Isomerization Reactions Research Articles

Theoretical Studies on the Isomerization Kinetics of Low-Lying Isomers of the SiC4H2 System.

The low-lying isomers of SiC4H2 are investigated to understand the kinetics of isomerization pathways using density functional theory. In our earlier work, we studied the various possible isomers (J. Phys. Chem. A, 2020, 124, 987-1002) and the chemical bonding of low-lying isomers of SiC4H2 (J. Phys. Chem. A, 2022, 126, 9366-9374). Among them, four isomers, 1-ethynyl-3-silacycloprop-1-en-3-ylidene (1), 3-silapent-1,4-diyn-3-ylidene (2), 1-silapent-1,2,3,4-tetraen-1-ylidene (4), and 1-silapent-2,4-diyn-1-ylidene (5) have already been identified in the laboratory. The previously known theoretical isomer 2-methylene-1-silabicyclo[1.1.0]but-1(3)-en-4-ylidene (3) and the newly identified unknown isomer through the present kinetic studies 5-silabicyclo[2.1.0]pent-1(4),2-dien-5-ylidene (N6) remain elusive in the laboratory to date. The isomerization pathways of the low-lying isomers of SiC4H2 are predicted through the transition state structures. Intrinsic reaction coordinate analysis identifies the minimum energy reaction pathways connecting the transition state from one isomer to another of the investigated system. The present kinetic data reveal the isomerization of global minimum energy isomer 1 to thermodynamically stable low-lying isomers, 2 and 5. Interestingly, isomer 3 interconverts to the experimentally known low-energy isomer 4, the second most thermodynamically stable isomer among them. The thermodynamic and kinetic parameters of the low-lying isomers of SiC4H2 are also documented in this work. The rate coefficient and equilibrium constant for isomerization reactions are calculated using the Rice-Ramsperger-Kassel-Marcus theory. The equilibrium constant delineates the difficulties in forming N6 and 3 through the isomerization pathways. Furthermore, ab initio molecular dynamics studies dictate the stability of low-lying isomers of SiC4H2 within the time scale of the simulation.

Catalytic pyrolysis of vegetable oils over NbOPO4 for SAF and green diesel production

Drop-in biofuels for aviation have been pointed out as a sustainable alternative for reducing the emission of greenhouse gases. These fuels are hydrocarbons and have a performance and composition similar to fossil fuels. Developing conversion routes for their production has stimulated different research projects and is considered one of the most challenging areas in biofuels research. In the present study, the liquid phase was analyzed and the biohydrocarbons were obtained from vegetable oils in the distillation range of gasoline, diesel, and aviation kerosene using NbOPO4 as a catalyst in the pyrolysis process. This catalyst was characterized by XRD and gas adsorption/desorption methods (BET, BJH, and TPD). The fatty materials' simultaneous deoxygenation, cracking, and partial isomerization reactions were performed at 350 °C under N2 atmosphere (10 bar), with up to 95% conversions. Considering the overlap of carbon chains in the fuel ranges, this study demonstrates that it is possible to obtain up to 21%, 83%, and 80% of biogasoline, Sustainable Aviation Fuel (SAF), and green diesel, respectively. The products were characterized by infrared spectroscopy and gas chromatography coupled with mass spectrometry. The freezing temperatures of the products without any purification treatment and their mixtures with fossil kerosene were measured by DSC. The SAF obtained after the distillation step, presented a freezing point of − 47 °C, in agreement with the standard ASTM 7566. This innovative, sustainable, and low-cost process is promising for industrial use, mainly because hydrogen was not used, and reactions were performed in a one-pot procedure.

Catalytic pyrolysis of harmful plastic waste to alleviate environmental impacts

Wax is a detrimental byproduct of plastic waste pyrolysis causing challenges upon its release into the environment owing to persistence and potential toxicity. In this study, the valorization of wax materials through conversion into BTEX (i.e., benzene, toluene, ethylbenzene, and xylene) was achieved via catalytic pyrolysis using zeolite-based catalysts. The potential of two types of waxes, spent wax (SW), derived from the pyrolysis of plastic waste, and commercial paraffin wax (PW), for BTEX generation, was investigated. Using HZSM-5, higher yields of oil (54.9 wt%) and BTEX (18.2 wt%) were produced from the pyrolysis of SW compared to PW (32.3 and 14.1 wt%, respectively). This is due to the improved accessibility of lighter hydrocarbons in SW to Brønsted and Lewis acid sites in HZSM-5 micropores, promoting cracking, isomerization, cyclization, Diels-Alder, and dehydrogenation reactions. Further, the use of HZSM-5 resulted in significantly larger yields of oil and BTEX from SW pyrolysis compared to Hbeta and HY. This phenomenon is ascribed to the well-balanced distribution of Brønsted and Lewis acid sites and the identical geometric structure of HZSM-5 micropores and BTEX molecules. The addition of Ga to HZSM-5 further led to 2.24% and 28.30% enhancements in oil and BTEX yields, respectively, by adjusting the acidity of the catalyst through the introduction of new Lewis acid sites. The regeneration of the Ga/HZSM-5 catalyst by removing deposited coke on the spent catalyst under air partially recovered catalytic activity. This study not only offers an efficient transformation of undesirable wax into valuable fuels but also provides an environmentally promising solution, mitigating pollution, contributing to carbon capture, and promoting a healthier and more sustainable environment. It also suggests future research directions, including catalyst optimization and deactivation management, feedstock variability exploration, and techno-economic analyses for sustainable wax conversion into BTEX via catalytic pyrolysis.

Synthesis of alkyl-branched fatty acids by isomerization on micro-mesoporous ferrierite-based zeolitic materials

The synthesis of alkyl-branched fatty acid methyl esters from fatty acids is of great interest to synthesize high value-added products. The isomerization reaction catalyzed by microporous zeolites is promising due to its high yields, but rapid deactivation is its main drawback. In this work, a micromesoporous ferrierite synthesized by a well-controlled alkaline recrystallization process, was evaluated as a catalyst in the isomerization of methyl oleate. Recrystallized materials present improved accessibility of acid sites at the mouth of 10-MR pores due to the increase of the mesopore surface. Methyl oleate isomerization has been implemented in batch conditions and in a continuous flow process with experiments lasting more than 3 days. Stable yield of branched isomers higher than 50% was reached at 285 °C and 3.5 h-1 WHSV. The catalytic results, combined with a detailed analysis of fresh and spent catalysts, has provided information on the phenomena involved, in particular deactivation.

The Ehrenfest force field: A perspective based on electron density functions

The topology of the Ehrenfest force field (EhF) is investigated as a tool for describing local interactions in molecules and intermolecular complexes. The EhF is obtained by integrating the electronic force operator over the coordinates of all but one electron, which requires knowledge of both the electron density and the reduced pair density. For stationary states, the EhF can also be obtained as minus the divergence of the kinetic stress tensor, although this approach leads to well-documented erroneous asymptotic behavior at large distances from the nuclei. It is shown that these pathologies disappear using the electron density functions and that the EhF thus obtained displays the correct behavior in real space, with no spurious critical points or attractors. Therefore, its critical points can be unambiguously obtained and classified. Test cases, including strained molecules, isomerization reactions, and intermolecular interactions, were analyzed. Various chemically relevant facts are highlighted: for example, non-nuclear attractors are generally absent, potential hydrogen–hydrogen interactions are detected in crowded systems, and a bifurcation mechanism is observed in the isomerization of HCN. Moreover, the EhF atomic basins are less charged than those of the electron density. Although integration of the EhF over regions of real space can also be performed to yield the corresponding atomic forces, several numerical drawbacks still need to be solved if electron density functions are to be used for that purpose. Overall, the results obtained support the Ehrenfest force field as a reliable descriptor for the definition of atomic basins and molecular structure.

Photochromic Nucleosides and Oligonucleotides.

Photochromism is a reversible phenomenon wherein a material undergoes a change in color upon exposure to light. In organic photochromes, this effect often results from light-induced isomerization reactions, leading to alterations in either the spatial orientation or electronic properties of the photochrome. The incorporation of photochromic moieties into biomolecules, such as proteins or nucleic acids, has become a prevalent approach to render these biomolecules responsive to light stimuli. Utilizing light as a trigger for the manipulation of biomolecular structure and function offers numerous advantages compared to other stimuli, such as chemical or electrical treatments, due to its non-invasive nature. Consequently, light proves particularly advantageous in cellular and tissue applications. In this review, we emphasize recent advancements in the field of photochromic nucleosides and oligonucleotides. We provide an overview of the design principles of different classes of photochromes, synthetic strategies, critical analytical challenges, as well as structure-property relationships. The applications of photochromic nucleic acid derivatives encompass diverse domains, ranging from the precise photoregulation of gene expression to the controlled modulation of the three-dimensional structures of oligonucleotides and the development of DNA-based fluorescence modulators. Moreover, we present a future perspective on potential modifications and applications.

Degradation and Transformation Mechanisms of Zanthoxylum Alkylamides Exposed to UVB Light.

Zanthoxylum oleoresin, a concentrated extract derived from Zanthoxylum bungeanum, is rich in non-volatile, intensely flavorful substances and amide compounds, such as hydroxy-α-sanshool, hydroxy-β-sanshool, and hydroxy-ε-sanshool. The production process of Zanthoxylum oleoresin remains unstandardized, and there is still a lack of research on the precise classification and quantification of its key chemical constituents, as well as the stability of these compounds when produced using different extraction methods. This study utilized preparative liquid chromatography to extract and purify amide compounds from Zanthoxylum oleoresin, successfully isolating three sanshools: hydroxy-α-sanshool, hydroxy-β-sanshool, and hydroxy-ε-sanshool. The stability of three these sanshools under UVB irradiation in different solvents was explored in UVB-simulated sunlight conditions to investigate the degradation or transformation mechanism of Zanthoxylum alkylamides under UVB irradiation. The findings indicate a rapid decrease in the hydroxy-α-sanshool content under UVB ultraviolet light, aligning with the second-order kinetics. This study revealed alterations in the contents of hydroxy-α-sanshool, hydroxy-β-sanshool, and hydroxy-ε-sanshool and the formation of a new compound following exposure to UVB light. This new compound, along with the three sanshools, possesses a uniform m/z 264 and shares similar chemical structures. Further analysis also uncovered that these compounds are capable of undergoing isomerization reactions under UVB irradiation. This demonstrates that UVB irradiation of certain intensities can modify the concentrations and chemical structures of these Zanthoxylum alkylamides. These insights offer crucial guidance for future studies on the processing and preservation of Zanthoxylum alkylamides and their derivatives.

Functionalised [Ge9Ni] Clusters as Homogeneous Single‐Site Catalysts for Olefin Isomerisation Reactions

AbstractSingle‐site catalysis is an increasingly vital strategy to optimise heterogeneous catalytic reactions. In an ideal case, the nature and the population of catalytically active sites are all identical, which is impossible to realise on a solid support. The idea of single site catalysts is transferred from heterogeneous to homogeneous catalysis by the incorporation of a transition metal with oxidation state 0 in the surface of a small, soluble, molecular, homoatomic germanium atom cluster that also comprises main group atoms with oxidation state close to zero. An optimised synthetic protocol for four cluster compounds Hyp3Et[Ge9Ni]PR3 (R=Ph, ptolyl, iPr, Me) is presented, in which the transition metal is embedded in Ge atoms of a polyhedral cluster. The products were characterised by NMR spectroscopy, LIFDI/MS and elemental analysis and also structurally characterised for R=iPr and Me by single crystal X‐ray structure determination comprising a closo‐[Ge9Ni] core. The catalytic potential is investigated for various olefin isomerisation reactions. The formation of an active metal site is realized by ligand dissociation, which is observed for PPh3. DFT calculations of Hyp3Et[Ge9Ni]PR3 (R=Ph, iPr, Me), Hyp3Et[Ge9Ni](tolyl) and Hyp3Et[Ge9Ni](C6H12) show that ligand exchange with substrate molecules occurs during the catalytic reaction. Since the same principle, namely a bare metal atom site at the cluster surface, applies as in single‐site heterogeneous catalysis and since in contrast to classical homogeneous catalysis the metal atom is embedded in a cluster surface, the abbreviation SSHoC for Single Site Homogeneous Catalyst is proposed for this catalyst type. Since each metal atom is catalytically active, SSHoC enable a considerable increase in efficiency of catalysts and thus allow for sustainable use of expensive and less abundant transition metals.

Development, intercomparison, and evaluation of an improved mechanism for the oxidation of dimethyl sulfide in the UKCA model

Abstract. Dimethyl sulfide (DMS) is an important trace gas emitted from the ocean. The oxidation of DMS has long been recognised as being important for global climate through the role DMS plays in setting the sulfate aerosol background in the troposphere. However, the mechanisms in which DMS is oxidised are very complex and have proved elusive to accurately determine in spite of decades of research. As a result the representation of DMS oxidation in global chemistry–climate models is often greatly simplified. Recent field observations and laboratory and ab initio studies have prompted renewed efforts in understanding the DMS oxidation mechanism, with implications for constraining the uncertainty in the oxidation mechanism of DMS as incorporated in global chemistry–climate models. Here we build on recent evidence and develop a new DMS mechanism for inclusion in the UK Chemistry Aerosol (UKCA) chemistry–climate model. We compare our new mechanism (CS2-HPMTF) to a number of existing mechanisms used in UKCA (including the highly simplified three-reactions–two-species mechanism used in CMIP6 studies with the model) and to a range of recently developed mechanisms reported in the literature through a series of global and box model experiments. Global model runs with the new mechanism enable us to simulate the global distribution of hydroperoxylmethyl thioformate (HPMTF), which we calculate to have a burden of 2.6–26 Gg S (in good agreement with the literature range of 0.7–18 Gg S). We show that the sinks of HPMTF dominate uncertainty in the budget, not the rate of the isomerisation reaction forming it and that, based on the observed DMS / HPMTF ratio from the global surveys during the NASA Atmospheric Tomography mission (ATom), rapid cloud uptake of HPMTF worsens the model–observation comparison. Our box model experiments highlight that there is significant variance in simulated secondary oxidation products from DMS across mechanisms used in the literature, with significant divergence in the sensitivity of the rates of formation of these products to temperature exhibited; especially for methane sulfonic acid (MSA). Our global model studies show that our updated DMS scheme performs better than the current scheme used in UKCA when compared against a suite of surface and aircraft observations. However, sensitivity studies underscore the need for further laboratory and observational constraints. In particular our results suggest that as a priority long-term DMS observations be made to better constrain the highly uncertain inputs into the system and that laboratory studies be performed that address (1) the uptake of HPMTF onto aerosol surfaces and the products of this reaction and (2) the kinetics and products of the following reactions: CH3SO3 decomposition, CH3S + O2, CH3SOO decomposition, and CH3SO + O3.

A theoretical calculation and kinetic modeling analysis of H-abstraction from 1-octene for subsequent isomerization and [formula omitted]-dissociation

As a main component of alkenes, 1-octene is the key important intermediate in the process of oxidation or pyrolysis of n-decane or higher alkanes. In this work, quantum chemistry calculations of the 1-octene combustion and pyrolysis were illustrated for the potential energy surface, including the H-abstraction reactions and subsequent isomerization and β-dissociation reactions. The primary β-dissociation reactions after the H-abstraction reaction involving H atoms and CH3/NH2/NO2 radicals are detailed description with kinetic calculation. The rate constants of the primary pyrolysis reaction of 1-octene from 300 to 2000K were determined with the RRKM theory combined with CTST. The results indicated the H-abstraction reactions adhere to the Evans-Polanyi principle within the calculation uncertainty 1 kcal/mol. The 3-site exhibited the highest competitiveness in the 1-octene H-abstraction process and dominated, whereas the 1-site is lack of competitiveness due to overcome higher energy barriers (2–4 kcal/mol). The five-membered or six-membered ring in the H atom transfer process served as the most favorable structure for the isomerization reaction of 1-octenyl because of the lowest transition state energy. The dissociation channel for converting 1-octenyl to various octadienes and H atoms consumed the least 1-octenyl in the dissociation process. Following temperatures exceeding 570K, INT6 exhibits a preeminent 1-octenyl consumption ratio, attributed to the synergy between the enthalpy and entropy effects. The modified kinetic data of 1-octene H-abstraction reactions by four radicals and the subsequent isomerization and β-dissociation was expanded from 300 to 2000K, 0.01–100 atm. The modified model shows the better experimental prediction capabilities.

Decomposition Kinetics of Perfluoro‐(4‐methyl‐2‐pentene) (C6F12) and its Extinguishing Performance Study

AbstractThe urgent desire for Halon substitution has initiated the development of potential environment friendly alternatives since the Montreal Protocol and subsequent amendments were introduced. Atmospheric degradable agents such as halogenated alkenes, fluorinated ketones and cyclic compounds have been extensively studied. Among them, perfluoro‐(4‐methyl‐2‐pentene) (C6F12) has an analogous structure to C6F12O (Novec 1230) and has been regarded as one kind of potential substitute. In this paper, the main decomposition kinetics of C6F12 are studied numerically. In addition, the interactions between C6F12 and flames are studied experimentally. The results show that the suppression effectiveness of C6F12 is comparable to C6F12O, and the temperature rise is not obvious. The low chemical reactivity is believed to be caused by the perfluorinated methyl and isopropyl groups adjacent to olefinic carbon decreasing the reactivity of C6F12 toward OH radicals. Chemical kinetics simulation indicates that H‐induced reactions dominate the initial decomposition of C6F12, and a lot of isomers will be formed via a series of isomerization reactions and subsequent decomposition reactions at high temperature. These findings suggest the promising applicability of C6F12 and worth further study.

Host-Guest-Interaction Enhanced Nitric Oxide Photo-Generation within a Pillar[5]arene Cavity for Antibacterial Gas Therapy.

Supramolecular macrocycles with intrinsic cavities have been widely explored as containers to fabricate versatile functional materials via specific host-guest recognitions. However, relatively few studies have focused on the modulation of guest reactivity within a macrocyclic cavity. Here, we demonstrate the confinement effect of pillar[5]arene with an electron-rich and precise cavity that can dramatically enhance guest photoactivity and nitric oxide (NO) generation upon visible light irradiation. Mechanism studies reveal that it is achieved through increasing the ground state nitro-aromatic torsion angle, suppressing the intersystem crossing relaxation path of the S1 state, and accelerating the isomerization reaction path of guest molecules. This NO-generating system displays broad-spectrum antibacterial, biofilm inhibition, and dispersal activities. Moreover, it can accelerate the healing of methicillin-resistant Staphylococcus aureus (MRSA)-infected wounds in vivo.

Why Nonheme Iron Halogenases Do Not Fluorinate C-H Bonds: A Computational Investigation.

Selective halogenation is necessary for a range of fine chemical applications, including the development of therapeutic drugs. While synthetic processes to achieve C-H halogenation require harsh conditions, enzymes such as nonheme iron halogenases carry out some types of C-H halogenation, i.e., chlorination or bromination, with ease, while others, i.e., fluorination, have never been observed in natural or engineered nonheme iron enzymes. Using density functional theory and correlated wave function theory, we investigate the differences in structural and energetic preferences of the smaller fluoride and the larger chloride or bromide intermediates throughout the catalytic cycle. Although we find that the energetics of rate-limiting hydrogen atom transfer are not strongly impacted by fluoride substitution, the higher barriers observed during the radical rebound reaction for fluoride relative to chloride and bromide contribute to the difficulty of C-H fluorination. We also investigate the possibility of isomerization playing a role in differences in reaction selectivity, and our calculations reveal crucial differences in terms of isomer energetics of the key ferryl intermediate between fluoride and chloride/bromide intermediates. While formation of monodentate isomers believed to be involved in selective catalysis is shown for chloride and bromide intermediates, we find that formation of the fluoride monodentate intermediate is not possible in our calculations, which lack additional stabilizing interactions with the greater protein environment. Furthermore, the shorter Fe-F bonds are found to increase isomerization reaction barriers, suggesting that incorporation of residues that form a halogen bond with F and elongate Fe-F bonds could make selective C-H fluorination possible in nonheme iron halogenases. Our work highlights the differences between the fluoride and chloride/bromide intermediates and suggests potential steps toward engineering nonheme iron halogenases to enable selective C-H fluorination.

BUTEN-1-in BUTEN-2-yə İZOMERLƏŞMƏSİ REAKSİYASINDA BİNAR KOBALT TƏRKİBLİ KATALİZATORLARIN AKTİVLİYİ

Hydrogen is considered the main source of chemical energy that can be converted directly into electricity and with the help of fuel cells into zero emissions of harmful waste, such as volatile organic compounds, nitrogen oxides and carbon. The production of hydrogen in accordance with the generation of energy using a fuel cell is considered a promising alternative to stationary and portable power generation plants in the future. Various methods are used to produce hydrogen: water electrolysis, gasification, partial oxidation of heavy oil and steam corrugation of organic compounds. Currently, in industry, about 90% of hydrogen is produced by the reaction of high-temperature steam reforming of light fractions of natural gas or oil. An attractive alternative to hydrogen production is the reaction of ethanol steam reforming. The activity of binary iron-cobalt, zinc-cobalt and magnesium-cobalt oxide catalysts in the reaction of isomerization of butene-1 to butenes-2 was studied. Iron-cobalt, zinc-cobalt and magnesium-cobalt oxide catalysts were prepared by co-precipitation from aqueous solutions of iron, zinc, magnesium and cobalt nitrate salts. The resulting mixture was evaporated at 95-100°C, then the resulting precipitate was dried at 100-120°C and then decomposed at 250°C until the nitrogen oxides were completely separated. The resulting solid was calcined at 700°C for 10 hours. Thus, 27 catalysts were prepared satisfying the following conditions: mA/NB, where A is Mo, Ti, Cu; cis, m,n=1-9, m+n=10. The isomerization reaction of butene-1 to butene-2 was carried out at a rate of 1200 h-1 volume of feedstock in the temperature range of 150-400°C. The reactor was filled with 5 mL of 1-2 mm thick catalyst and fed with a reaction mixture consisting of butene-1 and nitrogen. The ratio of butene-1 to nitrogen is 1:9. It is shown that with increasing cobalt content in the Fe-Co-o catalyst composition, its activity in the isomerization reaction reaches a maximum on catalysts with equivalent ratio of initial elements. The ratio of trans- and cis-butenes-2 practically does not change and is equal to 1.2. It is found that both Lewis acid centers and Bransted acid centers are present on the surface of this catalyst in approximately equal amounts. It was found that with increasing zinc content in the catalyst composition, the degree of butene-1 isomerization decreases and the ratio of trans-butene-2 to cis-butene-2 increases from 0 to 2.1, indicating the presence of Lewis and Bronsted acid centers on the surface of Zn-Co-O catalysts. It is found that for the Mg-Co-O catalytic system the ratio of trans and cis butenes-2 yields varies in the range 0.8-1.5. It is shown that, in the whole temperature range studied, the atomic ratio of cobalt and magnesium has a strong influence on their activity in the reaction of Butene-1 isomerization into Butene-2. It is found that the number of Lewis and Bronsted acid centers is practically independent of the composition of the Mg-Co-O catalytic system. Based on the study of the reaction of butene-1 to butene-2 isomerization on binary cobalt-containing catalysts, it was found that the samples with equimolar ratios of primary elements in Fe-Co-O catalysts, as well as samples with one of the predominant elements in the catalytic systems Zn-Co-O and Mg-Co-O are active. Keywords: Isomerization, butene-1, butenes-2, binary catalysts Cobalt oxide.

Atmospheric Chemistry of NH2SO3H in Polluted Areas: An Unexpected Isomerization of NH2SO3H in Acid-Polluted Regions.

NH2SO3H is an effective nucleation agent for the formation of atmospheric aerosols and cloud particles. So, the ammonolysis of SO3 to form NH2SO3H without and with neutral (H2O) and basic (NH3) trace gases has been extensively investigated. However, the acidic trace gas X (X = H2SO4 and CH3SO3H)-assisted ammonolysis of SO3 is still up for debate. In this work, a comprehensive theoretical investigation of X-assisted ammonolysis of SO3 and its reverse reaction (the isomerization of NH2SO3H to form SO3-···NH3+) was carried out in the gas phase and at the air-water interface. The gas-phase results show that X-assisted isomerization of NH2SO3H to form SO3-···NH3+ is more energetically and kinetically favorable than its reverse reaction and the isomerization of NH2SO3H in the presence of H2O and NH3. Such unexpected findings revealed that gas-phase NH2SO3H is highly reactive in the presence of acidic trace gas in contrast to the high stability of NH2SO3H in neutral and basic conditions. At the air-water interface, the X-assisted isomerization reaction of NH2SO3H involves multiple water molecules. The loop structure of the reaction center (X···NH2SO3H···3H2O) promotes the transfer of protons in the water molecules to form the SO3-···NH3+ ion pair, which can then interact with several interfacial water molecules to form ammonium bisulfate. These interfacial reaction channels follow a stepwise mechanism and proceed at the picosecond time-scale. The findings of this study will contribute to a better understanding of the atmospheric behavior of NH2SO3H in polluted acidic trace gases.

Amide isomerization pathways: Electronic and structural background of protonation- and deprotonation-mediated cis-trans interconversions.

The cis-trans isomerization of amide bonds leads to wide range of structural and functional changes in proteins and can easily be the rate-limiting step in folding. The trans isomer is thermodynamically more stable than the cis, nevertheless the cis form can play a role in biopolymers' function. The molecular system of N-methylacetamide · 2H2O is complex enough to reveal energetics of the cis-trans isomerization at coupled cluster single-double and coupled cluster single-double and perturbative triple [CCSD(T)] levels of theory. The cis-trans isomerization cannot be oversimplified by a rotation along ω, since this rotation is coupled with the N-atom pyramidal inversion, requesting the introduction of a second dihedral angle "α." Full f(ω,α) potential energy surfaces of the different amide protonation states, critical points and isomerization reaction paths were determined, and the barriers of the neutral, O-protonated and N-deprotonated amides were found too high to allow cis-trans interconversion at room temperature: ∼85, ∼140, and ∼110 kJ mol-1, respectively. For the N-protonated amide bond, the cis form (ω = 0°) is a maximum rather than a minimum, and each ω state is accessible for less than ∼10 kJ mol-1. Here we outline a cis-trans isomerization pathway with a previously undescribed low energy transition state, which suggests that the proton is transferred from the more favorable O- to the N-protonation site with the aid of nearby water molecules, allowing the trans → cis transition to occur at an energy cost of ≤11.6 kJ mol-1. Our results help to explain why isomerase enzymes operate via protonated amide bonds and how N-protonation of the peptide bond occurs via O-protonation.

Atmospheric photooxidation and ozonolysis of sabinene: reaction rate coefficients, product yields, and chemical budget of radicals

Abstract. The oxidation of sabinene by the hydroxyl radical (OH) and ozone (O3) was investigated under atmospherically relevant conditions in the atmospheric simulation chamber SAPHIR (Simulation of Atmospheric Photochemistry In a Large Reaction Chamber) at Forschungszentrum Jülich, Germany. The rate coefficients of the reactions of sabinene with OH and with O3 were determined. The temperature dependence between 284 to 340 K of the rate coefficient of the reaction of sabinene with OH, kSAB+OH, was measured for the first time using an OH reactivity instrument, resulting in an Arrhenius expression of (1.67 ± 0.16) × 10−11 × exp((575 ± 30)/T) cm3 s−1. The values agree with those determined in chamber experiments in this work and reported in the literature for ∼ 298 K within the uncertainties of measurements. The ozonolysis reaction rate coefficient of sabinene (kSAB+O3) determined in chamber experiments at a temperature of (278 ± 2) K is (3.4 ± 0.8) × 10−17 cm3 s−1, which is 58 % lower than the value reported in the literature for room temperature. The measurement of products from the oxidation of sabinene by OH resulted in an acetone yield of (21 ± 15) %, a formaldehyde yield of (46 ± 25) %, and a sabinaketone yield of (18 ± 16) %. All yields determined in the chamber experiments agree well with values from previous laboratory studies within their uncertainties. In addition, the formaldehyde yield determined in this study is consistent with that predicted by the sabinene OH-oxidation mechanism which was devised from quantum chemical calculations by Wang and Wang (2018), whereas the acetone yield is about 15 % higher than that predicted by the mechanism. In the ozonolysis experiments, the analysis of product measurements results in an acetone yield of (5 ± 2) %, a formaldehyde yield of (48 ± 15) %, a sabinaketone yield of (31 ± 15) %, and an OH radical yield of (26 ± 29) %. The OH radical yield is lower than expected from the theoretical mechanism in Wang and Wang (2017), but the value still agrees within the uncertainty. An analysis of the chemical budget of OH radicals was performed for the chamber experiments. The analysis reveals that the destruction rate of the OH radical matches the production rate of OH, suggesting that there is no significant missing OH source for example from isomerization reactions of peroxy radicals for the experimental conditions in this work.

The exergy analysis of low carbon or carbon free fuels: Methane, methanol, and hydrogen under engine like conditions

The carbon dioxide (CO2) emitted from the combustion of fossil fuels is considered the main contributor to the warming of the climate system. Low carbon or carbon free fuels like methane, methanol, and hydrogen have gained a lot of attention on their ability of CO2 reduction in Internal Combustion (IC) engines. In this context, improving thermal efficiency at reduced levels of emissions by adopting low carbon fuels is meaningful in the current situation. However, the exergy loss of unconstrained combustion degrades the exergy work transform in IC engines. The present study focuses on the exergy loss and the thermomechanical exergy of these low carbon or carbon free dual fuels blended with high reactive n-heptane as the diesel surrogate under engine like conditions. Results reveal that these fuels lower the exergy loss by reducing the rate of n-heptane dehydrogenation and isomerization reaction. In addition, hydrogen in blends shows more influence on exergy loss reduction than methanol and methane. Using low carbon or carbon free fuels in blends improves the thermomechanical exergy distribution at 1000–2000 K combustion range. This is due to the fact that the reactions concerning radicals are more active than a traditional isooctane/n-heptane mixture, especially under low initial temperature and rich mixture conditions. Methanol in blend promotes the thermomechanical exergy at 1500–2000 K temperature range due to its highly intense H2O2-OH, CH2O-CO oxidation reaction, especially for the rich mixture. Furthermore, with regard to these low carbon fuels, one unit increment of thermomechanical exergy leads to approximately 0.65–1.2% improvement of second law efficiency. In conclusion, the optimization of low carbon fuels on exergy is mainly reflected in the low temperature and rich mixture combustion conditions, thus, the use of low carbon fuels does improve exergy use in combustion processes where the mixture is not uniformly distributed.

Unit cell distortion of ferrierite induced by carbonaceous deposits during skeletal isomerization of 1-butene

In this study, high temperature and ambient X-ray powder diffraction (XRD) are employed to differentiate the impacts of temperature and carbon deposition on the catalytic activity of the zeolite H-ferrierite (H-FER) via changes in the unit cell structure. XRD serves as a straightforward tool to distinguish these influences on the lattice geometry of H-FER, which ultimately affects catalytic activity. Due to its industrial relevance, the isomerization reaction of 1-butene to iso-butene is selected. Pristine H-FER undergoes a thermal contraction when heated from room temperature to 450 °C. However, internal carbonaceous deposits (i.e., deposits inside the micropores of the zeolite) counterbalance this effect, and the unit cell expands anisotropically during the reaction startup phase. Internal carbon deposits act as the driving force for this expansion and counteract external forces. During catalyst deactivation (time on stream >100 h), a slight unit cell contraction (i.e., ΔV = −6 Å3) is observed that stems from external carbon deposit condensation (i.e., deposits on the crystal surface). Characterization of an oxidatively regenerated catalyst supports this finding by proving this structural distortion to be nearly reversible. This study concludes that internal carbon species are primarily responsible for structural distortion and counteract thermal contraction, whereas external coke species are the driving force for catalyst deactivation.

Experimental and mechanistic study on isothermal oxidative pyrolysis of oil shale

The reasonable introduction of oxygen to the in situ conversion process of oil shale can effectively reduce energy consumption. However, current studies on oxidative pyrolysis often proceed under non-isothermal conditions with sharp fluctuations in temperature which hinders the accurate determination of the reaction process and the corresponding temperature in the system. In this work, we study the isothermal oxidative pyrolysis behavior of two oil shales differing in total organic carbon (TOC) at constant temperatures. The results indicated that with the increase in reaction temperature, low-temperature oxidative pyrolysis (LTOP) and high-temperature oxidative pyrolysis (HTOP) can be distinguished, where the transition temperature is approximately 340 ℃. During the LTOP, kerogen undergoes low-temperature oxidation, including oxygen addition, isomerization, and decomposition reactions. As for the HTOP, high-temperature oxidation coexists with the pyrolysis of kerogen and the subsequent coke oxidation. The pyrolysis reaction gradually dominates the entire reaction with increasing temperature. Accordingly, a temperature-dependent reaction model of oxidative pyrolysis of oil shale in isothermal conditions was validated and the real-time change in the reaction rates was obtained. Also, oil shale with a higher TOC has a more intense reaction and a higher transition temperature of HTOP and LTOP. This work provides a theoretical basis for the further application of oxidative pyrolysis in oil shale exploitation.