Showing papers on "Elementary reaction published in 2018"
11 Jun 2018
TL;DR: An alternative approach to predicting electron sources and sinks using recurrent neural networks, specifically long short-term memory (LSTM) architectures, operating directly on SMILES strings is discussed, which has shown promising preliminary results.
Abstract: Reaction predictor is an application for predicting chemical reactions and reaction pathways. It uses deep learning to predict and rank elementary reactions by first identifying electron sources and sinks, pairing those sources and sinks to propose elementary reactions, and finally ranking the reactions by favorability. Global reactions can be identified by chaining together these elementary reaction predictions. We carefully curated a data set consisting of over 11 000 elementary reactions, covering a broad range of advanced organic chemistry. Using this data for training, we demonstrate an 80% top-5 recovery rate on a separate, challenging benchmark set of reactions drawn from modern organic chemistry literature. A fundamental problem of synthetic chemistry is the identification of unknown products observed via mass spectrometry. Reaction predictor includes a pathway search feature that can help identify such products through multi-target mass search. Finally, we discuss an alternative approach to predicting electron sources and sinks using recurrent neural networks, specifically long short-term memory (LSTM) architectures, operating directly on SMILES strings. This approach has shown promising preliminary results.
141 citations
TL;DR: In this article, the effect of the size of Cu clusters on the binding strengths of reactants and reaction intermediates as well as the activation barriers for the elementary reaction steps underlying CO2 hydrogenation was investigated.
114 citations
TL;DR: In this article, the active phase, kinetics, and reaction mechanism of the water-gas-shift (WGS) reaction over the Rh-ZrO2 interface was explored.
Abstract: The industrially important water–gas-shift (WGS) reaction is a complex network of competing elementary reactions in which the catalyst is a multicomponent system consisting of distinct domains. Herein, we have combined density functional theory calculations with microkinetic modeling to explore the active phase, kinetics, and reaction mechanism of the WGS over the Rh–ZrO2 interface. We have explicitly considered the support and metal and their interface and find that the Rh–ZrO2 interface is far more active toward WGS than Rh(111) facets, which are susceptible to CO poisoning. CO2 forming on the zirconia support rapidly transforms into formate. These findings demonstrate the central role of the interface in the water–gas-shift reaction and the importance of modeling both the support and the metal in bifunctional systems.
52 citations
TL;DR: Mechanistic studies establish that the proton and electron transfer to disparate sites in a single concerted kinetic step, via multi-site concerted proton-electron transfer, represents a new elementary reaction step available to C–H bonds.
Abstract: Oxidative activation and reductive formation of C–H bonds are crucial in many chemical, industrial, and biological processes. Reported here is a new strategy for these transformations, using a form of proton-coupled electron transfer (PCET): intermolecular electron transfer coupled to intramolecular proton transfer with an appropriately placed cofactor. In a fluorenyl-benzoate, the positioned carboxylate facilitates rapid cleavage of a benzylic C–H bond upon reaction with even weak 1 e − oxidants, for example, decamethylferrocenium. Mechanistic studies establish that the proton and electron transfer to disparate sites in a single concerted kinetic step, via multi-site concerted proton-electron transfer. This work represents a new elementary reaction step available to C–H bonds. This strategy is extended to reductive formation of C–H bonds in two systems. Molecular design considerations and possible utility in synthetic and enzymatic systems are discussed.
46 citations
TL;DR: This study used ab initio quantum mechanical calculations to identify the thermodynamically preferable elementary reaction pathways of hydroxyl radical-induced acetone degradation and solved the ordinary differential equations for each species participating in the elementary reactions to predict the concentration profiles for acetone and its transformation byproducts in an aqueous phase UV/hydrogen peroxide AOP.
Abstract: Advanced oxidation processes (AOPs) that produce highly reactive hydroxyl radicals are promising methods to destroy aqueous organic contaminants. Hydroxyl radicals react rapidly and nonselectively with organic contaminants and degrade them into intermediates and transformation byproducts. Past studies have indicated that peroxyl radical reactions are responsible for the formation of many intermediate radicals and transformation byproducts. However, complex peroxyl radical reactions that produce identical transformation products make it difficult to experimentally study the elementary reaction pathways and kinetics. In this study, we used ab initio quantum mechanical calculations to identify the thermodynamically preferable elementary reaction pathways of hydroxyl radical-induced acetone degradation by calculating the free energies of the reaction and predicting the corresponding reaction rate constants by calculating the free energies of activation. In addition, we solved the ordinary differential equatio...
44 citations
TL;DR: The results indicate that the initial reaction pathway of the CL-20 molecule is the cleavage of the N-NO2 bond at different densities and that the frequency of N- NO2 bond breakage decreases at high density, which leads to a decrease in the number of final products.
Abstract: The explosive detonation reaction occurs when explosives are compressed by different shock strengths, and the degree of compression affects the chemical reaction of the detonation process. The thermal decomposition mechanism of explosives under different compression densities has thus attracted significant research interest, and a better understanding of this mechanism would be helpful for determining the mechanism of the detonation reaction of explosives. In this study, a e-CL-20 supercell was constructed, and the thermal decomposition was calculated at different compression densities and temperatures using molecular dynamics simulations based on the ReaxFF-lg reactive force field. We analyzed the effect of density on the main elementary reaction, which consists of the initial reaction and the formation of final products. In addition, we studied the effect of density on the generation of clusters and the reaction kinetics of the thermal decomposition. The results indicate that the initial reaction pathway of the CL-20 molecule is the cleavage of the N-NO2 bond at different densities and that the frequency of N-NO2 bond breakage decreases at high density. As the density increases, clusters easily form and are resistant to decomposition at the later stage of thermal decomposition, which eventually leads to a decrease in the number of final products. Increasing the initial density of CL-20 significantly increases the reaction rate of the initial decomposition but hardly changes the activation energy of the decomposition.
42 citations
TL;DR: In this paper, a combination of experiments, density functional theory (DFT) and kinetic calculations was used to systematically understand the detailed chemistry of heterogeneous mercury reaction with HBr over Fe2O3 surface.
39 citations
TL;DR: In this article, the effect of CO2 dilution on laminar premixed n-heptane/air flames were performed via the outwardly propagating spherical flames and the simulation of one-dimensional planar flames.
39 citations
TL;DR: In this paper, the NH 3 -SCR reaction mechanism of NO over CeO 2 /TiO 2 (110) surface is investigated using density functional theory (DFT) calculations.
36 citations
TL;DR: In this article, it was shown that the assembly of Si tetrahedra onto MgOH2 surfaces is a rate-limiting elementary reaction in the production of serpentine and/or talc from olivine.
35 citations
TL;DR: In this paper, the first detailed kinetic model of the low-temperature oxidation of tetrahydrofuran has been developed from ab initio calculations at the CBS-QB3 level of theory.
TL;DR: The results show that the water- assisted OH + CH2NH reaction cannot accelerate the reaction because the dominated water-assisted process depends parametrically on water concentration, and the overall reaction rate coefficients are smaller.
Abstract: In recent work, there has been considerable speculation about the atmospheric reaction of methylenimine (CH2NH), because this compound is highly reactive, soluble in water, and sticky, thus posing severe experimental challenges. In this work, we have revisited the kinetics of the OH + CH2NH reaction assisted by a single water molecule. The potential energy surfaces (PESs) for the water-assisted OH + CH2NH reaction were calculated using the CCSD(T)//BH&HLYP/aug-cc-pVTZ levels of theory. The rate coefficients for the bimolecular reaction pathways CH2NHH2O + OH and CH2NH + H2OHO were computed using canonical variational transition state theory (CVT) with small curvature tunneling correction. The reaction without water has four elementary reaction pathways, depending on how the hydroxyl radical approaches CH2NH. In all cases, the reaction begins with the formation of a single pre-reactive complex before producing abstraction and addition products. When water is added, the products of the reaction do not change, and the reaction becomes quite complex, yielding four different pre-reactive complexes and eight reaction pathways. The calculated rate coefficient for the OH + CH2NH (water-free) reaction at 300 K is 1.7 × 10-11 cm3 molecule-1 s-1 and for OH + CH2NH (water-assisted), it is 5.1 × 10-14 cm3 molecule-1 s-1. This result is similar to the isoelectronic analogous reaction OH + CH2O (water-assisted). In general, the effective rate coefficients of the water-assisted reaction are 2∼3 orders of magnitude smaller than water-free. Our results show that the water-assisted OH + CH2NH reaction cannot accelerate the reaction because the dominated water-assisted process depends parametrically on water concentration. As a result, the overall reaction rate coefficients are smaller.
TL;DR: In this paper, the effect of applied electrode potential on the linear Bronsted-Evans-Polanyi relation between the activation energy and the reaction energy across metal surfaces was investigated. And the potential-dependent activation energies used to construct the BEP relations were calculated using a Marcus theory based approach.
TL;DR: The potential energy surface involved in the thermal decomposition of 1-propanol radicals was investigated in detail using automated codes (tsscds2018 and Q2DTor) and results show significant differences between the values of the rate constants calculated with the two VTST approaches.
Abstract: The potential energy surface involved in the thermal decomposition of 1-propanol radicals was investigated in detail using automated codes (tsscds2018 and Q2DTor). From the predicted elementary reactions, a relevant reaction network was constructed to study the decomposition at temperatures in the range 1000-2000 K. Specifically, this relevant network comprises 18 conformational reaction channels (CRCs), which in general exhibit a large wealth of conformers of reactants and transition states. Rate constants for all the CRCs were calculated using two approaches within the formulation of variational transition-state theory (VTST), as incorporated in the TheRa program. The simplest, one-well (1W) approach considers only the most stable conformer of the reactant and that of the transition state. In the second, more accurate approach, contributions from all the reactant and transition-state conformers are taken into account using the multipath (MP) formulation of VTST. In addition, kinetic Monte Carlo (KMC) simulations were performed to compute product branching ratios. The results show significant differences between the values of the rate constants calculated with the two VTST approaches. In addition, the KMC simulations carried out with the two sets of rate constants indicate that, depending on the radical considered as reactant, the 1W and the MP approaches may display different qualitative pictures of the whole decomposition process.
TL;DR: In this article, the effects of CO2 and H2O on premixed turbulent oxy-fuel combustion (case oxy-H2O, case oxy-CO2) in a constant vessel are investigated.
TL;DR: This study narrows the gap between gas-phase and condensed-phase studies on methane conversion and suggests that the monofunctional mechanism probably operates in non-oxidative methane aromatization.
Abstract: The underlying mechanism for non-oxidative methane aromatization remains controversial owing to the lack of experimental evidence for the formation of the first C-C bond. For the first time, the elementary reaction of methane with atomic clusters (FeC3- ) under high-temperature conditions to produce C-C coupling products has been characterized by mass spectrometry. With the elevation of temperature from 300 K to 610 K, the production of acetylene, the important intermediate proposed in a monofunctional mechanism of methane aromatization, was significantly enhanced, which can be well-rationalized by quantum chemistry calculations. This study narrows the gap between gas-phase and condensed-phase studies on methane conversion and suggests that the monofunctional mechanism probably operates in non-oxidative methane aromatization.
TL;DR: This work underscores the importance of understanding the PCET mechanism for controlling metal hydride reactivity, which could lead to superior catalyst design for fuel production and oxidation, through quantification of the relative contributions of parallel stepwise and concerted mechanisms toward net product formation.
Abstract: Catalytic processes to generate (or oxidize) fuels such as hydrogen are underpinned by multiple proton-coupled electron transfer (PCET) steps that are associated with the formation or activation of metal-hydride bonds. Fully understanding the detailed PCET mechanisms of metal hydride transformations holds promise for the rational design of energy-efficient catalysis. Here we investigate the detailed PCET mechanisms for the activation of the transition metal hydride complex CpW(CO)2(PMe3)H (Cp = cyclopentadienyl) using stopped-flow rapid mixing coupled with time-resolved optical spectroscopy. We reveal that all three limiting PCET pathways can be accessed by changing the free energy for elementary proton, electron, and proton-electron transfers through the choice of base and oxidant, with the concerted pathway occurring exclusively as a secondary parallel route. Through detailed kinetics analysis, we define free energy relationships for the kinetics of elementary reaction steps, which provide insight into the factors influencing reaction mechanism. Rate constants for proton transfer processes in the limiting stepwise pathways reveal a large reorganization energy associated with protonation/deprotonation of the metal center (λ = 1.59 eV) and suggest that sluggish proton transfer kinetics hinder access to a concerted route. Rate constants for concerted PCET indicate that the concerted routes are asynchronous. Additionally, through quantification of the relative contributions of parallel stepwise and concerted mechanisms toward net product formation, the influence of various reaction parameters on reactivity are identified. This work underscores the importance of understanding the PCET mechanism for controlling metal hydride reactivity, which could lead to superior catalyst design for fuel production and oxidation.
TL;DR: In this article, a typical bio-oil model compound was selected to study its detailed behavior in decomposition over an active stepped Ni surface by density functional theory calculations, and the adsorption geometries and energies of various intermediates were reported.
TL;DR: In this paper, a C++ program was developed to assess ReaxFF trajectories and to analyze elementary reactions involved in the CO 2 gasification process of lignite, and the calculated overall enthalpy and entropy changes could clearly explain the experimental data.
TL;DR: In this paper, a systematic study of O2 reduction on the (1.1) facets of Pt3Sn based on periodic density functional theory (DFT) calculation is presented. And the authors verify the important role of tin in improving the catalytic activity of ORR, indicating the introduction of Sn significantly decreases the activation energy barrier.
TL;DR: In this paper, the authors investigated the sensitivity of the ignition delay time, τign, of n-butanol to uncertainties in the rate-rule kinetic rate parameters, at various initial temperatures (600-1000 K), pressures (10-80 bar), and equivalence ratios (05-20) and showed that τign is mostly sensitive to variations in four dominant reaction classes.
TL;DR: In this article, the effect of cooling rate on the reaction of volatiles from low-rank coal pyrolysis is studied using ReaxFF molecular dynamics simulations, and the activation energy for the secondary reactions of nascent tar is determined by fixed-temperature simulations from 1800 to 2800 K and is found to be 24.4
TL;DR: In this article, a packed-bed reactor containing γ-Al2O3 was used to accelerate oleic acid with ethanol under subcritical and supercritical conditions, and an Eley-Rideal (ER) reaction mechanism was proposed to describe each elementary reaction step.
Abstract: Esterification of oleic acid with ethanol was conducted under subcritical and supercritical conditions in a packed-bed reactor containing γ-Al2O3. The presence of γ-Al2O3 significantly improved the reaction rate such that the 42% yield achieved at 325 °C, 200 bar, and 1-min residence time without the alumina was increased to 98% at the same conditions when alumina was present. The catalytic capacity was attributed to Lewis acid sites on the surface of alumina, and non Bronsted acid sites were detected. Experiments to study the kinetics were executed at a pressure of 200 bar, elevated temperatures (200, 225, 275, 300, and 325 °C), and residence times of half to 8 minutes. Mass transfer limitations were estimated to be negligible via the Mears and Weisz-Prater criteria. Kinetic analysis based on the one-step model demonstrates that the overall reaction was endothermic, and an Eley-Rideal (ER) reaction mechanism was proposed to describe each elementary reaction step. The stability of γ-Al2O3 on product conversion was tested via a 25 h operation under 325 °C, 200 bar, and 1 min residence time, and decrease of the conversion was not observed. However, results of the catalyst analytical characterization shows a decrease of the acid site density and surface area, supporting the occurrence of catalyst degradation. The addition of water slightly decreased the yield, while the pressure change from 200 to 100 bar did not have an obvious effect on the conversion.
TL;DR: In this paper, an expanded version of a recently developed reaction mechanism describing liquid-phase decomposition of RDX was validated by a comparison of experimental results obtained from confined rapid thermolysis at various set temperatures.
TL;DR: Using a radio-frequency ion trap to study ion-molecule reactions under isolated conditions, a direct experimental determination of reaction rate constants for the sequential oxidation of iodine anions by ozone at room temperature (300 K) is reported.
Abstract: Using a radio-frequency ion trap to study ion-molecule reactions under isolated conditions, we report a direct experimental determination of reaction rate constants for the sequential oxidation of iodine anions by ozone at room temperature (300 K). The results are R1: I- + O3 → IO- + O2, k1 = (7 ± 2) × 10-12 cm3 s-1; R2: IO- + O3 → IO2- + O2, k2 = (10 ± 2) × 10-9 cm3 s-1; R3: IO2- + O3 → IO3- + O2, k3 = (16 ± 2) × 10-9 cm3 s-1. More oxidized forms such as IO4- and IO5- were not observed. Additionally, we performed quantum chemical calculations to elucidate the energetics of these oxidation reactions.
TL;DR: In this paper, the surface effect of sodium borohydride (NaBH4) has been investigated theoretically on Au and Pt (111) noble metal surfaces by periodic density functional theory calculations.
TL;DR: In this article, the authors performed density functional theory calculations on the decomposition of formic acid over a stepped Ni surface and obtained the zero-point energy corrected activation energies and reaction energies.
TL;DR: In this article, a single reaction framework comprising multistep mechanism, based on suitable modification of double-trap kinetic model, was presented to capture the key characteristics of the ORR, CV and oxide growth.
TL;DR: In this article, the elementary reactions of CH3OH on TiO2 surface were investigated through visualization of specific elementary steps by highly controllable electron and hole injection using scanning tunneling microscopy.
Abstract: Direct visualization and comparison of the elementary reactions induced by electrons and holes are of importance for finding a way to conduct chemical reactions and reaction sequences in a controllable manner. As a semiconductor, TiO2 provides a playground to perform the measurements, and moreover, the information can be useful for design of high-performance TiO2-based catalysts and photocatalysts. Here, we present our investigation on the elementary reactions of CH3OH on TiO2 surface through visualization of specific elementary steps by highly controllable electron and hole injection using scanning tunneling microscopy. The distinct sequential routes and their kinetics, namely, breaking C–O and O–H bonds by electrons and breaking O–H and C–H bonds by holes, respectively, have been experimentally identified and well elucidated by density functional theory calculations. Our nonlocal h-injection experimental and theoretical results suggest that the delocalized holes in the TiO2 substrate should be responsib...
TL;DR: This work provides the first comparative study on the reactivity of single noble-metal atoms (Rh, Pt) on the same cluster support (Al2O4-) and can be helpful for rational design of single-atom catalysts for selective methane conversion.
Abstract: Studying the elementary reactions of single-noble-metal-atom-doped species can give theoretical guidance for the design of related single-atom catalysis. Using a combination of mass spectrometry and density functional theory calculations, the reaction of RhAl2O4- with the most stable alkane molecule CH4 under thermal conditions has been studied. The methane tends to be converted into syngas (free H2 and adsorbed CO) with activation of four C-H bonds. In sharp contrast, formaldehyde was generated in the previously reported reaction of PtAl2O4- with CH4. Density functional theory calculations show that the difference in reactivity between RhAl2O4- and PtAl2O4- is found to be due to a higher energy barrier of the third C-H bond activation for the Pt analogue. This work provides the first comparative study on the reactivity of single noble-metal atoms (Rh, Pt) on the same cluster support (Al2O4-) and can be helpful for rational design of single-atom catalysts for selective methane conversion.