Gas-phase Reactions Research Articles

Area-Selective Atomic Layer Deposition of ZnO on Si\SiO2 Modified with Tris(dimethylamino)methylsilane.

Delayed atomic layer deposition (ALD) of ZnO, i.e., area selective (AS)-ALD, was successfully achieved on silicon wafers (Si\SiO2) terminated with tris(dimethylamino)methylsilane (TDMAMS). This resist molecule was deposited in a home-built, near-atmospheric pressure, flow-through, gas-phase reactor. TDMAMS had previously been shown to react with Si\SiO2 in a single cycle/reaction and to drastically reduce the number of silanols that remain at the surface. ZnO was deposited in a commercial ALD system using dimethylzinc (DMZ) as the zinc precursor and H2O as the coreactant. Deposition of TDMAMS was confirmed by spectroscopic ellipsometry (SE), X-ray photoelectron spectroscopy (XPS), and wetting. ALD of ZnO, including its selectivity on TDMAMS-terminated Si\SiO2 (Si\SiO2\TDMAMS), was confirmed by in situ multi-wavelength ellipsometry, ex situ SE, XPS, and/or high-sensitivity/low-energy ion scattering (HS-LEIS). The thermal stability of the TDMAMS resist layer, which is an important parameter for AS-ALD, was investigated by heating Si\SiO2\TDMAMS in air and nitrogen at 330 °C. ALD of ZnO takes place more readily on Si\SiO2\TDMAMS heated in the air than in N2, suggesting greater damage to the surface heated in the air. To better understand the in situ ALD of ZnO on Si\SiO2\TDMAMS and modified (thermally stressed) forms of it, the ellipsometry results were plotted as the normalized growth per cycle. Even one short pulse of TDMAMS effectively passivates Si\SiO2. TDMAMS can be a useful, small-molecule inhibitor of ALD of ZnO on Si\SiO2 surfaces.

Photoaging of phenolic secondary organic aerosol in the aqueous phase: evolution of chemical and optical properties and effects of oxidants

Abstract. While gas-phase reactions are well established to have significant impacts on the mass concentration, chemical composition, and optical properties of secondary organic aerosol (SOA), the aqueous-phase aging of SOA remains poorly understood. In this study, we performed a series of long-duration photochemical aging experiments to investigate the evolution of the composition and light absorption of the aqueous SOA (aqSOA) from guaiacyl acetone (GA), a semivolatile phenolic carbonyl that is common in biomass burning smoke. The aqSOA was produced from reactions of GA with hydroxyl radical (•OH-aqSOA) or a triplet excited state of organic carbon (3C∗-aqSOA) and was then photoaged in water under conditions that simulate sunlight exposure in northern California for up to 48 h. The effects of increasing aqueous-phase •OH or 3C∗ concentration on the photoaging of the aqSOA were also studied. High-resolution aerosol mass spectrometry (HR-AMS) and UV–Vis spectroscopy were utilized to characterize the composition and the light absorptivity of the aqSOA and to track their changes during aging. Compared to •OH-aqSOA, the 3C∗-aqSOA is produced more rapidly and shows less oxidation, a greater abundance of oligomers, and higher light absorption. Prolonged photoaging promotes fragmentation and the formation of more volatile and less light-absorbing products. More than half of the initial aqSOA mass is lost, and substantial photobleaching occurs after 10.5 h of prolonged aging under simulated sunlight illumination for 3C∗-aqSOA and 48 h for •OH-aqSOA. By performing positive matrix factorization (PMF) analysis of the combined HR-AMS and UV–Vis spectral data, we resolved three generations of aqSOA with distinctly different chemical and optical properties. The first-generation aqSOA shows significant oligomer formation and enhanced light absorption at 340–400 nm. The second-generation aqSOA is enriched in functionalized GA species and has the highest mass absorption coefficients in 300–500 nm, while the third-generation aqSOA contains more fragmented products and is the least light absorbing. These results suggest that intermediately aged phenolic aqSOA is more light absorbing than other generations, and that the light absorptivity of phenolic aqSOA results from a competition between brown carbon (BrC) formation and photobleaching, which is dependent on aging time. Although photoaging generally increases the oxidation of aqSOA, a slightly decreased O/C of the •OH-aqSOA is observed after 48 h of prolonged photoaging with additional •OH exposure. This is likely due to greater fragmentation and evaporation of highly oxidized compounds. Increased oxidant concentration accelerates the transformation of aqSOA and promotes the decay of BrC chromophores, leading to faster mass reduction and photobleaching. In addition, compared with •OH, photoaging by 3C∗ produces more low-volatility functionalized products, which counterbalances part of the aqSOA mass loss due to fragmentation and evaporation.

Unveiling the complexity of non-oxidative coupling of methane: A simplified kinetics approach

This study presents the generation and optimization of a detailed mechanism for the non-oxidative coupling of methane (NOCM), consisting of 1,112 species and 106,877 gas-phase reactions using Reaction Mechanism Generator. A mechanism reduction, performed in the Ansys Workbench, focused on four target products and employed various reduction methods, yielding an optimal skeletal mechanism of 178 species and 9,695 gas-phase reactions. Methane conversion and species concentration were evaluated using Chemkin-pro software, accounting for six independent variables, with temperature and pressure exhibiting the most significant impact. We observed that the formation rate of light hydrocarbons is higher at the initial position of the reaction, whereas aromatic hydrocarbons form more readily as reactor length increases and temperatures rise. Therefore, tailored reactor size and reaction conditions could improve the yield of C2 products under NOCM conditions. Further, a smaller reactor size at high temperatures and low pressure may optimize the formation of light hydrocarbons. Through sensitivity analyses, we identified critical reactions for product formation and aromatic coke precursors, thereby providing insights into gas-phase non-oxidative methane conversion. Further optimization studies are needed to investigate parameter interactions, aiming for optimal conversion with higher yields of target species and minimal aromatic coke precursors. These findings can inform effective reactor design and optimization strategies.

Temperature Distribution Assessment in Gas–Solid Reactive and Nonreactive Systems Heated by Microwaves

Research studies have shown that selective heating of different phases in gas–solid systems exposed to microwave (MW) irradiation leads to a temperature gradient between gas and MW absorber solids. This can suppress undesired secondary gas-phase reactions and yield apparent kinetic improvements and energy savings. However, the effects of reaction exothermicity/endothermicity, gas velocity, temperature probe location at the microscale, and MW penetration depth in fixed beds on the temperature difference between solid and gas phases exposed to MWs and temperature distribution in the fixed beds are poorly understood. Highlighting these effects was targeted in this study and accomplished with the help of multiphysics simulations by COMSOL Multiphysics (5.6). The corresponding COMSOL model was initially verified and validated by data collected from literature studies and those obtained experimentally by the authors in this study. Simulation results from the verified and validated model indicated that the temperature gradient between the gas and the MW absorber solid increases by increasing the gas velocity or switching from an endothermic reaction to no reaction and/or an exothermic reaction. In addition, the collected results show nonuniform temperature distribution in a fixed-bed reactor made of MW absorber particles irradiated by MWs due to the limited penetration depth of MWs and hotspot formation. This operational deficiency makes the large-scale design of this type of reactor very challenging for MW heating-assisted reactions.

Experimental and Computational Research on the Effect of Flow Distribution on the Growth of Boron Nitride Nanotubes by Chemical Vapor Deposition

The chemical vapor deposition (CVD) method offers a controlled and scalable approach for the large-scale production of boron nitride nanotubes (BNNTs). However, current research on BNNTs has been primarily focused on identifying precursors that can facilitate the efficient preparation of pure BNNTs. The collection of BNNTs remains a significant challenge due to the complex nature of the CVD reaction, which is influenced by the variable location of the floating aggregation of BNNTs generated from the gas phase reaction. To address this issue, we utilized B/MgO/Fe2O3, a well-established CVD precursor, to grow BNNTs in a horizontal furnace. Through experimental and computational fluid dynamics simulation analysis, we identified the impact of vortex generation and low flow rate on the floating height of BNNTs. Using the insights gained from these investigations, we designed a more efficient growth setup, laying the foundation for achieving larger-scale BNNT production in the future.

Trends of inorganic sulfur and nitrogen species at an urban site in western Canada (2004–2018)

A 15-year (2004–2018) record of measurement data of gaseous and particulate sulfur and nitrogen pollutants in air collected at an urban site in Burnaby in western Canada was analyzed for generating their decadal trends using three different methods, including linear regression, Mann-Kendall test and Theil-Sen trend estimator (MK-TS), and ensemble empirical mode decomposition (EEMD). Annual mean concentration of SO2 and SO42− decreased by about 59% and 42%, respectively, during the 15-year period. The slower decreases of SO42− than SO2 were mainly caused by the increased O3 concentration and temperature in spring and summer, which promoted conversion of SO2 to SO42− through gas-phase reaction, and by the increased aerosol pH value and availability of H2O2 in winter, which enhanced aqueous-phase SO42− formation. Accordingly, the sulfur oxidation ratio (SOR) increased by 23% or more in spring, summer, and winter during the 15-year period. Annual mean concentrations of NO2 and NO3− declined by 36% and 38%, respectively, during this period. On seasonal basis, NO3− decreased faster than NO2 in autumn and slower in winter. The non-linear responses of NO3− to NO2 concentration decreases were more evident in winter than the other seasons, partly due to the increased particulate NO3− fraction caused by decreased temperature, increased aerosol pH value, and enhanced NO3− formation caused by increased O3 concentrations. Annual mean concentration of NH3 showed small increases due to stable NH3 emission and reduced conversion of NH3 to NH4+. NH4+ concentration decreased by 51% during the 15-year period. These results suggest that reduced oxidants levels are likely responsible for weakened formation of secondary inorganic aerosols, besides emission reductions for SO2 and NO2.

Protonation Isomer Specific Ion-Molecule Radical Reactions.

Through a combination of ion-mobility filtering and laser-equipped quadrupole ion-trap mass spectrometry, the gas-phase reaction kinetics of two protonation isomers of the distonic-radical quinazoline cation are independently measured with ethylene. For these radical addition reactions, protonation site variations drive significant changes in nearby radical reactivity, and this is primarily due to through-space electrostatic effects. Furthermore, quantum chemical methods specifically designed for calculating long-range interactions, such as double-hybrid density functional theory, are required to rationalize the experimentally measured difference in reactivity.

Mist cannon trucks can exacerbate the formation of water-soluble organic aerosol and PM2.5 pollution in the road environment

Abstract. Mist cannon trucks have been widely applied in megacities in China to reduce the road dust, since they are considered to be more water saving and efficient than the traditional sprinkling trucks. However, their effect on the formation of water-soluble organic compounds and the pollution control of fine particles (PM2.5) remains unknown. We characterized the variations of chemical compositions in PM2.5 collected on the road sides during the simulated operations of mist cannon truck and traditional sprinkling truck via Fourier transform ion cyclotron resonance mass spectrometry and ion chromatography. The mass concentrations of water-soluble organic carbon in PM2.5 showed a significant increase (62 %–70 %) after air spraying. Furthermore, we found that water-soluble organic compounds, particularly organic nitrates, increased significantly via the interactions of reactive gas-phase organics, atmospheric oxidants and aerosol liquid water after air spraying, although the air spraying had a better effect on suppressing road dust than the ground aspersion. Moreover, the formation of PM2.5 on the road segment where the mist cannon truck passed by was promoted, with an increase of up to 13 % in mass concentration after 25–35 min, on average. Thus, the application of mist cannon trucks potentially worsens the road atmospheric environment through the increase in PM2.5 levels and the production of a large number of water-soluble organic compounds in PM2.5. The overall results provide not only valuable insights to the formation processes of water-soluble organic compounds associated with aerosol liquid water in the road environment but also management strategies to regulate the operation of mist cannon trucks in China.

Biomass Combustion in the Helically Coiled Domestic Boiler Combined with the Equilibrium/Chemical Kinetics CFD Approach

In the face of threats related to energy supply and climate change, the use of biomass is gaining importance, particularly in distributed energy systems. Combustion of biomass, including residue biomass, is considered one of the routes to increase the share of renewables in energy generation. The modeling of gaseous phase reactions remains crucial in predicting the combustion behavior of biomass and pollutant emissions. However, their simulation becomes a challenging task due to the computational cost. This paper presents a numerical analysis of the combustion process of a gas mixture released during biomass decomposition in a domestic 25 kW coil-type boiler. Three types of biogenic fuels were taken into consideration. The work aimed at examining the available tools for modeling gas burning, thus the geometry of the system was limited only to the 2D case. The thermodynamic equilibrium composition of pyrolysis gas was determined and implemented in Ansys to simulate the process. The computational results showed the potential of detailed, but reduced, combustion mechanisms of CH4/CO/H2 mixtures in predicting the main process features. The mechanism involving 85 reactions appeared to be more reliable compared to that comprising 77 reactions, particularly for volatiles with higher H2 content, whilst offering an acceptable calculation time. The burning characteristics obtained for volatiles with less CH4 and more H2 are in good agreement with the real operation conditions reported for the boiler.

Updating and evaluating the NH3 gas-phase chemical mechanism of MOZART-4 in the WRF-Chem model

The accuracy of determining atmospheric chemical mechanisms is a key factor in air pollution prediction, pollution-cause analysis and the development of control schemes based on air quality model simulations. However, the reaction of NH3 and OH to generate NH2 and its subsequent reactions are often ignored in the MOZART-4 chemical mechanism. To solve this problem, the gas-phase chemical mechanism of NH3 was updated in this study. Response surface methodology (RSM), integrated gas-phase reaction rate (IRR) diagnosis and process analysis (PA) were used to quantify the influence of the updated NH3 chemical mechanism on the O3 simulated concentration, the nonlinear response relationship of O3 and its precursors, the chemical reaction rate of O3 generation and the meteorological transport process. The results show that the updated NH3 chemical mechanism can reduce the error between the simulated and observed O3 concentrations and better simulate the O3 concentration. Compared with the Base scenario (original chemical mechanism simulated), the first-order term of NH3 in the Updated scenario (updated NH3 chemical mechanism simulated) in RSM passed the significance test (p < 0.05), indicating that NH3 emissions have an influence on the O3 simulation, and the effects of the updated NH3 chemical mechanism on NOx-VOC-O3 in different cities are different. In addition, the analysis of chemical reaction rate changes showed that NH3 can affect the generation of O3 by affecting the NOx concentration and NOx circulation with radicals of OH and HO2 in the Updated scenario, and the change of pollutant concentration in the atmosphere leads to the change of meteorological transmission, eventually leading to the reduction of O3 concentration in Beijing. In conclusion, this study highlights the importance of atmospheric chemistry for air quality models to model atmospheric pollutants and should attract more research focus.

Vaporization of Intact Neutral Biomolecules Using Laser-Based Thermal Desorption.

The production of a clean neutral molecular sample is a crucial step in many gas-phase spectroscopy and reaction dynamics experiments investigating neutral species. Unfortunately, conventional methods based on heating cannot be used with most nonvolatile biomolecules due to their thermal instability. In this paper, we demonstrate the application of laser-based thermal desorption (LBTD) to produce neutral molecular plumes of biomolecules such as dipeptides and lipids. Specifically, we report mass spectra of glycylglycine, glycyl-l-alanine, and cholesterol obtained using LBTD vaporization, followed by soft femtosecond multiphoton ionization (fs-MPI) at 400 nm. For all molecules, the signal from the intact precursor ion was observed, highlighting the softness and applicability of the LBTD and fs-MPI approach. In more detail, cholesterol underwent hardly any fragmentation. Both dipeptides fragmented significantly, although mostly through only a single channel, which we attribute to the fs-MPI process.

Numerical simulation and validation of reaction mechanism for the Siemens process in silicon production

The Siemens process is the most widely used silicon production routine for photovoltaic applications. However, the complicated chemistry in deposition reactor has not yet been fully understood, although there has been much research on it. In this paper, a modified reaction kinetic model was proposed to represent the gas phase and surface reactions based on the literature. The thermodynamic properties of gas phase components were re-evaluated by density functional theory since the reverse reaction rate of gas phase reaction is highly dependent on Gibbs energy change (ΔG). Most of our efforts were devoted to validating the reaction mechanism. The modified reaction model was analyzed and compared with published experimental data in two different reaction paths, deposition from trichlorosilane and etch from hydrochloride, respectively. The epitaxial growth rate could be predicted by the modified reaction model under a wide range of operating conditions.The results could provide a theoretical basis for possible computational fluid dynamics (CFD) simulations of industrial Siemens reactor in the future.

The Fate of Water in Hydrogen-Based Iron Oxide Reduction.

Gas-solid reactions are important for many redox processes that underpin the energy and sustainability transition. The specific case of hydrogen-based iron oxide reduction is the foundation to render the global steel industry fossil-free, an essential target as iron production is the largest single industrial emitter of carbon dioxide. This perception of gas-solid reactions has not only been limited by the availability of state-of-the-art techniques which can delve into the structure and chemistry of reacted solids, but one continues to miss an important reaction partner that defines the thermodynamics and kinetics of gas phase reactions: the gas molecules. In this investigation, cryogenic-atom probe tomography is used to study the quasi in situ evolution of iron oxide in the solid and gas phases of the direct reduction of iron oxide by deuterium gas at 700°C. So far several unknown atomic-scale characteristics are observed, including, D2 accumulation at the reaction interface; formation of a core (wüstite)-shell (iron) structure; inbound diffusion of D through the iron layer and partitioning of D among phases and defects; outbound diffusion of oxygen through the wüstite and/or through the iron to the next free available inner/outer surface; and the internal formation of heavy nano-water droplets at nano-pores.

NOx and N2O emissions from Ca-rich fuel conversion in oxyfuel circulating fluidized bed combustion

A part of Estonia’s power generation is based on oil shale combustion in thermal power plants, resulting in high CO2 and pollutant emissions. The presented paper is a part of long-term experimental work to investigate the combustion of Ca-rich oil shale for air and oxyfuel combustion environments. The experiments were performed in a 60 kWth circulating fluidized bed (CFB) test facility under air, O2/CO2, and O2 with the recycled flue gas (RFG) modes of inlet O2 as vol.% ranging from 21% to 52%, and with RFG of 50% and 87%. The influence of different combustion atmospheres, excess oxygen volumetric ratios in the primary oxidizer (λPr), and dense bed temperatures (TDB), on the total NOx and N2O formations, have been studied in detail.The results show that specific concentrations of NOx (per MJ) were reduced by 14% in O2/CO2 mode, and 22% in O2/RFG compared to air combustion. N2O emissions were increased in OXY21 mode and reduced significantly from 20 mg/MJ to 4 mg/MJ with increasing inlet O2% to 52% under O2/CO2. NOx and N2O emissions were the lowest of all combustion experiments at high inlet O2% with RFG application (OXY87 + RFG). Under all tested atmospheres, NOx emissions were increasing with increasing excess oxygen in the primary oxidizer. N2O enhanced with excess oxygen, accounted for the homogenous gas-phase reaction of volatile-N and the heterogenous reaction of char-N. NOx emissions at low operating TDB were decreasing with increasing bed temperatures and increased at higher TDB > 770 °C in air combustion and TDB > 850 °C in OXY30. N2O emissions were slightly decreased at higher bed temperatures under air and O2/CO2 modes. Combustion efficiency enhanced at higher inlet O2% resulting in lower CO concentrations with reduced fuel burnout, and increased NOx and N2O emissions.Overall, NOx and N2O formations were more affected by the condition of operating parameters under both air and oxyfuel combustion experiments, and the obtained results are giving confidence that oxyfuel technology does not influence the release of nitrogen emissions from oil shale combustion. To lower the cost of CO2 impurities removal, the optimal operating conditions during oil shale CFB combustion can be confirmed after considering the discussed parameters. However, further CO2 purification resulting from oil shale oxyfuel flue gas stream is possibly required.

The Function of Hydrotalcite as a Support Material in the Alcohol-Assisted Catalytic CO2 Hydrogenation Reaction.

The traditional CO2 hydrogenation reaction in gas phase always requires harsh reaction conditions to activate CO2 , resulting in huge energy consumption. However, with the assistance of 1-butanol solvent, catalytic CO2 hydrogenation can be proceeded at a mild condition of 170 °C and 30 bars. To further improve the catalytic performance of the widely studied Cu-ZnO-ZrO2 catalyst (CZZ), the catalysts were modified by incorporating hydrotalcite (HTC) as a support material. The addition of HTC significantly improved the copper dispersion and surface area of the catalyst. The performance of CZZ-HTC catalysts was investigated at varying weight percentages of HTC, and all showed higher space-time yield of methanol (STYMeOH ) compared to the commercial catalyst. Notably, CZZ-6HTC exhibited the highest methanol selectivity, further highlighting the beneficial role of HTC as a support material.

Non-oxidative coupling of methane over Mo-doped CeO2 catalysts: Understanding surface and gas-phase processes

Direct catalytic non-oxidative coupling is a promising route for the valorization of abundant methane. Understanding the mechanism is difficult because reactions at the surface of the catalyst and in the gas phase via radicals are important at the high temperatures employed. Herein, a series of Mo-doped CeO2 samples with isolated Mo sites were prepared by flame spray pyrolysis method and screened for their performance in non-oxidative coupling of methane. The selectivity to value-added C2 hydrocarbons (ethane and ethylene) among gas-phase products could reach 98%. During the reaction, the isolated Mo-oxo species in the as-prepared catalyst are reduced and convert into Mo (oxy-)carbide species, which act as the active sites for methane activation. By varying the available catalyst-free gas volume along the length of the reactor, we studied the contribution of gas-phase reactions in the formation of different products. Ethane is the primary product of non-oxidative methane coupling and, at least, a part of ethylene and most of benzene is formed through gas-phase chemistry. This work provides insights into the design of efficient catalysts for non-oxidative coupling of methane and highlights the importance of reducing the free volume in the reactor to limit secondary gas-phase reactions.

The GRETOBAPE Gas-phase Reaction Network: The Importance of Being Exothermic

The gas-phase reaction networks are the backbone of astrochemical models. However, due to their complexity and nonlinear impact on the astrochemical modeling, they can be the first source of error in the simulations if incorrect reactions are present. Over time, following the increasing number of species detected, astrochemists have added new reactions, based on laboratory experiments and quantum mechanics (QM) computations, as well as reactions inferred by chemical intuition and the similarity principle. However, sometimes no verification of their feasibility in the interstellar conditions, namely their exothermicity, was performed. In this work, we present a new gas-phase reaction network, GRETOBAPE, based on the KIDA2014 network and updated with several reactions, cleaned from endothermic reactions not explicitly recognized as such. To this end, we characterized all the species in the GRETOBAPE network with accurate QM calculations. We found that ∼5% of the reactions in the original network are endothermic, although most of them are reported as barrierless. The reaction network of Si-bearing species is the most impacted by the endothermicity cleaning process. We also produced a cleaned reduced network, GRETOBAPE-red, to be used to simulate astrochemical situations where only C-, O-, N-, and S-bearing species with less than six atoms are needed. Finally, the new GRETOBAPE network, its reduced version, and the database with all the molecular properties are made publicly available. The species property database can be used in the future to test the feasibility of possibly new reactions.

Methane Activation by [OsC3]+: Implications for Catalyst Design.

Gas-phase reactions of [OsC3]+ with methane at ambient temperature have been studied by using quadrupole-ion trap mass spectrometry combined with quantum chemical calculations. The comparison of [OsC3]+ with the product clusters revealed significant changes in cluster reactivity. In particular, with different ligands, the cluster may produce multiple products or, alternatively, just a single product. Theoretical calculations reveal the influence of electronic features such as molecular polarity index, charge and spin distribution, and HOMO-LUMO gap on the reactivity of the Os complexes. Fundamentally, it is the polarity of the clusters that leads to the cluster reactivity in the methane activation. Furthermore, reducing the local polarity of the catalyst active site may be one means of reducing the number of byproducts in the reaction.

Native and compactly folded in-solution conformers of pepsin are revealed and distinguished by mass spectrometric ITEM-TWO analyses of gas-phase pepstatin A - pepsin complex binding strength differences.

Pepsin, because of its optimal activity at low acidic pH, has gained importance in mass spectrometric proteome research as a readily available and easy-to-handle protease. Pepsin has also been study object of protein higher-order structure analyses, but questions about how to best investigate pepsin in-solution conformers still remain. We first determined dependencies of pepsin ion charge structures on solvent pH which indicated the in-solution existence of (a) natively folded pepsin (N) which by nanoESI-MS analysis gave rise to a narrow charge state distribution with an 11-fold protonated most intense ion signal, (b) unfolded pepsin (U) with a rather broad ion charge state distribution whose highest ion signal carried 25 protons, and (c) a compactly folded pepsin conformer (C) with a narrow charge structure and a 12-fold protonated ion signal in the center of its charge state envelope. Because pepsin is a protease, unfolded pepsin became its own substrate in solution at pH 6.6 since at this pH some portion of pepsin maintained a compact/native fold which displayed enzymatic activity. Subsequent mass spectrometric ITEM-TWO analyses of pepstatin A - pepsin complex dissociation reactions in the gas phase confirmed a very strong binding of pepstatin A by natively folded pepsin (N). ITEM-TWO further revealed the existence of two compactly folded in-solution pepsin conformers (Ca and Cb) which also were able to bind pepstatin A. Binding strengths of the respective compactly folded pepsin conformer-containing complexes could be determined and apparent gas phase complex dissociation constants and reaction enthalpies differentiated these from each other and from the pepstatin A - pepsin complex which had been formed from natively folded pepsin. Thus, ITEM-TWO turned out to be well suited to pinpoint in-solution pepsin conformers by interrogating quantitative traits of pepstatin A - pepsin complexes in the gas phase.

Reliable Gas Phase Reaction Rates at Affordable Cost by Means of the Parameter-Free JunChS-F12 Model Chemistry.

A recently developed strategy for the computation at affordable cost of reliable barrier heights ruling reactions in the gas phase (junChS, [Barone, V.; J. Chem. Theory Comput. 2021, 17, 4913-4928]) has been extended to the employment of explicitly correlated (F12) methods. A thorough benchmark based on a wide range of prototypical reactions shows that the new model (referred to as junChS-F12), which employs cost-effective revDSD-PBEP86-D3(BJ) reference geometries, has an improved performance with respect to its conventional counterpart and outperforms the most well-known model chemistries without the need of any empirical parameter and at an affordable computational cost. Several benchmarks show that revDSD-PBEP86-D3(BJ) structures and force fields provide zero point energies and thermal contributions, which can be confidently used, together with junChS-F12 electronic energies, for obtaining accurate reaction rates in the framework of the master equation approach based on the ab initio transition-state theory.