Gas-phase Formation Research Articles

Towards improving the characteristics of high-energy pyrazines and their N-oxides.

Based on the methods of quantum chemistry and atom-atom potentials, the molecular and crystal structure of a number of high-energy pyrazines was modeled: unsubstituted diazines, as well as fully nitrated 1,4-diazabenzenes, their oxides and polymorphs. The enthalpies of formation, densities of molecular crystals, and some performance characteristics of these compounds were determined. The parameters of decomposition of substances were estimated. It has been established that tetranitropyrazine-1,4-dioxide has maximum energy content and excellent performance characteristics, which determine the prospects for using this compound as a high-energy one in the considered series of compounds. In this work, DFT calculations were conducted through the software Gaussian 09 using B3LYP functional with basis set aug-cc-PVDZ and the Grimme dispersion correction D2. For crystal structure optimization, the atom-atom potential methods with PMC program (Packing of Molecules in Crystal) were used. Charges for molecular electrostatic potential were fitted by FitMEP and enthalpies of formation in gas phase were assessed by G3B3.

The response of daytime nitrate formation to source emissions reduction based on chemical kinetic and thermodynamic model

Particulate nitrate is an important component of particulate matter and poses a significant threat to the ecosystem and human health. The gas-phase formation pathway of nitrate is extremely important, which mainly comprises the NO2 oxidation process triggered by OH radicals and the nitrate partitioning process. The response of nitrate to source emission reduction during different pollution periods remains unclear. Here, we applied the chemical kinetic and thermodynamics model to explore the importance oxidation process and partitioning process during different pollution periods based on high-time resolution observation data. The result indicated that with the aggravation of pollution, the partitioning process gradually ceases to be a limiting step in the formation of nitrates. The results of the influencing factor analysis indicate that NO2 concentration and aerosol pH values play a more significant role in the formation of nitrates. Specifically, during the clean period, nitrate formation is sensitive to both NO2 concentration and pH values, but during the pollution period, it becomes sensitive only to NO2 concentration. By combining source apportionment, we explored the response of nitrate formation to source emission reduction, and the results showed that the control of vehicle exhaust emissions and coal combustion sources is more effective in mitigating nitrate pollution. Additionally, this study also emphasized the importance of early prevention and control of pollution sources. This research provides scientific evidence for the precise management and control of nitrates.

Correlations among complex organic molecules around protostars: Effects of physical structure

Context. Complex organic molecules have been observed toward many protostars. Their column density ratios are generally constant across protostellar systems, with some low-level scatter. However, the scatter in the column density ratio of formamide (NH2CHO) to methanol (CH3OH), NNH2CHO/NCH3OH, is one of the highest compared to other ratios. The larger scatter for NNH2CHO/NCH3OH (or weak correlation of these two molecules) is sometimes interpreted as evidence of gas-phase formation of NH2CHO. Aims. In this work, we propose an alternative interpretation in which this scatter is produced by differences in the snowline locations related to differences in binding energies of these species (formamide typically has a ≳2000 K larger binding energy than methanol) and the small-scale structure of the envelope and the disk system. Therefore, we do not include chemistry in our models in order to isolate the effect of physical factors. We also include CH3CN in our work as a control molecule, as it has a similar binding energy to CH3OH. Methods. We used radiative transfer models to calculate the emission from NH2CHO, CH3OH, and CH3CN in protostellar systems with and without disks. The abundances of these species were parameterized in our models, and we fit the calculated emission lines to find the column densities and excitation temperatures of these species, as done in real observations. Results. Given the difference in binding energies of NH2CHO and CH3OH, we find the gas-phase NNH2CHO/NCH3OH needs to be multiplied by a correction factor of approximately ten in order to give the true abundance ratio of these two species in the ices. This factor is much smaller (i.e., ~2) for NCH3CN/NCH3OH (the control molecule). We find that models with different disk sizes, luminosities, and envelope masses produce a scatter in this correction factor, and hence in NNH2CHO/NCH3OH comparable with that of observations. The scatter in NNH2CHO/NCH3OH is larger than that of NCH3CN/NCH3OH in models consistent with the observations. However, the scatter in the models for NCH3CN/NCH3OH is smaller than observations by a factor of around two, as expected from the similar binding energies of CH3OH and CH3CN pointing to the need for some chemical effects in the gas or ice to explain the observed ratios. We show that the scatter in NNH2CHO/NCH3OH will be lower than previously measured if we correct for the difference in sublimation temperatures of these two species in observations of ~40 protostellar systems with ALMA. Conclusions. The scatter in NNH2CHO/NCH3OH (or the ratio of any two molecules with a large binding energy difference) can be partially explained by the difference in their binding energies. Correction for this bias makes the scatter in this ratio similar to that in ratios of other complex organics in the observations, making NH2CHO a “normal” molecule. Therefore, we conclude that gas-phase chemistry routes for NH2CHO are not necessary to explain the larger scatter of NNH2CHO/NCH3OH compared with other ratios.

Gas-Phase Formation of Sulfurous Acid (H2SO3) in the Atmosphere.

Sulfurous acid (H2SO3) is known to be thermodynamically instable decomposing into SO2 and H2O. All attempts to detect this elusive acid in solution failed up to now. Reported H2SO3 formation from an experiment carried out in a mass spectrometer as well as results from theoretical calculations, however, indicated a possible kinetic stability in the gas phase. Here, it is shown experimentally that H2SO3 is formed in the OH radical-initiated gas-phase oxidation of methanesulfinic acid (CH3S(O)OH) at 295±0.5 K and 1 bar of air with a molar yield of %. Further main products are SO2, SO3 and methanesulfonic acid. CH3S(O)OH represents an important intermediate product of dimethyl sulfide oxidation in the atmosphere. Global modeling predicts an annual H2SO3 production of ∼8 million metric tons from the OH+CH3S(O)OH reaction. The investigated H2SO3 depletion in the presence of water vapor results in k(H2O+H2SO3) <3×10-18 cm3 molecule-1 s-1, which indicates a lifetime of at least one second for atmospheric humidity. This work provides experimental evidence that H2SO3, once formed in the gas phase, is kinetically stable enough to allow its characterization and subsequent reactions.

Gas-Phase Preparation of the 14π Hückel Polycyclic Aromatic Anthracene and Phenanthrene Isomers (C14H10) via the Propargyl Addition-BenzAnnulation (PABA) Mechanism.

Polycyclic aromatic hydrocarbons (PAHs) imply the missing link between resonantly stabilized free radicals and carbonaceous nanoparticles, commonly referred to as soot particles in combustion systems and interstellar grains in deep space. Whereas gas phase formation pathways to the simplest PAH - naphthalene (C10H8) - are beginning to emerge, reaction pathways leading to the synthesis of the 14π Hückel aromatic PAHs anthracene and phenanthrene (C14H10) are still incomplete. Here, by utilizing a chemical microreactor in conjunction with vacuum ultraviolet (VUV) photoionization (PI) of the products followed by detection of the ions in a reflectron time-of-flight mass spectrometer (ReTOF-MS), the reaction between the 1'- and 2'-methylnaphthyl radicals (C11H9⋅) with the propargyl radical (C3H3⋅) accesses anthracene (C14H10) and phenanthrene (C14H10) via the Propargyl Addition-BenzAnnulation (PABA) mechanism in conjunction with a hydrogen assisted isomerization. The preferential formation of the thermodynamically less stable anthracene isomer compared to phenanthrene suggests a kinetic, rather than a thermodynamics control of the reaction.

Gas-phase formation of fullerene/9-hydroxyfluorene cluster cations

ABSTRACT In interstellar environment, fullerene species readily react with large molecules (e.g. polycyclic aromatic hydrocarbons, PAHs and their derivatives) in the gas phase, which may be the formation route of carbon dust grains in space. In this work, the gas-phase ion–molecule collision reaction between fullerene cations (${\rm C}_{n}\, ^+$, n = 32, 34,…, 60) and functionalized PAH molecules (9-hydroxyfluorene, C13H10O) are investigated both experimentally and theoretically. The experimental results show that fullerene/9-hydroxyfluorene cluster cations are efficiently formed, leading to a series of large fullerene/9-hydroxyfluorene cluster cations (e.g. [(C13H10O)C60]+, [(C13H10O)3C58]+, and [(C26H18O)(C13H10O)2C48]+). The binding energies and optimized structures of typical fullerene/9-hydroxyfluorene cluster cations were calculated. The bonding ability plays a decisive role in the cluster formation processes. The reaction surfaces, modes, and combination reaction sites can result in different binding energies, which represent the relative chemical reactivity. Therefore, the geometry and composition of fullerene/9-hydroxyfluorene cluster cations are complicated. In addition, there is an enhanced chemical reactivity for smaller fullerene cations, which is mainly attributed to the newly formed deformed carbon rings (e.g. 7 C-ring). As part of the co-evolution network of interstellar fullerene chemistry, our results suggest that ion–molecule collision reactions contribute to the formation of various fullerene/9-hydroxyfluorene cluster cations in the interstellar medium, providing insights into different chemical reactivity caused by oxygenated functional groups (e.g. hydroxyl, OH, or ether, C-O-C) on the cluster formations.

Spectroscopic Characterization of Highly Excited Neutral Chromium Tricarbonyl.

Spectroscopic characterization of highly excited neutral transition-metal complexes is important for understanding the multifaceted reaction mechanisms between metals and ligands. In this work, the reactions of neutral chromium atoms with carbon monoxide were probed by size-specific infrared spectroscopy. Interestingly, Cr(CO)3 was found to have an unprecedented 7A2″ septet excited state rather than the singlet ground state. A combination of experiment and theory shows that the gas-phase formation of this highly excited Cr(CO)3 is facile both thermodynamically and kinetically. Electronic structure and bonding analyses indicate that the valence electrons of Cr atoms in the septet Cr(CO)3 are in a relatively stable configuration, which facilitate the highly excited structure and the planar geometric shape (D3h symmetry). The observed septet Cr(CO)3 affords a paradigm for exploring the structure, properties, and formation mechanism of a large variety of excited neutral compounds.

Advective and diffusive gas phase transport in vadose zones: Importance for defining vapour risks and natural source zone depletion of petroleum hydrocarbons

Quantifying the interlinked behaviour of the soil microbiome, fluid flow, multi-component transport and partitioning, and biodegradation is key to characterising vapour risks and natural source zone depletion (NSZD) of light non-aqueous phase liquid (LNAPL) petroleum hydrocarbons. Critical to vapour transport and NSZD is transport of gases through the vadose zone (oxygen from the atmosphere, volatile organic compounds (VOCs), methane and carbon dioxide from the zone of LNAPL biodegradation). Volatilisation of VOCs from LNAPL, aerobic biodegradation, methanogenesis and heat production all generate gas pressure changes that may lead to enhanced gas fluxes apart from diffusion. Despite the importance of the gaseous phase dynamics in the vadose zone processes, the relative pressure changes and consequent scales of advective (buoyancy and pressure driven) / diffusive transport is less studied. We use a validated multi-phase multi-component non-isothermal modelling framework to differentiate gas transport mechanisms. We simulate a multicomponent unweathered gasoline LNAPL with high VOC content to maximise the potential for pressure changes due to volatilisation and to enable the joint effects of methanogenesis and shallower aerobic biodegradation of vapours to be assessed, along with heat production. Considering a uniform fine sand profile with LNAPL resident in the water table capillary zone, results suggest that biodegradation plays the key role in gas phase formation and consequent pressure build-up. Results suggest that advection is the main transport mechanism over a thin zone inside the LNAPL/capillary region, where the effective gaseous diffusion is very low. In the bulk of the vadose zone above the LNAPL region, the pressure change is minimal, and gaseous diffusion is dominant. Even for high biodegradation rate cases, pressure build-up due to heat generation (inducing buoyancy effects) is smaller than the contribution of gas formation due to biodegradation. The findings are critical to support broader assumptions of diffusive transport being dominant in vapour transport and NSZD assessments.

Computational insight into the crystal structures of cubane and azacubanes.

Using quantum chemistry and atom-atom potential methods, the molecular and crystal structures of cubane 1 and all types of unsubstituted azacubanes 2-22 were calculated. Alternative possible polymorphs of cubane 1 have been proposed. The thermochemical properties of azacubanes in the gas and solid phases were assessed. Thermodynamic aspects of stability are considered, and a significant decrease in stability is revealed upon transition from cubane 1 to octaazacubane 22. It has been shown that the density and energetic properties of azacubanes depend nonlinearly on the number of nitrogen atoms in the structure and the density of octaazacubane 22 at room temperature is 1.546gcm-3, which is significantly lower than the previously given estimate. In this work, DFT calculations were conducted through the software Gaussian 09 using B3LYP functional with basis set aug-cc-PVDZ and the Grimme dispersion correction D2. For crystal structure optimization, the atom-atom potential methods with PMC (packing of molecules in crystal) program were used. Charges for molecular electrostatic potential were fitted by FitMEP, and enthalpies of formation in gas phase were assessed by G3B3.

Chemical differences among collapsing low-mass protostellar cores

Context. Organic features lead to two distinct types of Class 0/I low-mass protostars: hot corino sources exhibiting abundant saturated complex organic molecules (COMs) and warm carbon-chain chemistry (WCCC) sources exhibiting abundant unsaturated carbon-chain molecules. Some observations suggest that the chemical variations between WCCC sources and hot corino sources are associated with local environments and the luminosity of protostars. Aims. We aim to investigate the physical conditions that significantly affect WCCC and hot corino chemistry, as well as to reproduce the chemical characteristics of prototypical WCCC sources and hybrid sources, where both carbon-chain molecules and COMs are abundant. Methods. We conducted a gas-grain chemical simulation in collapsing protostellar cores, adopting a selection of typical physical parameters for the fiducial model. By adjusting the values of certain physical parameters, such as the visual extinction of ambient clouds (AVamb), cosmic-ray ionization rate (ζ), maximum temperature during the warm-up phase (Tmax), and contraction timescale of protostars (tcont), we studied the dependence of WCCC and hot corino chemistry on these physical parameters. Subsequently, we ran a model with different physical parameters to reproduce scarce COMs in prototypical WCCC sources. Results. The fiducial model predicts abundant carbon-chain molecules and COMs. It also reproduces WCCC and hot corino chemistry in the hybrid source L483. This suggests that WCCC and hot corino chemistry can coexist in some hybrid sources. Ultraviolet (UV) photons and cosmic rays can boost WCCC features by accelerating the dissociation of CO and CH4 molecules. On the other hand, UV photons can weaken the hot corino chemistry by photodissociation reactions, while the dependence of hot corino chemistry on cosmic rays is relatively complex. The value of Tmax does not affect any WCCC features, while it can influence hot corino chemistry by changing the effective duration of two-body surface reactions for most COMs. The long tcont can boost WCCC and hot corino chemistry by prolonging the effective duration of WCCC reactions in the gas phase and surface formation reactions for COMs, respectively. The scarcity of COMs in prototypical WCCC sources can be explained by insufficient dust temperatures in the inner envelopes that are typically required to activate hot corino chemistry. Meanwhile, the high ζ and the long tcont favors the explanation for scarce COMs in these sources. Conclusions. The chemical differences between WCCC sources and hot corino sources can be attributed to the variations in local environments, such as AVamb and ζ, as well as the protostellar property, tcont.

Mechanism and direct kinetic study on the homogeneous gas-phase formation of 1,2,3,4,5,6,7-HpCN from the self-condensation of 2,3,4,5-TeCPRs and cross-condensation of PCPR with 2,3,4,5-TeCPR

Polychlorinated naphthalenes (PCNs) are ubiquitous in the environment and have received extensive public concern due to their dioxin-like properties. 1,2,3,4,5,6,7-heptachloronaphthalene (1,2,3,4,5,6,7-HpCN) is one of the most toxic and detected PCNs. The chlorophenoxy radical (CPR) was demonstrated as a crucial intermediate specie for PCN formation in thermal and combustion conditions. In this study, the homogeneous gas-phase formations of PCNs from the self-condensation of 2,3,4,5-terachlorophenoxy radicals (2,3,4,5-TeCPRs) and the cross-condensation of pentachlorophenoxy radical (PCPR) with 2,3,4,5-TeCPR were researched by using the direct density functional theory (DFT) kinetic calculation. The possible formation mechanisms were investigated at the MPWB1K/6–311+G(3df,2p)//MPWB1K/6–31+G(d,p) level. The rate constants and their temperature dependence of key elementary reactions were deduced over the temperature range of 600–1200 K. This study reveals that pathways concluding with Cl elimination are energetically favored over those ending with H elimination. 1,2,3,4,5,6,7-HpCN is the main PCN product from the self-condensation of 2,3,4,5-TeCPRs, whereas 1,2,3,4,5,6,7-HpCN and 1,2,3,4,5,6,8-HpCN are the major PCN products from the cross-condensation of PCPR and 2,3,4,5-TeCPR. 1,2,3,4,5,6,7-HpCN is preferentially formed from self-condensation of 2,3,4,5-TeCPRs. Multi-chlorine substitutions suppress the PCN formations. The findings discussed in this paper aim to provide detailed insights into the formation mechanism of highly chlorinated PCNs from chlorophenol precursors under thermal and combustion conditions.

A density functional theory study on the gas-phase formation of InGaN by metalorganic chemical vapor deposition

Density functional theory was used to analyze the formation of InGaN from trimethyl indium (TMIn) and trimethyl gallium (TMGa) by metalorganic chemical vapor deposition in ammonia in terms of oligomerization reactions of the nitrides and the elimination reactions of the oligomers formed. The reaction pathways were assumed by reference to previous studies, and their free energy and energy barrier characteristics were calculated for different temperatures. The results indicated that, in the oligomerization reactions, the decomposition temperature of dimers is higher than that of trimers; dimethyl indium nitride (DMInNH2) is more prone to polymerization than dimethyl gallium nitride (DMGaNH2); and oligomerization of DMInNH2 is more likely to occur. In the elimination reactions, when the reaction temperature is high, oligomers tend to generate [MMXNH2][MMXNH2] (MM = monomethyl; X = In or Ga) first by intramolecular elimination, and then generate the stable products [XNHNH2][XNHNH2] by intermolecular elimination of NH3. However, when the reaction temperature is low, [X(NH2)3][X(NH2)3] is generated by intermolecular elimination.

Lattice energy and specific impulse to hydrazoic acid, Pb, Ag and Cu azides

In this work, the gas phase formation enthalpies (kJmol−1) to hydrazoic acid, Pb, Ag and Cu azides were calculated as: HN3 = 310.00; AgN3 = 509.70, Pb(N3)2 = 768.37 and Cu(N3)2 = 855.45. The respective phase transition enthalpies (kJmol−1) are: HN3(l) → HN3(g) = 9.70; AgN3 (s) → AgN3 (g) = 198.70; Pb(N3)2(s) → Pb(N3)2(g) = 284.37 and Cu(N3)2(s) → Cu(N3)2(g) = 268.45. By using and empirical equation, the detonation velocity to cupric azide, Cu(N3)2 (s), was estimated as 7340 ms−1.

Shocking Sgr B2 (N1) with its own outflow

Aims. Because studies on complex organic molecules (COMs) in high-mass protostellar outflows are sparse, we want to investigate how a powerful outflow, such as that driven by the exciting source of the prominent hot core Sagittarius B2(N1), influences the gas molecular inventory of the surrounding medium with which it interacts. Identifying chemical differences to the hot core unaffected by the outflow and what causes them may help to better understand molecular segregation in other star-forming regions. Methods. We made use of the data taken as part of the 3 mm imaging spectral-line survey Re-exploring Molecular Complexity with ALMA (ReMoCA). We studied the morphology of the emission regions of simple and complex molecules in Sgr B2 (N1). For a selection of twelve COMs and four simpler species, spectra were modelled under the assumption of local thermodynamic equilibrium and population diagrams were derived at two positions, one in each lobe of the outflow. From this analysis, we obtained rotational temperatures and column densities. Abundances were subsequently compared to predictions of astrochemical models and to observations of L1157-B1, a position located in the well-studied outflow of the low-mass protostar L1157, and the source G+0.693-0.027 (G0.693), located in the Sgr B2 molecular cloud complex, which are other regions whose chemistry has been impacted by shocks. Results. Integrated intensity maps of SO and SiO emission reveal a bipolar structure with blue-shifted emission dominantly extending to the south-east from the centre of the hot core and red-shifted emission to the north-west. The morphology of both lobes is complex but can roughly be characterised by an emission component at a larger opening angle, containing most of the emission, and narrower features. The wider-angle component is also prominently observed in emission of S-bearing molecules and species that only contain N as a heavy element, including COMs, but also CH3OH, CH3CHO, HNCO, and NH2CHO. Rotational temperatures are found in the range of ~ 100–200 K. Abundances of N-bearing molecules with respect to CH3OH are enhanced in the outflow component compared to N1S, a position that is not impacted by the outflow. A comparison of molecular abundances with G+0.693–0.027 and L1157-B1 does not show any correlations, suggesting that a shock produced by the outflow impacts Sgr B2 (N1)’s material differently or that the initial conditions were different. Conclusions. The short distance of the analysed outflow positions to the centre of Sgr B2 (N1) lead us to propose a scenario in which a phase of hot-core chemistry (i.e. thermal desorption of ice species and high-temperature gas-phase chemistry) preceded a shock wave. The subsequent compression and further heating of the material resulted in the accelerated destruction of (mainly O-bearing) molecules. Gas-phase formation of cyanides seems to be able to compete with their destruction in the post-shock gas. The abundances of cyanopolyynes are enhanced in the outflow component pointing to (additional) gas-phase formation, possibly incorporating atomic N sourced from ammonia in the post-shock gas. To confirm such a scenario, chemical shock models need to be run that take into account the pre- and post-shock conditions of Sgr B2 (N1). In any case, the results provide new perspectives on shock chemistry and the importance of the environment in which it occurs.

Isomerism of : Accurate structural, energetic, and spectroscopic characterization.

A recent work [Ye et al. Mon. Not. R. Astron. Soc. 2023, 525, 1158] on the gas-phase formation of t-HC(O)SH, already detected in the interstellar medium, pointed out that the trans form of HC(S)OH is a potential candidate for astronomical observations. Prompted by these results, the family of isomers has been investigated from an energetic point of view using a double-hybrid density functional in combination with a partially augmented triple-zeta basis set. This preliminary study showed that the most stable species of the family are the cis and trans forms of HC(O)SH and HC(S)OH. For their structural and spectroscopic characterization, a composite scheme based on coupled cluster (CC) calculations that incorporates up to the quadruple excitations and accounts for the extrapolation to the complete basis set limit and core correlation effects has been employed. This approach opens to the prediction of rotational constants with an accuracy of 0.1%. A hybrid scheme, based on harmonic frequencies computed using the CC singles, doubles and a perturbative treatment of triples method (CCSD(T)) in conjunction with a quadruple-zeta basis set, allowed us to obtain fundamental vibrational frequencies with a mean absolute error of about 1%.

Update for Isomerization Stabilization Energies: The Fulvenization Approach.

An alternative approach for calculating aromatic stabilization energies is proposed based on transforming an (anti)aromatic ring into a fulvene isomer. This fulvenization process gives a value of 34.05 kcal·mol-1 for benzene in the singlet state and a value of -17.85 kcal·mol-1 in the triplet state. Additionally, it is possible to use experimental values (as long as they exist) for the calculation as the gas-phase formation enthalpies of benzene and fulvene, whose difference is 33.72 kcal·mol-1. On the other hand, this same approach has been evaluated on several six-membered rings, including those persubstituted, biradicals, azines, and inorganic analogues, giving results in agreement with those reported in the literature using different criteria. Additionally, it is possible to differentiate the aromaticity of the rings in polycyclic aromatic hydrocarbons according to Clar's rules. Assigning the (anti)aromatic character in various nonbenzenoid rings (neutral and charged), except for five- and seven-membered rings, is also possible. The construction of the fulvene isomers in PAHs is set such that nonaromaticity-related effects are not considered. The results show that the fulvenization approach is an effective and efficient approach that can serve as an alternative or complement to existing tools.

Evaluation of two-phase sample transport upon ablation of gelatin as a proxy for soft biological matrices using nanosecond laser ablation – inductively coupled plasma – mass spectrometry

BackgroundRecent papers on LA-ICP-MS have reported that certain elements are transported in particulate form, others in gaseous form and still others in a combination of both upon ablation of C-based materials. These two phases display different transport behaviour characteristics, potentially causing smearing in elemental maps, and could be processed differently in the ICP which raises concerns as to accuracy of quantification and emphasizes the need for matrix-matching of external standards. This work aims at a better understanding of two-phase sample transport by evaluating the peak profile changes observed upon varying parameters such as laser energy density and wavelength. ResultsIt is demonstrated that upon ablation of gelatin, elements are transported predominantly in particulate phase, but already at low laser energy density, a significant fraction of some elements is transported in the gaseous phase, which is even more expressed at higher energy density. This behaviour is element-specific since the ratio of the signal intensity for the analyte element transported in gas phase to the total signal intensity was 0 % for 23Na, 43 % for 66Zn and as high as 95 % for 13C using a 193 nm laser. The results also suggest an effect of the laser wavelength, as all elements show either the same or higher amount of gas phase formation upon ablating with 213 nm versus 193 nm. It was even established that elements that fully occur in particulate form upon ablation using 193 nm laser radiation are partly converted into gaseous phase when using 213 nm. SignificanceThis work provides a thorough investigation of the underexposed phenomenon of two-phase sample transport upon ablation of biological samples upon via LA-ICP-MS. It is shown that for some elements a fraction of the ablated material is converted and transported in the gas phase, which can lead to significant smearing effects. As such, it is important to evaluate element-specific peak profiles on beforehand and, if necessary, adapt instrument settings and slow down data acquisition.

Effects of isoprene on the ozonolysis of Δ3-carene and β-caryophyllene: Mechanisms of secondary organic aerosol formation and cross-dimerization

Elucidating the mutual effects between the different volatile organic compounds (VOCs) is crucial for comprehending the formation mechanism of atmospheric secondary organic aerosols (SOA). Here, the mixed VOCs experiments of isoprene and Δ3-carene/β-caryophyllene were carried out in the presence of O3 using an indoor smog chamber. The suppression effect of isoprene was recognized by the scanning mobility particle sizer spectrometer, online vacuum ultraviolet free electron laser (VUV-FEL) photoionization aerosol mass spectrometry, and quantum chemical calculations. The results indicate that the suppression effect of isoprene on the ozonolysis of Δ3-carene and β-caryophyllene shows fluctuating and monotonous trends, respectively. The carbon content of the precursor could be the main factor for regulating the strength of the suppression effect. Plausible structures and formation mechanisms of several new products generated from the single VOC precursor and VOC-cross-reaction are proposed, which enrich the category of VOC oxidation products. Meanwhile, a new dimerization mechanism of the RO2 + R'O2 reaction is suggested, which offers an intriguing perspective on the gas phase formation process of particle phase accretion products. The present findings provide valuable insights into clarifying the pivotal roles played by isoprene in the interplay between different VOCs and understanding of SOA formation mechanisms of VOC mixtures, especially nearby the emission origins.

Numerical gas flow simulation performed to analyze advective gas flow in a compacted clay material

Burial of nuclear waste in a clay formation can result in the generation of significant amount of hydrogen and other gases due to corrosion of metallic materials under anoxic conditions, radioactive decay of waste, and radiolysis of water. If the rate of gas production exceeds the rate of gas diffusion within the clay porewater, formation and accumulations of a discrete gas phase will occur until the gas pressure becomes large enough to surpass the surrounding material gas-entry pressure; at that critical point occurrence of dilating and advective gas flow is expected [1]. Precompacted bentonite is suggested as a sealing material for the isolation of boreholes, disposal galleries, and deposition holes in a deep geological disposal facility for radioactive waste. Both formation of new porosity and the spread of dilatant pathways have been related to the advective flow of gas in bentonite [2]. Understanding of the processes and mechanisms involved is a key aspect when evaluating the impact of gas flow on repository design and construction of any future facility. A series of gas injection tests on compacted bentonite were carried out at the British Geological Survey within DECOVALEX-2023 (D-23) Task B: MAGIC and LASGIT. The project focuses on the development of new numerical techniques for the quantitative prediction of gas flow in repository systems.&#x0D; DECOVALEX-2023 Task B started with Modelling Advection of Gas In Clays (MAGIC). The planned approach of the task includes different stages, with an initial conceptual model development phase. In this work, the D-23 MAGIC has been studied based on the same methodology as previously applied in the work for DECOVALEX-2019 Task A - stage 1A (ENGINEER) using the same material properties and modelling strategy [3, 4]. Based on the actual dimensions of the sample, the 3D finite element model includes the geometry of the sample, the injection rod, the shape of the injection tip, the back pressure and the two filters associated to the location of the sensors. Hexahedral elements have been generated utilizing the CODE_BRIGHT software [5], and the random heterogeneity have been defined considering 3 different materials (M1-M2-M3). Layer-by-layer random permeability distribution was assumed (see Figure 1) applying 2/3–1/6–1/6 weighting for intrinsic permeabilities (ki) equal to 1×10-21, 1×10-20 and 1×10-19 m2, for materials M1, M2 and M3, respectively, as well as incorporating different embedded fracture parameters.&#x0D; LASGIT (LArge Scale Gas Injection Tests modelling) is a full-scale demonstration experiment operated by SKB at the Äspö Hard Rock Laboratory Hard Rock Laboratory at a depth of 420 m [6]. The 3D finite element model geometry is generated according to the real dimension of the test. Tests measurements included pressure and rate of gas inflow, gas outflow volume and pore pressure observed at various points of the sample. The gas test 1 of the LASGIT experiment is modeled in this study using a multiphase flow in porous media approach with the aim of facilitating the creation of favorable gas migration routes assuming permeability heterogeneity (Figure 1(b)), the same strategy as the MAGIC random permeability distribution outlined above. In this case, a coupled hydro-gas 3D FEM numerical model has been developed by the UPC/Andra research team to simulate the gas flow tests using the computer software CODE_BRIGHT, including initial permeability heterogeneity throughout the model. The numerical formulation also includes embedded fractures. The proposed model has been able to reproduce satisfactorily the observed behavior of the test measurements.

Revised gas-phase formation network of methyl cyanide: the origin of methyl cyanide and methanol abundance correlation in hot corinos

ABSTRACT Methyl cyanide (CH3CN) is one of the most abundant and widely spread interstellar complex organic molecules (iCOMs). Several studies found that, in hot corinos, methyl cyanide and methanol abundances are correlated suggesting a chemical link, often interpreted as a synthesis of them on the interstellar grain surfaces. In this article, we present a revised network of the reactions forming methyl cyanide in the gas phase. We carried out an exhaustive review of the gas-phase CH3CN formation routes, propose two new reactions, and performed new quantum mechanics calculations of several reactions. We found that 13 of the 15 reactions reported in the databases KIDA and UDfA have incorrect products and/or rate constants. The new corrected reaction network contains 10 reactions leading to methyl cyanide. We tested the relative importance of those reactions in forming CH3CN using our astrochemical model. We confirm that the radiative association of CH3+ and HCN, forming CH3CNH+, followed by the electron recombination of CH3CNH+, is the most important CH3CN formation route in both cold and warm environments, notwithstanding that we significantly corrected the rate constants and products of both reactions. The two newly proposed reactions play an important role in warm environments. Finally, we found a very good agreement between the CH3CN predicted abundances with those measured in cold (∼10 K) and warm (∼90 K) objects. Unexpectedly, we also found a chemical link between methanol and methyl cyanide via the CH$_{3}^{+}$ ion, which can explain the observed correlation between the CH3OH and CH3CN abundances measured in hot corinos.