Showing papers on "Elementary reaction published in 2016"

Catalytic conjunctive cross-coupling enabled by metal-induced metallate rearrangement

Abstract: Transition metal catalysis plays a central role in contemporary organic synthesis. Considering the tremendously broad array of transition metal-catalyzed transformations, it is remarkable that the underlying elementary reaction steps are relatively few in number. Here, we describe an alternative to the organometallic transmetallation step that is common in many metal-catalyzed reactions, such as Suzuki-Miyaura coupling. Specifically, we demonstrate that vinyl boronic ester ate complexes, prepared by combining organoboronates and organolithium reagents, engage in palladium-induced metallate rearrangement wherein 1,2-migration of an alkyl or aryl group from boron to the vinyl α-carbon occurs concomitantly with C-Pd σ-bond formation. This elementary reaction enables a powerful cross-coupling reaction in which a chiral Pd catalyst merges three simple starting materials-an organolithium, an organoboronic ester, and an organotriflate-into chiral organoboronic esters with high enantioselectivity.

Quantum and classical dynamics of reactive scattering of H2 from metal surfaces.

Abstract: We review the state-of-the art in dynamics calculations on the reactive scattering of H2 from metal surfaces, which is an important model system of an elementary reaction that is relevant to heterogeneous catalysis. In many applications, quantum dynamics and classical trajectory calculations are performed within the Born–Oppenheimer static surface model. However, ab initio molecular dynamics (AIMD) is finding increased use in applications aimed at modeling the effect of surface phonons on the dynamics. Molecular dynamics with electronic friction has been used to model the effect of electron–hole pair excitation. Most applications are still based on potential energy surfaces (PESs) or forces computed with density functional theory (DFT), using a density functional within the generalized gradient approximation to the exchange–correlation energy. A new development is the use of a semi-empirical version of DFT (the specific reaction parameter (SRP) approach to DFT). We also discuss the accurate methods that have become available to represent electronic structure data for the molecule–surface interaction in global PESs. It has now become possible to describe highly activated H2 + metal surface reactions with chemical accuracy using the SRP-DFT approach, as has been shown for H2 + Cu(111) and Cu(100). However, chemical accuracy with SRP-DFT has yet to be demonstrated for weakly activated systems like H2 + Ru(0001) and non-activated systems like H2 + Pd(111), for which SRP DFs are not yet available. There is now considerable evidence that electron–hole pair (ehp) excitation does not need to be modeled to achieve the (chemically) accurate calculation of dissociative chemisorption and scattering probabilities. Dynamics calculations show that phonons can be safely neglected in the chemically accurate calculation of sticking probabilities on cold metal surfaces for activated systems, and in the calculation of a number of other observables. However, there is now sufficient evidence to suggest that the decision on whether or not to neglect phonons should be taken with care, with appropriate consideration of the observable to be computed and of the relevant surface temperature. AIMD calculations have provided valuable insights into the mechanisms that are operative in the dissociative adsorption and absorption of hydrogen on/in precovered metal surfaces. Classical and quantum dynamics calculations have shown that the reaction probability of H2 on Pt surfaces consisting of (100) steps and (111) terraces can to a very good approximation be computed as a weighted average of the reactivities on the steps and terraces. Progress obtained with dynamics calculations on the scattering of H2 from alloys and from simple low index metal surfaces is also reported. Insights that may be obtained on the reactivity of a metal surface from the prominent presence of out-of-plane diffraction or, conversely, the complete absence of diffraction, are discussed. A new field has been opened up by experiments on H2 scattering from surfaces at fast grazing incidence, and we discuss new predictions regarding diffraction and dissociative scattering of H2 under such conditions.

Uncertainty Quantification Framework Applied to the Water–Gas Shift Reaction over Pt-Based Catalysts

Abstract: This paper presents a systematic approach to quantify uncertainties of various quantities of interest (QoIs) in catalysis determined by microkinetic models developed from first principles. One of the main sources of uncertainty in any microkinetic simulation is attributed to the exchange-correlation approximations in density functional theory (DFT) used to calculate the rate constants for all elementary reaction steps within transition state theory. These DFT approximations are at the core of significant discrepancies between computational simulations and experimental measurements. Therefore, any model calculation should be accompanied by a measure of uncertainty. This work uses probability to represent uncertainties and latent variable models to develop probabilistic models that account for errors and correlations in DFT energies. These probabilistic models are further constrained to known reaction thermodynamics, and then propagated to QoIs such as turnover frequency (TOF), apparent activation barrier, ...

Faraday efficiency and mechanism of electrochemical surface reactions: CO2 reduction and H2 formation on Pt(111)

Abstract: An atomic scale model of the electrical double layer is used to calculate the mechanism and rate of electrochemical reduction of CO2 as well as H2 formation at a Pt(111) electrode. The water layer contains solvated protons and the electrode has excess electrons at the surface. Density functional theory within the generalized gradient approximation is used to describe the electronic structure while the mechanism and activation energy of the various elementary reactions is obtained by calculating minimum energy paths using the nudged elastic band method. The applied electrical potential is deduced from the calculated work function. The optimal reaction mechanism for CO2 reduction to either methane or methanol is found and the estimated rate compared with that of the competing reaction, H2 formation. When the free energy of only the intermediates and reactants is taken into account, not the activation energy, Pt(111) would seem to be a good electrocatalyst for CO2 reduction, significantly better than Cu(111). This, however, contradicts experimental findings. Detailed calculations reported here show that the activation energy for CO2 reduction is high for both Heyrovsky and Tafel mechanisms on Pt(111) in the relevant range of applied potential. The rate-limiting step of the Heyrovsky mechanism, *COOH + H+ + e− → *CO + H2O, is estimated to have an activation energy of 0.95 eV at −0.9 V vs. standard hydrogen electrode. Under the same conditions, the activation energy for H2 formation is estimated to be only 0.5 eV. This explains why attempts to reduce CO2 using platinum electrodes have produced only H2. A comparison is made with analogous results for Cu(111) [J. Hussain et al., Procedia Comput. Sci., 2015, 51, 1865] where a reaction mechanism with low activation energy for CO2 electroreduction to methane was identified. The difference between the two electrocatalysts is discussed.

Elucidating the field influence on the energetics of the methane steam reforming reaction: A density functional theory study

Abstract: To help realize lower operating-temperatures for the highly endothermic Ni-catalytic methane steam reforming (MSR) process, we focused on elucidating the influence of an applied electric field on the energetics of the said reaction. Two aspects were considered in this study: the electric field effects on (i) the adsorption and electronic properties of the MSR-involved species, and (ii) the overall MSR energy profile. Our results show that for Ni-based MSR processes, a positive field strengthens the adsorption of the reactants, promotes product desorption, impedes coke formation, lowers the overall energy profiles and consequently, reduces the temperature requirements for the overall MSR-on-Ni reaction. Based on our phase diagram obtained from first principles, we show that CO can be obtained from the dehydrogenation of COH and CHO at moderate hydrogen partial pressure values with a negative field, while methanol is formed on the surface via hydroxyl oxidation of CH 3 at high hydrogen partial pressures and positive field values. This investigation suggests ways to facilitate the MSR reforming reaction in the presence of an electric field and also points toward a number of elementary reactions that need to be considered for establishing microkinetic model studies.

Study of Hydrogen Peroxide Reactions on Manganese Oxides as a Tool To Decode the Oxygen Reduction Reaction Mechanism

Abstract: Hydrogen peroxide has been detected as a reaction intermediate in the electrochemical oxygen reduction reaction (ORR) on transition-metal oxides and other electrode materials. In this work, we studied the electrocatalytic and catalytic reactions of hydrogen peroxide on a set of Mn oxides, Mn2O3, MnOOH, LaMnO3, MnO2, and Mn3O4, that adopt different crystal structures to shed light on the mechanism of the ORR on these materials. We then combined experiment with kinetic modeling with the objective to correlate the differences in the ORR activity to the kinetics of the elementary reaction steps, and we uncovered the importance of structural and compositional factors in the catalytic activity of the Mn oxides. We concluded that the exceptional activity of Mn2O3 in the ORR is due to its high catalytic activity both in the reduction of oxygen to hydrogen peroxide and in the decomposition of the latter, and furthermore, we proposed a tentative link between crystal structure and reactivity.

Addition-Elimination or Nucleophilic Substitution? Understanding the Energy Profiles for the Reaction of Chalcogenolates with Dichalcogenides.

Abstract: We have quantum chemically explored the mechanism of the substitution reaction between CH3X(-) and the homo- and heterodichalcogenides CH3X'X″CH3 (X, X', X″ = S, Se, Te) using relativistic density functional theory at ZORA-OLYP/TZ2P and COSMO for simulating the effect of aqueous solvation. In the gas phase, all substitution reactions proceed via a triple-well addition-elimination mechanism that involves a stable three-center intermediate. Aqueous solvation, in some cases, switches the character of the mechanism to double-well SN2 in which the stable three-center intermediate has become a labile transition state. We rationalize reactivity trends and some puzzling aspects of these elementary reactions, in particular, vanishing activation energies and ghost three-center intermediates, using the activation strain model (ASM).

A base-mediated self-propagative Lossen rearrangement of hydroxamic acids for the efficient and facile synthesis of aromatic and aliphatic primary amines

Abstract: A variety of aromatic and aliphatic hydroxamic acids were converted to the corresponding primary amines via base-mediated rearrangement. This rearrangement could proceed with less than 1 equiv. of K2CO3 in polar solvents under thermal conditions with no external reagents. This rearrangement has several features including no external activating agents needed for promoting the rearrangement, less than one equivalent of a base is sufficient for the reaction, and a clean reaction in which only carbon dioxide is produced as a by-product. A self-propagating mechanism via an isocyanate intermediate is proposed and elementary reaction steps, namely, chain propagation reactions are supported by experiments.

First Principles Calculations for Hydrogenation of Acrolein on Pd and Pt: Chemoselectivity Depends on Steric Effects on the Surface.

Abstract: The chemoselective hydrogenation of acrolein on Pt(111) and Pd(111) surfaces is investigated employing density functional theory calculations. The computed potential energy surfaces together with the analysis of reaction mechanisms demonstrate that steric effects are an important factor that governs chemoselectivity. The reactions at the C=O functionality require more space than the reactions at the C=C functionality. Therefore the formation of allyl alcohol is more favorable at low coverage, while the reduction of the C=C bond and the formation of propanal becomes kinetically more favorable at higher coverage. The elementary reaction steps are found to follow different reaction mechanisms, which are identified according to terminology typically used in organometallic catalysis. The transition state scaling (TSS) relationship is demonstrated and the origin of multiple TSS lines is linked to variation of an internal electronic structure of a carbon skeleton.

Density functional theory (DFT) studies of CO oxidation reaction on M13 and Au18M clusters (M = Au, Ag, Cu, Pt and Pd): the role of co-adsorbed CO molecule

Abstract: Using the icosahedra M13 (M = Au, Ag, Cu, Pt, Pd) and hetero-atom doped Au18M (M = Ag, Cu, Pt, Pd) clusters as model systems, we have systematically investigated the role of the co-adsorbed CO molecule played in the CO oxidation reaction on the basis of density functional theory (DFT) calculations. The results indicate that the co-adsorbed CO molecule at a triangular active site can induce the dissociation of the OCOO* intermediate via a tri-molecular reaction route. This mechanism is also validated on other larger single doped gold alloy clusters such as AunAg and AunCu (n = 32–34, 54). The underlying reason for promoting the oxidation effect of a co-adsorbed CO molecule is unraveled. It is found that the relatively weaker d–π* back bonding of CO on group 11 elements like Au, Ag and Cu may increase its electrophilic activity, which can facilitate the dissociation of nearby OCOO* intermediates. For the CO molecule that is bounded to the Pd and Pt atoms, it can also induce the dissociation of OCOO* intermediate, but shows weaker electrophilic activity. By explicitly considering the elementary reaction steps in a Kinetic Monte Carlo (KMC) simulation, we have shown that the tri-molecular reaction route is an alternative reaction channel of CO oxidation, which is competitive to the conventional bi-molecular route on a doped Au18M cluster.

Highly Active and Stable CeO2-Promoted CuCl2/Al2O3 Oxychlorination Catalysts Developed by Rational Design Using a Rate Diagram of the Catalytic Cycle

Abstract: In this study, we have developed a method to predict the steady-state rate and Cu oxidation state during ethylene oxychlorination from a reaction rate diagram of the individual steps involved in the catalytic oxychlorination cycle. The steady state of the redox cycle is represented by a cross point of the reaction rates of the reduction and oxidation steps as a function of the Cu2+ in the rate diagram. Transient kinetics of elementary reactions and steady-state kinetics of the overall catalytic cycle were investigated in an operando study using combined mass and UV–vis-NIR spectrophotometry. The catalytic consequence of the promoters was then evaluated in terms of reduction and oxidation activity as well as number of active sites, site activity, and the catalyst oxidation state at steady state. Results revealed that the neat CuCl2 catalysts operated at low Cu2+ at the steady-state conditions with stoichiometric feed composition, as a result of relatively low oxidation rate of Cu1+. As a consequence of a h...

Revealing Stepwise Mechanisms in Dipolar Cycloaddition Reactions: Computational Study of the Reaction between Nitrones and Isocyanates

Abstract: The mechanism of cycloaddition reactions of nitrones with isocyanates has been studied using density functional theory (DFT) methods at the M06-2X/cc-pVTZ level of theory. The exploration of the potential energy surfaces associated with two reactive channels leading to 1,2,4-oxadiazolidin-5-ones and 1,4,2-dioxazolidines revealed that the cycloaddition reaction takes place through a concerted mechanism in gas phase and in apolar solvents but a stepwise mechanism in polar solvents. In stepwise mechanisms, the first step of the reaction is a rare case in which the nitrone oxygen acts as a nucleophile by attacking the central carbon atom of the isocyanate (interacting with the π-system of the C═O bond) to give an intermediate. The corresponding transition structure is stabilized by an attractive electrostatic interaction favored in a polar medium. The second step of the reaction is the rate-limiting one in which the formation of 1,2,4-oxadiazolidin-5-ones or 1,4,2-dioxazolidines is decided. Calculations indic...

Silane-initiated nucleation in chemically active plasmas: validation of density functionals, mechanisms, and pressure-dependent variational transition state calculations

Abstract: The growth of anionic silicon hydride clusters is a critically important process in nanodusty plasmas. In the current study, we focus on the formation of homologs of silylene (Sin+1H2n+2−, n = 3, 4) and silyl (SinH2n+1−, n = 4, 5) anions via anion–neutral reaction pathways. Species like silyl or silylene anions and their related elementary reactions, which are involved in the formation of silicon hydride clusters, were not used in developing exchange–correlation (xc) density functionals (i.e., they were not included in the training set of semiempirical density functionals); therefore, we explored the accuracy of various widely used xc density functionals based on reaction energies and barrier heights. Among the 21 density functionals we tested, M06-2X has the best performance for a hybrid functional, and MN15-L has the best performance for a local functional. Thermal rate constants of the elementary reactions involved in the reaction mechanism are calculated using M06-2X and multistructural canonical variational transition state theory with the small-curvature tunneling approximation (MS-CVT/SCT). The pressure dependence of unimolecular isomerization reactions is treated with system-specific quantum RRK theory (SS-QRRK) and the Lindemann–Hinshelwood mechanism.

Theoretical and Experimental Studies on Elementary Reactions in Living Radical Polymerization via Organic Amine Catalysis

Abstract: The reaction mechanism of living radical polymerization using organic catalysts, a reversible complexation mediated polymerization (RCMP), was studied using both theoretical calculations and experiments. The studied catalysts are tetramethylguanidine (TMG), triethylamine (TEA), and thiophene. Methyl 2-iodoisobutyrate (MMA-I) was used as the low-molar-mass model of the dormant species (alkyl iodide) of poly(methyl methacrylate) iodide (PMMA-I). For the reaction of MMA-I with TEA to generate MMA• and •I-TEA radicals (activation process), the Gibbs activation free energy for the inner-sphere electron transfer mechanism was calculated to be 39.7 kcal mol–1, while the observed one was 25.1 kcal mol–1. This difference of the energies suggests that the present RCMP proceeds via the outer-sphere electron transfer mechanism, i.e., single-electron transfer (SET) reaction from TEA to MMA-I to generate MMA• and •I-TEA radicals. The mechanism of the deactivation process of MMA• to generate MMA-I was also theoretically...

Spinodal decomposition of chemically reactive binary mixtures

Abstract: We simulate the influence of a reversible isomerization reaction on the phase segregation process occurring after spinodal decomposition of a deeply quenched regular binary mixture, restricting attention to systems wherein material transport occurs solely by diffusion. Our theoretical approach follows a diffuse-interface model of partially miscible binary mixtures wherein the coupling between reaction and diffusion is addressed within the frame of nonequilibrium thermodynamics, leading to a linear dependence of the reaction rate on the chemical affinity. Ultimately, the rate for an elementary reaction depends on the local part of the chemical potential difference since reaction is an inherently local phenomenon. Based on two-dimensional simulation results, we express the competition between segregation and reaction as a function of the Damkohler number. For a phase-separating mixture with components having different physical properties, a skewed phase diagram leads, at large times, to a system converging to a single-phase equilibrium state, corresponding to the absolute minimum of the Gibbs free energy. This conclusion continues to hold for the critical phase separation of an ideally perfectly symmetric binary mixture, where the choice of final equilibrium state at large times depends on the initial mean concentration being slightly larger or less than the critical concentration.

Performance of First-Principles-Based Reaction Class Transition State Theory

Abstract: Performance of the Reaction Class Transition State Theory (RC-TST) for prediction of rates constants of elementary reactions is examined using data from its previous applications to a number of different reaction classes. The RC-TST theory is taking advantage of the common structure denominator of all reactions in a given family combined with structure activity relationships to provide a rigorous theoretical framework to obtain rate expression of any reaction within a reaction class in a simple and cost-effective manner. This opens the possibility for integrating this methodology with an automated mechanism generator for “on-the-fly” generation of accurate kinetic models of complex reacting systems.