclination in P1 favors water movement in relation to P2. The regolith in P1 shows greater mineral alteration with increased pedogenic development.

Structure, spectroscopic signatures, and formation of hydroxy-azirine: a potential interstellar prebiotic molecule

ABSTRACT Hydroxy-azirine (C2H3NO) is a -OH derivative of azirine (C2H3N), molecule that has been the subject of several unfruitful searches in space. Hydroxy-azirine is an isomer of the detected prebiotic species methyl isocyanate, CH3NCO, and glycolonitrile, HOCH2CN, as well as the yet undetected imine acetaldehyde, NHCHCHO. However, the lack of preliminary spectroscopic data on hydroxy-azirine has prevented its astronomical search. The aim of this study is to provide high-level theoretical spectroscopic signatures of the most stable hydroxy-azirine isomers to enable their eventual interstellar search. A total of 12 isomers have been characterized for hydroxy-azirine and their isomerization processes have been analysed at the CCSD(T)-F12/cc-pVTZ-F12 level. The most stable structures are 3-hydroxy-2H-azirine (I) and 2-hydroxy-2H-azirine (II) in their syn- and anti-configurations, which are suggested as the most relevant candidates for laboratory and interstellar detection. To ease their identification by means of rotational spectroscopy, we report a set of the required spectroscopic parameters using state-of-the-art composite and coupled-cluster approaches. For astronomical purposes, we provide a complete line list for I-syn and I-antihydroxy-azirine up to 50 GHz, which takes the hyperfine structure into account, and will be essential to hunt for these interstellar candidates experimentally. In addition, anharmonic vibrational frequencies and intensities are reported to predict a trustworthy vibrational spectra and to estimate the vibrational partition function. Finally, we analyse the possibility of formation of hydroxy-azirine from the reaction of azirine with the hydroxyl radical in the gas-phase and on the surface of ices, finding for the latter a feasible formation route under interstellar conditions.

Tailoring porosity and acidity of Sn-modified ZSM-5 for co-aromatization of methanol with 2-methylfuran

It is of great interest to explore sustainable routes for producing aromatics from renewable biomass resources. Herein, co-aromatization of 2-methylfuran (2-MF) with methanol was investigated at 500 °C over the hierarchical Sn-containing ZSM-5 zeolites. The results demonstrated that the Sn-containing samples showed higher aromatics selectivity than the Sn-free counterpart, due to that the increase of Lewis acidic sites caused by Sn incorporation contributed to Diels-Alder (DA) reaction of 2-MF with alkenes. The Sn-containing ZSM-5 was modified by the alkaline treatment to form hierarchical structure. The newly formed mesopores facilitated the mass transport and promoted coke tolerance capacity, thus greatly improving aromatics selectivity and catalytic stability. In addition, the route of aromatics formation were detailed by isotopic experiment and theoretical calculations. The results showed that the individual conversion of 2-MF, alkene/aromatics methylations and DA reaction might co-occur during co-feeding of 13C-methanol with 2-MF, while the individual conversion of 13C-methanol reaction seems to be partly inhibited. The theoretical calculations disclosed that the cycloaddition of furans with alkene was the rate-determining step for aromatics formation via the DA route.

Immobilization of the Amidohydrolase MxcM and Its Application for Biocatalytic Flow Synthesis of Pseudochelin A

The chemical synthesis of heterocycles typically requires elevated temperature and acid or base addition to form the desired product. Moreover, these reactions often involve hazardous reagents, which is why biocatalytic routes for heterocycle formation have gained increasing attention. In recent years, several enzymes belonging to the amidohydrolase superfamily have been identified to generate heterocycles via cyclocondensation reactions. Of particular interest is the amidohydrolase MxcM, which catalyzes the formation of an imidazoline moiety in the biosynthesis of the anti-inflammatory natural product pseudochelin A. In this study, we present a concept for the immobilization of this enzyme using a fused hexahistidine tag for fixation onto a solid, porous carrier. Notably, the immobilization improves the enzyme’s tolerance to organic solvents. The immobilized MxcM exhibits a residual activity of 169% in the polar solvent acetonitrile compared to the free enzyme, and the storage stability in the presence of 20 vol% acetonitrile was ameliorated. In addition, an immobilized enzyme reactor (IMER) was designed that can be operated under flow conditions. The MxcM-IMER retains its biocatalytic activity and mechanic stability over the tested operation time. These results provide important insights for the integration of heterocycle-forming amidohydrolases in chemical processes.

Rare earth element signature modifications induced by differential acid alteration of rocks in the Iberian Pyrite Belt

The efficient use of REE as tracers of weathering processes requires further data acquisition to establish robust connections between mechanisms, mineral assemblages and REE concentration patterns. Here, results are provided corresponding to rocks from four locations in the Iberian Pyrite Belt that were altered with different intensity by sulfuric fluids. The acidic alteration was mild (pH 5–7) in Calañas and El Villar, while strong (pH below 3 to 5) in Quebrantahuesos and Tharsis. The main REE reservoir before alteration were phyllosilicates (and plagioclase/feldspar in El Villar only). Mild alteration generated mainly kaolinite while illite and some plagioclase/feldspar were preserved. Mildly altered rocks approximately preserved the relative REE concentrations of their precursors, interpreted as indicating a low fluid/rock ratio environment where binding sites in minerals outnumbered those in fluids. Strong acid alteration produced alunite, jarosite, beudantite and goethite, with partial illite preservation. Where illite and/or goethite were major minerals, this alteration generated REE patterns enriched in light REE over heavy REE or did not modify the relative REE concentrations of the original rock. However, samples where REE concentrations were controlled by alunite or jarosite had LREE > > HREE. In alunite the maximum concentration corresponded to Nd and Sm. In jarosite concentrations decreased from La to Lu, except that a positive Eu anomaly was preserved from the precursor. These REE concentration patterns are interpreted as due to crystallographic factors, controlled by the best fit of REE with charge 3+ in the 12-fold coordination site of K in jarosite and alunite. This site is larger in jarosite and fits La best, whereas the smaller site in alunite best fits Nd and Sm. Goethite REE signature is variable, perhaps due to two formation routes, (1) silicates ➔ goethite and (2) silicates ➔ jarosite ➔ goethite. Our interpretations of REE concentration in alunite, jarosite and goethite are novel and provide new routes to further develop REE as tracers of acid alteration processes.

Undiscovered Spin Trapping Artifacts in Persulfate Oxidation Processes: Implications for Identification of Hydroxyl or Sulfate Radicals in Water

Electron paramagnetic resonance (EPR) coupled with the spin trapping technique is widely used to identify radicals and has become a tool that is being heavily relied on to probe the reaction mechanism in advanced oxidation processes (AOPs). However, the unexpected spin trapping artifacts in a highly oxidizing environment may mislead the mechanistic interpretations of AOPs. Here, we report previously unrevealed reactions between peroxydisulfate (PDS) and 5,5-dimethyl-1-pyrroline N-oxide (DMPO) or N-tert-butyl-α-phenylnitrone (PBN). The mainly detectable adducts are attributed to DMPO-OH and PBN-OH. Through radical quenching experiments, the nonradical generation pathways of DMPO-OH and PBN-OH are validated, and the 17O incorporation experiment demonstrates that H2O plays a critical role in DMPO-OH production. The formation routes of the primary artifact signals (DMPO-OH, DMPO-SO4, PBN-OH, and PBN-SO4) are further proposed by applying diverse solvents. PDS may directly oxidize DMPO to generate DMPO-SO4 in acetonitrile or acetone solvent, while DMPO-OH is yielded using H2O as the solvent. The generation of PBN-OH or PBN-SO4 follows the inverted spin trapping mechanism (a combination of one-electron oxidation and nucleophilic addition). These findings manifest the necessity of selecting the proper spin traps and performing careful control tests during exploration of the catalytic oxidation mechanism under highly oxidizing conditions.

Chemical Insights into the Formation of Colloidal Iridium Nanoparticles from In Situ X-ray Total Scattering: Influence of Precursors and Cations on the Reaction Pathway.

Iridium nanoparticles are important catalysts for several chemical and energy conversion reactions. Studies of iridium nanoparticles have also been a key for the development of kinetic models of nanomaterial formation. However, compared to other metals such as gold or platinum, knowledge on the nature of prenucleation species and structural insights into the resultant nanoparticles are missing, especially for nanoparticles obtained from IrxCly precursors investigated here. We use in situ X-ray total scattering (TS) experiments with pair distribution function (PDF) analysis to study a simple, surfactant-free synthesis of colloidal iridium nanoparticles. The reaction is performed in methanol at 50 °C with only a base and an iridium salt as precursor. From different precursor salts─IrCl3, IrCl4, H2IrCl6, or Na2IrCl6─colloidal nanoparticles as small as Ir∼55 are obtained as the final product. The nanoparticles do not show the bulk iridium face-centered cubic (fcc) structure but show decahedral and icosahedral structures. The formation route is highly dependent on the precursor salt used. Using IrCl3 or IrCl4, metallic iridium nanoparticles form rapidly from IrxClyn- complexes, whereas using H2IrCl6 or Na2IrCl6, the iridium nanoparticle formation follows a sudden growth after an induction period and the brief appearance of a crystalline phase. With H2IrCl6, the formation of different Irn (n = 55, 55, 85, and 116) nanoparticles depends on the nature of the cation in the base (LiOH, NaOH, KOH, or CsOH, respectively) and larger particles are obtained with larger cations. As the particles grow, the nanoparticle structure changes from partly icosahedral to decahedral. The results show that the synthesis of iridium nanoparticles from IrxCly is a valuable iridium nanoparticle model system, which can provide new compositional and structural insights into iridium nanoparticle formation and growth.

Exploring the genesis of spiral galaxies

Context. A tantalizing enigma in extragalactic astronomy concerns the chronology and driving mechanisms of the build-up of late-type galaxies (LTGs). The standard scenario envisages two formation routes, with classical bulges (CBs) assembling first in a quick and violent quasi-monolithic episode followed by gradual disk assembly, and pseudo-bulges (PBs) progressively forming over gigayear-long timescales through gentle gas inflow from the disk and in situ star formation. The expectation from this antagonistic rationale is the segregation of present-day LTG bulges into two evolutionary distinct groups, which is in sharp contrast with recent observations. Aims. The present study aims for a thorough investigation of the star formation history (SFH) of LTGs with its ultimate goal being to outline a coherent framework for the formation and evolution of spiral galaxies and their main stellar components. Methods. Using population spectral synthesis models, we analyse the spatially resolved SFH of bulges and disks of 135 LTGs from the CALIFA survey, covering the relevant range in LTG mass. Complementarily, characteristic physical properties of bulges and disks, such as mean colours, mass- and light-weighted stellar age and metallicity, and EW(Hα), were contrasted with predictions from evolutionary synthesis models, by adopting exponentially declining SFHs with e-folding times τ between 0.1 and 20 Gyr. Results. Analysis of the SFH of roughly half a million spaxels consistently reveals that the main physical and evolutionary properties of both bulges and disks are continuously distributed across present-day total stellar mass ℳ⋆, T. The τ in spiral galaxies with log(ℳ⋆, T) > 10 increases from the centre to the periphery, suggesting that these systems grow in an inside-out fashion. Quite importantly, the radial gradient of τ in an individual galaxy increases with increasing ℳ⋆, T, which is consistent with a high bulge-to-disk age contrast in high-mass spirals, while lower-mass LTGs display roughly the same τ throughout their entire radial extent, with intermediate mass galaxies in between. Predictions obtained through evolutionary synthesis are overall consistent with observed properties. Finally, bulges and disks of higher mass galaxies exhibit shorter formation timescales as compared to their lower mass counterparts. Conclusions. Collectively, the obtained results evince a coherent and unified picture for the formation and evolution of LTGs, in which PBs and CBs denote extremities of a continuous mass sequence. Our findings are consistent with the framework where bulges are assembled jointly with their parent disks by gradual inside-out growth, at a pace that is regulated by the depth of the galactic potential. This postulate is further supported by the fact that the revealed correlations are entirely devoid of a bimodality, as it would be expected if CBs and PBs were to emerge from two distinct formation routes.

Significant influence of water molecules on the SO3 + HCl reaction in the gas phase and at the air-water interface.

The products resulting from the reactions between atmospheric acids and SO3 have a catalytic effect on the formation of new particles in aerosols. However, the SO3 + HCl reaction in the gas-phase and at the air-water interface has not been considered. Herein, this reaction was explored exhaustively by using high-level quantum chemical calculations and Born Oppenheimer molecular dynamics (BOMD) simulations. The quantum calculations show that the gas-phase reaction of SO3 + HCl is highly unlikely to occur under atmospheric conditions with a high energy barrier of 22.6 kcal mol-1. H2O and (H2O)2 play obvious catalytic roles in reducing the energy barrier of the SO3 + HCl reaction by over 18.2 kcal mol-1. The atmospheric lifetimes of SO3 show that the (H2O)2-assisted reaction dominates over the H2O-assisted reaction within the altitude range of 0-5 km, whereas the H2O-assisted reaction is more favorable within an altitude range of 10-50 km. BOMD simulations show that H2O-induced formation of the ClSO3-⋯H3O+ ion pair and HCl-assisted formation of the HSO4-⋯H3O+ ion pair were identified at the air-water interface. These routes followed a stepwise reaction mechanism and proceeded at a picosecond time scale. Interestingly, the formed ClSO3H in the gas phase has a tendency to aggregate with sulfuric acids, ammonias, and water molecules to form stable clusters within 40 ns simulation time, while the interfacial ClSO3- and H3O+ can attract H2SO4, NH3, and HNO3 for particle formation from the gas phase to the water surface. Thus, this work will not only help in understanding the SO3 + HCl reaction driven by water molecules in the gas-phase and at the air-water interface, but it will also provide some potential routes of aerosol formation from the reaction between SO3 and inorganic acids.

Determination of the influence of water on the SO3 + CH3OH reaction in the gas phase and at the air-water interface.

Liu et al. (Proc. Natl. Acad. Sci. U. S. A, 2019, 116, 24966-24971) showed that at an altitude of 0 km, the reaction of SO3 with CH3OH to form CH3OSO3H reduces the amount of H2SO4 produced by the hydrolysis of SO3 in regions polluted with CH3OH. However, the influence of the water molecule has not been fully considered yet, which will limit the accuracy of calculating the loss of SO3 in regions polluted with CH3OH. Here, the influence of water molecules on the SO3 + CH3OH reaction in the gas phase and at the air-water interface was comprehensively explored by using high-level quantum chemical calculations and Born-Oppenheimer molecular dynamics (BOMD) simulations. Quantum chemical calculations show that both pathways for the formation of CH3OSO3H and H2SO4 with water molecules have greatly lowered energy barriers compared to the naked SO3 + CH3OH reaction. The effective rate coefficients reveal that H2O-catalyzed CH3OSO3H formation (a favorable route for CH3OSO3H formation) can be competitive with H2O-assisted H2SO4 formation (a favorable process for H2SO4 formation) at high altitudes up to 15 km. BOMD simulations found that H2O-induced formation of the CH3OSO3-⋯H3O+ ion pair and CH3OH-assisted formation of HSO4- and H3O+ ions were observed at the droplet surface. These interfacial routes followed a loop-structure or chain reaction mechanism and proceeded on a picosecond time scale. These results will contribute to better understanding of SO3 losses in the polluted areas of CH3OH.

Synthesis, formation mechanism, and intrinsic physical properties of several As/P-containing MAX phases

• Molten-salt sintering is applicable to P-based MAX phases • Solid-state sintering is suitable to As-based MAX phases • Route 1: NbP+Nb+C → Nb 3 P 2 C is most favorable to obtain pure Nb 3 P 2 C • Molten-salt sintering do well on all routes, especially for Nb 2 PC+NbP→Nb 3 P 2 C • Various intrinsic physical properties of 321 phases are firstly reported 321 phases are an atypical series of MAX phases, in which A = As/P, with superior elastic properties, featuring in the MA-triangular-prism bilayers in the crystal structure. Until now, besides Nb 3 As 2 C, the pure phases of the other 321 compounds have not been realized, hampering the study of their intrinsic properties. Here, molten-salt sintering (MSS) and solid-state synthesis (SSS) were applied to synthesize As/P-containing 321 phases and 211 phases. Analyzing the phase composition of the end-product via multiple-phase Rietveld refinement, we found that MSS can effectively improve the purity of P-containing MAX phases, with the phase content up to 99% in Nb 3 P 2 C and 75.4(5)% in Nb 2 PC. In contrast, MSS performed poorly on As-containing MAX phases, only 8.9(4)% for Nb 3 As 2 C and 64(2)% for Nb 2 AsC, as opposed to the pure phases obtained by SSS. The experimental analyses combined with first-principles calculations reveal that the dominant formation route of Nb 3 P 2 C is through NbP + Nb + C → Nb 3 P 2 C. Moreover, we found that the benefits of MSS on P-containing MAX phases are on the facilitation of three considered chemical reaction routes, especially on Nb 2 PC + NbP → Nb 3 P 2 C. Furthermore, the intrinsic physical properties and Fermi surface topology of two 321 phases consisting of electron, hole, and open orbits are revealed theoretically and experimentally, in which the electron carriers are dominant in electrical transport. The feasible synthesis methods and the formation mechanism are instructive to obtain high-purity As/P-containing MAX phases and explore new MAX phases. Meanwhile, the intrinsic physical properties will give great support for future applications on 321 phases.

A combined crossed molecular beam and theorerical study of the O(3P,1D) + acrylonitrile (CH2CHCN) reactions and implications for combustion and extraterrestrial environments.

Acrylonitrile (CH2CHCN) is ubiquitous in space (molecular clouds, solar-type star forming regions, and circumstellar envelopes) and is also abundant in the upper atmosphere of Titan. The reaction O(3P) + CH2CHCN can be of relevance in the chemistry of the interstellar medium because of the abundance of atomic oxygen. The oxidation of acrylonitrile is also important in combustion as the thermal decomposition of pyrrolic and pyridinic structures present in fuel-bound nitrogen generates many nitrogen-bearing compounds, including acrylonitrile. Despite its relevance, limited information exists on this reaction. We report a combined experimental and theoretical investigation of the reactions of acrylonitrile with both ground 3P and excited 1D atomic oxygen. From product angular and time-of-flight distributions in crossed molecular beam experiments with mass spectrometric detection at a collision energy, Ec, of 31.4 kJ mol-1, we have identified the primary reaction products and determined their branching fractions (BFs). Theoretical calculations of the relevant triplet and singlet potential energy surfaces (PESs) were performed to interpret the experimental results and elucidate the reaction mechanism. Adiabatic statistical calculations of product BFs for the decomposition of the main triplet and singlet intermediates have been carried out. Combining the experimental and theoretical results, we conclude that the O(3P) reaction leads to two main product channels: (i) CH2CNH (ketenimine) + CO (dominant with a BF of 0.87 ± 0.05), formed via efficient intersystem crossing from the entrance triplet PES to the underlying singlet PES, and (ii) HCOCHCN + H (minor, with a BF of 0.13 ± 0.05), occurring adiabatically on the triplet PES. Our study suggests the inclusion of this reaction as a possible destruction pathway of CH2CHCN and a possible formation route of CH2CNH in the interstellar medium. The O(1D) + CH2CHCN reaction mainly leads to the formation of CH2CNH + CO adiabatically on the singlet PES. This result can improve models related to the chemistry of interstellar ice and cometary comas, where O(1D) reactions can play a role. Overall, our results are expected to be useful for improving the models of combustion and extraterrestrial environments.

Gas-phase formation of glycolonitrile in the interstellar medium.

Our automated reaction discovery program, AutoMeKin, has been utilized to investigate the formation of glycolonitrile (HOCH2CN) in the gas phase under the low temperatures of the interstellar medium (ISM). The feasibility of a proposed pathway depends on the absence of barriers above the energy of reactants and the availability of the suggested precursors in the ISM. Based on these criteria, several radical-radical reactions and a radical-molecule reaction have been identified as viable formation routes in the ISM. Among the radical-radical reactions, OH + CH2CN appears to be the most relevant, considering the energy of the radicals and its ability to produce glycolonitrile in a single step. However, our analysis reveals that this reaction produces hydrogen isocyanide (HNC) and formaldehyde (CH2O), with rate coefficients ranging from (7.3-11.5) × 10-10 cm3 molecule-1 s-1 across the temperature range of 10-150 K. Furthermore, the identification of this remarkably efficient pathway for HNC elimination from glycolonitrile significantly broadens the possibilities for any radical-radical mechanism proposed in our research to be considered as a feasible pathway for the formation of HNC in the ISM. This finding is particularly interesing given the persistently unexplained overabundance of hydrogen isocyanide in the ISM. Among the radical-molecule reactions investigated, the most promising one is OH + CH2CHNH, which forms glycolonitrile and atomic hydrogen with rate coefficients in the range (0.3-6.6) × 10-10 cm3 molecule-1 s-1 within the 10-150 K temperature range. Our calculations indicate that the formation of both hydrogen isocyanide and glycolonitrile is efficient under the harsh conditions of the ISM.

On the role of HNNO in NO[formula omitted] formation

The formation of nitrogen oxides (NOx) during combustion is a topic of substantial fundamental and practical interest, given the complex nature of its formation kinetics and the fact that, as a highly regulated pollutant emission, it is a major constraint in engineering design. To date, there are four known mechanisms by which the strong N–N bond can be broken to facilitate NOx formation from N2 present in air. Here we posit and explore the possibility of a new NOx formation route mediated by an HNNO intermediate whose reactions with common combustion species break the N–N bond. Altogether, we present results from master equation (ME) calculations for HNNO formation from H + N2O (+M), ab initio electronic structure and RRKM/ME calculations for HNNO + O2, and simulations of NO profiles in freely propagating flames using a newly constructed HNNO kinetic sub-model. Our ME results for the H + N2O reaction indicate that HNNO is the favored product channel at lower temperatures and higher pressures – e.g. favored over all other products up to ∼1100 K and over NH + NO up to ∼1500 K above 10 atm. Our ab initio electronic structure calculations for trans-HNNO + O2 show a barrier for abstraction to HO2 + N2O of 18.2 kcal/mol and a barrier for addition of 27.0 kcal/mol to form an HN(OO)NO which can decompose to NO + HNO2 over a barrier of 32.3 kcal/mol (cis-HNNO + O2 shows similar reactivity). Altogether, our rate constant calculations and kinetic modeling, which also includes estimated rate constants for HNNO + radical reactions, suggest that HNNO + O2 mainly recycles HNNO back to N2O but is sufficiently slow that the primary fate of HNNO in many combustion situations likely involves reactions with radical species, which appear likely to occur quickly and with high NOx yields.

New prebiotic molecules in the interstellar medium from the reaction between vinyl alcohol and CN radicals: unsupervised reaction mechanism discovery, accurate electronic structure calculations and kinetic simulations.

Vinyl alcohol (VyA) and cyanide (CN) radicals are relatively abundant in the interstellar medium (ISM). VyA is the enolic tautomer of acetaldehyde and has two low-lying conformers, characterized by the syn or anti placement of hydroxyl hydrogen with respect to the double bond. In this paper, we present a gas-phase model of the barrierless reactions of both VyA's conformers with CN employing accurate quantum chemical computations in the framework of a master equation approach based on the transition state theory. Our results indicate that both VyA conformers feature a similar reactivity with CN, starting with a barrierless addition to the double bond and followed by different isomerization, dissociation, and/or hydrogen elimination steps. The rate constants computed for temperatures up to 600 K show that several reaction channels are open even under the harsh conditions of the ISM, with the favoured one providing the first feasible formation route of a prebiotic molecule not yet detected in the ISM, namely cyanoacetaldehyde. This finding suggests looking for cyanoacetaldehyde in regions where both VyA and CN have already been detected, like, e.g., Sagittarius B2N or G+0.693-0.027.

Mapping NGC 7027 in New Light: CO+ and HCO+ Emission Reveal Its Photon- and X-Ray-dominated Regions

The young and well-studied planetary nebula (PN) NGC 7027 harbors significant molecular gas that is irradiated by luminous, pointlike UV (central star) and diffuse (shocked nebular) X-ray emission. This nebula represents an excellent subject to investigate the molecular chemistry and physical conditions within photon- and X-ray-dominated regions (PDRs and XDRs). As yet, the exact formation routes of CO+ and HCO+ in PN environments remain uncertain. Here we present ∼2″ resolution maps of NGC 7027 in the irradiation tracers CO+ and HCO+ obtained with the IRAM NOEMA interferometer, along with SMA CO and HST 2.12 μm H2 data for context. The CO+ map constitutes the first interferometric map of this molecular ion in any PN. Comparison of CO+ and HCO+ maps reveals strikingly different emission morphologies, as well as a systematic spatial displacement between the two molecules; the regions of brightest HCO+, found along the central waist of the nebula, are radially offset by ∼1″ (∼900 au) outside the corresponding CO+ emission peaks. The CO+ emission furthermore precisely traces the inner boundaries of the nebula’s PDR (as delineated by near-IR H2 emission), suggesting that central star UV emission drives CO+ formation. The displacement of HCO+ radially outward with respect to CO+ is indicative that dust-penetrating soft X-rays are responsible for enhancing the HCO+ abundance in the surrounding molecular envelope, forming an XDR. These interferometric CO+ and HCO+ observations of NGC 7027 thus clearly establish the spatial distinction between the PDR and XDR formed (respectively) by intense UV and X-ray irradiation of molecular gas.

Emission, partition, and formation pathway of polychlorinated dibenzo-p-dioxins and dibenzofurans during co-disposal of industrial waste with municipal solid waste

Growing amounts of industrial waste (IW) are being co-disposed in municipal solid waste incineration (MSWI) plants, whereas few studies have been conducted on the characterization of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) during co-incineration. The emission characteristics and formation pathways of PCDD/Fs are systematically analyzed during the co-disposal of MSW and IW of the textile/food industry in a full-scale MSWI plant. The emission of toxic PCDD/Fs in the chimney (CH) is far lower than the national standard (<0.005 ng I-TEQ/Nm3) in co-disposal, as well as fly ash (FA) (<50 ng I-TEQ/kg). Co-incinerating IW of the textile industry significantly influences the PCDD/Fs (total 136 kinds of congeners) emission in boiler outlet (BO), CH and FA, while co-incinerating IW of the food industry causes less impact. IW co-disposal also enables PCDD/Fs migration from BO into FA, probably attributing to chlorine (Cl) increment in IW. Furthermore, the distribution patterns of PCDD/F isomers are analogous under MSWI and IW co-disposal conditions. De novo synthesis in all states is generally the dominant formation route of PCDD/Fs, possibly due to the longer dwell time of carbon residues, and de novo synthesis is strengthened by IW co-disposal. Chlorophenol (CP)-route synthesis and dibenzofuran (DF) chlorination contribute less, which are slightly reduced by co-incinerating IW, while dibenzodioxin (DD) chlorination is somewhat strengthened. Major air pollutants in co-disposal, including CO, NOx, HCl and particulate matter (PM) are poorly related to PCDD/Fs (−0.33 <R<0.52), while SO2 is negatively correlated with PCDD/F homologues, indicating potential suppression by sulfur (S) in IW.

On synthesis, stability and superconductivity of ThNb2H12under pressure: ab-inito calculations

Systematic study employing first-principle electronic structure method along with the evolutionary structure search algorithm as implemented in the universal structure predictor evolutionary Xtallography (USPEX) code has been carried out to look for the possibility of synthesising the ternary hydride (ThxNbyHz) from constituent's thorium, niobium and hydrogen. Our study suggests that the ternary hydride with formula ThNb2H12 will be globally stable above 12.5 GPa indicating the possibility of its formation from constituents Th, Nb, H and binary compounds of Th-H and Nb-H. Further, though the global stability of this compound is predicted to be beyond 12.5 GPa, it is metastable at 0 GPa. We have theoretically explored different formation routes to synthesize this compound. The zero-pressure monoclinic structure (Cm) of metastable ThNb2H12 undergoes a transformation to hexagonal structure (P6/mmm) at 3.5 GPa and this hexagonal structure (P6/mmm) transforms to other hexagonal type structure (P6/m) at 50 GPa. The high pressure structural sequence so predicted through static lattice calculations has been further substantiated by confirming the elastic and lattice dynamic stability of each structure in the pressure regime of their stability. Employing Allen–Dynes modified McMillan [P. B. Allen and R. C. Dynes, Phys. Rev. B 12, 905 (1975)] formula along with the fixed Coulomb pseudopotential of 0.1 and calculated electron-phonon coupling parameter, the superconductive temperature has also been calculated for this compound. Our analysis indicates that this material will behave as superconductor with transition temperature (Tc) ∼ 25 K at 15 GPa, which, will reduce to 4 K upon further compression to 100 GPa. Our analysis on lattice thermal conductivity of this compound in P6/mmm phase, at 15 GPa, shows a significantly large anisotropy in conductivity tensor, especially at lower temperatures.

Glycolaldehyde formation mediated by interstellar amorphous ice: a computational study

ABSTRACT Glycolaldehyde (HOCH2CHO) is the most straightforward sugar detected in the Interstellar Medium (ISM) and participates in the formation pathways of molecules fundamental to life, red such as ribose and derivatives. Although detected in several regions of the ISM, its formation route is still debated and its abundance cannot be explained only by reactions in the gas phase. This work explores a new gas-phase formation mechanism for glycolaldehyde and compares the energy barrier reduction when the same route happens on the surface of amorphous ices. The first step of the mechanism involves the formation of a carbon–carbon bond between formaldehyde (H2CO) and the formyl radical (HCO), with an energy barrier of 27 kJ mol−1 (gas-phase). The second step consists of barrierless hydrogen addition. Density functional calculations under periodic boundary conditions were applied to study this reaction path on 10 different amorphous ice surfaces through an Eley–Rideal type mechanism. It was found that the energy barrier is reduced on average by 49 per cent, leading in some cases to a 100 per cent reduction. The calculated adsorption energy of glycolaldehyde suggests that it can be promptly desorbed to the gas phase after its formation. This work, thus contributes to explaining the detected relative abundances of glycolaldehyde and opens a new methodological framework for studying the formation routes for Complex Organic Molecules (COMs) in interstellar icy grains.