Transition State Theory Research Articles

10-Fold Increase in Hydrogen Atom Transfer Reactivity for a Series of S = 1 FeIV═O Complexes Over the S = 2 [(TQA)FeIV═O]2+ Complex via Entropic Lowering of Reaction Barriers by Secondary Sphere Cycloalkyl Substitution.

Nonheme iron enzymes utilize S = 2 iron(IV)-oxo intermediates as oxidants in biological oxygenations. In contrast, corresponding synthetic nonheme FeIV═O complexes characterized to date favor the S = 1 ground state that generally shows much poorer oxidative reactivity than their S = 2 counterparts. However, one intriguing exception found by Nam a decade ago is the S = 1 [FeIV(O)(Me3NTB)]2+ complex (Me3NTB = [tris((N-methyl-benzimidazol-2-yl)methyl)amine], 1O) with a hydrogen atom transfer (HAT) reactivity that is 70% that of the S = 2 [FeIV(O)(TQA)]2+ complex (TQA = tris(2-quinolylmethyl)amine, 3O). In our efforts to further explore this direction, we have unexpectedly uncovered a family of new S = 1 complexes with HAT reaction rates beyond the currently reported limits in the tripodal ligand family, surpassing oxidation rates found for the S = 2 [FeIV(O)(TQA)]2+ complex by as much as an order of magnitude. This is achieved simply by replacing the secondary sphere methyl groups of the Me3NTB ligand with larger cycloalkyl-CH2 (R groups in 2OR) moieties ranging from c-propylmethyl to c-hexylmethyl. These 2OR complexes show Mössbauer data at 4 K and 1H NMR spectra at 193 and 233 K that reveal S = 1 ground states, in line with DFT calculations. Nevertheless, they give rise to the most reactive synthetic nonheme oxoiron(IV) complexes found to date within the tripodal ligand family. Our DFT study indicates transition state stabilization through entropy effects, similar to enzymatic catalysis.

Transformation behavior and toxicity assessment of beaytlmethodeyammonNium chbride (BAC-12) disinfectant during hospital wastewater treatment.

This work focused on the transformation behavior of the emerging beaytlmethodeyammonium chbride (BAC-12) disinfectant existed in the treatment of medical sewage during its disinfection treatment. The degradation ability of ozone (O3) to BAC-12 was the best, followed by UV/NaOCl, UV, and NaOCl. The enhancement of BAC-12 in UV/NaOCl system is caused by the combined effect of UV photolysis, reactive chlorine species (RCS), and •OH. The transformation products of BAC-12 in the disinfection treatment were detected, and the chemical structure of products was rationalized by frontier molecular orbital and transition state theory methodologies. According to the ecological structure-activity relationship (ECOSAR) assessment, the intermediates of BAC-12 in UV, NaOCl, and UV/NaOCl treatments had lower half lethal concentration (LC50) and chronic toxicity (ChV) values with a higher ecotoxicity than BAC-12. O3 disinfection treatment of these toxic intermediates can significantly reduce the toxicity of the BAC-12 solution. This work provides necessary information on the potential environmental risks of BAC-12 arising from different disinfection methods in the treatment of medical wastewater.

An accurate full-dimensional potential energy surface of the hydrogen sulfide dimer system and its kinetics for the hydrogen exchange channel

The hydrogen sulfide cluster exhibits weak interaction, which plays a crucial role in biomolecules, superconductivity, etc. Hydrogen sulfide dimer, (H2S)2, as a relatively simple system containing this interaction, is a preferred subject for studying this weak interaction and has attracted significant attention recently. In this work, an accurate full-dimensional potential energy surface (PES) for the (H2S)2 system is reported using the permutationally invariant polynomial-neural network (PIP-NN) method. The energies for the sampled 106,952 configurations are calculated using the CCSD(T)-F12a/AVTZ method, with the Boys-Bernardi counterpoise method used to eliminate basis set superposition errors (BSSE). The total root-mean square error (RMSE) of the final PIP-NN PES is only 0.019 kcal mol−1. The PES can reproduce well the structures, energies, and harmonic frequencies of the nine stationary points, five stationary isomers and four transition states. Two new hydrogen exchange reaction channels are included in the current PES. The rate coefficients of the hydrogen exchange channels are calculated using both conventional transition state theory (TST) and canonical variational theory (CVT) with the quantum tunneling correction treated by the zero-curvature tunneling (ZCT) and small curvature tunneling (SCT) methods.

Organophosphate Hydrolysis by a Designed Metalloenzyme: Impact of Mutations Explained.

The efficient design of novel enzymes has been attainable only by a combination of theoretical approaches and experimental refinement, suggesting inadequate performance of de novo design protocols. Based on the analysis of the evolutionary trajectory of a designed organophosphate hydrolase, this work aimed at developing and validating the improved theoretical models describing the catalytic activity of five enzyme variants (including wild-type as well as theoretically derived and experimentally refined enzymes) performing the hydrolysis of diethyl 7-hydroxycoumarinyl phosphate. The following aspects possibly important for enzyme design were addressed: the level of theory sufficient for a reliable description of enzyme-reactant interactions, the issue of ground state (GS) destabilization versus transition state (TS) stabilization, and the derivation of the proper side chain rotamers of amino acid residues. For enzyme variants analyzed herein, differential transition state stabilization (DTSS, i.e., preferential TS binding by an enzyme over the GS binding) calculated with a non-empirical model of the interaction energy (i.e., multipole electrostatic plus approximate dispersion terms, MED) displayed a superior performance in ranking the enzyme catalytic activity. The MED DTSS-based systematic rotamer refinement performed with an efficient scanning procedure and accounting for long-range interaction energy terms is an important step capable of unlocking the full potential impact of the given residue that could be otherwise overlooked with a conventional static approach featuring optimization to the nearby minimum. While TS stabilization is the main factor contributing to the increased catalytic activity of the de novo-designed variant studied in this work, directed evolution refinement appears to impact the catalytic activity of another enzyme variant analyzed herein via GS destabilization.

Predictive evaluation of hydrogen diffusion coefficient on Pd(111) surface by path integral simulations using neural network potential

Quantum diffusion of hydrogen (H) on palladium (Pd) (111) was studied using two types of path integral simulations: quantum transition state theory (QTST) and ring polymer molecular dynamics (RPMD). The use of an artificial neural network potential trained by density functional theory calculations has made it feasible to perform path integral simulations considering nuclear quantum effects (NQEs) of a many-body Pd-H system with accuracy. The QTST result has shown a clear non-Arrhenius behavior in the diffusion coefficient (D) of H below the temperature of 150–200 K due to the NQEs. Comparing the D on Pd surface and bulk Pd, it was found that surface and bulk diffusion are competitive at high temperature. Consistent with this, it was observed in the RPMD simulation at 800 K that a part of the quantum trajectories of H on the surface bifurcates to the Pd subsurface. As the temperature decreases, the surface diffusion becomes much faster than the bulk diffusion. Furthermore, distinct quantum behaviors of H were identified at the surface and within the bulk. The D values obtained from this study using a “flexible” Pd surface model differed considerably from those previously reported using “frozen” Pd surface models, indicating an important contribution from Pd vibrations coupled with H diffusion. Published by the American Physical Society 2024

Polyethylene Glycol‐Based Refolding Kinetic Modulation of CRABP I Protein

ABSTRACTCrowding environment has a significant impact on the folding and stability of protein in biological systems. In this work, we have used four different sizes of a molecular crowder, polyethylene glycol (PEG), to analyze the unfolding and refolding kinetics of an iLBP protein, CRABP I, using urea as chemical denaturant. In general, the stability of the native state of the protein is boosted by the presence of crowding agents in the solution. However, our findings show that not only the type of crowder but also the crowder size played a key role in the effects of excluded volume. In case of lower molecular weight of PEG (M.W. 400), even at 200 g/L concentration, only the viscosity effect is observed, whereas for higher molecular weight of PEG (M.W. 1000), both the viscosity effect and excluded volume effect are noticed, and even at a higher concentration (200 g/L) of PEG 1000, the excluded volume predominates over the viscosity effect. Using the transition state theory, we were also able to determine the free energies of activation for the unfolding and refolding studies from their respective rate constants. Additionally, MD simulation studies provide strong support for our experimental observation. Analysis of secondary structure propensity (SSP) reveals a marked decline in the presence of structural elements (β‐sheet, β‐bridge, turn, and α‐helix) from 81% to 43% over the 1 μs time scale unfolding MD simulation under 8 M urea conditions. Conversely, in a 200 ns refolding simulation, the rate of refolding notably increases at a concentration of 200 g/L PEG 1000.

Positive activation entropy of Bacillus circulans xylanase catalyzed ONPX2 hydrolysis: A mechanistic and engineering study

Transition state (TS) stabilization by enzymes greatly accelerates catalytic reactions. For some enzymes, the TS complex has entropy higher than enzyme substrate (ES) complex. But the origin of favorable entropy remains unclear. In this work, we studied the mechanism of Bacillus Circulans xylanase (BCX) 11 catalyzed o-nitrophenyl β-xylobioside (ONPX2) glycoside hydrolysis. The catalytic reaction exhibits a positive activation entropy, and an increase in ionic strength leads to a decrease in entropy without affecting the activation free energy, indicating that the entropy is predominantly influenced by electrostatic forces. Moreover, NMR measurements of electrostatic attractions within the active site demonstrate a positive entropy, aligning with molecular dynamics (MD) simulations showing that electrostatic interactions contribute to the entropic stabilization of the TS complex. These findings suggest that the positive entropy primarily originates from alterations in electrostatic interactions due to the formation of the oxocarbenium ion at C1 in the TS. Differences of electrostatic interactions in ES and TS modify hydrogen bonding of surrounding residues in the active site which causes their side chain dynamics and thus conformational entropy changes. Residues critical for the positive activation entropy are identified. A new BCX mutant with an increased activation entropy and catalytic activity is found.

Kinetic study of hydrogen abstraction and unimolecular decomposition reactions of diethylamine during pyrolysis and oxidation

Diethylamine (DEA) represents a nitrogen-containing bio-oil model compound and an amine-model compound during the co-combustion of ammonia and hydrocarbons. Kinetics of the hydrogen abstraction reactions by H, O, OH, O2, HO2, CH3, and C2H5, and unimolecular decomposition reactions including 1,3-elimination, 1,2-hydrogen transfer, and C-N/C-C bond dissociations of DEA are theoretically investigated. Using the DLPNO-CCSD(T)/CBS(T-Q) method as a benchmark, cost-effective M06-2X/def2-TZVP method is selected for kinetic investigation. High-pressure-limit rate constants for the reactions with tight saddle points are determined by canonical variational transition-state theory with the multi-structural torsional anharmonicity and small-curvature tunneling. Hydrogen abstraction reactions at the C site connecting to the N site dominant in the low-to-mediate temperature range. Pressure dependent rate constants for the unimolecular decomposition reactions are determined by SS-QRRK/MSC-Dean approach. 1,3-elimination and C-C dissociation reactions dominant at the low-to-mediate and high temperature ranges, respectively.

Theoretical research of the NO and N2O reduction by char in oxy-fuel CFBC: Influence of H2O

Oxy-fuel CFBC has lower NOx emissions compared to air combustion, with the high H2O content affecting NO and N2O reduction. This study, using density functional theory and transition state theory, investigates the impact of H2O on the char model at the electronic level and its subsequent influence on NO and N2O reduction. Results reveal that H2O adsorbs onto the active site on char, forming hydroxyl groups and free H atoms. The unstable O atom in –OH group leads to benzene ring rearrangement in the char model, creating additional active sites. NO reduction follows two paths: producing CO(R1) or CO2(R2). For N2O reduction(R3), N2O reduction precedes benzene ring cracking, with CO2 production providing more active sites. In R1, CO formation limits reactive sites and increases activation energy, while R2 and R3 favor CO2 formation. Reaction pathways leading to CO2 formation show lower activation energies and higher pre-exponential factors for N2 release.

Hydrogen Atom Abstraction Reaction from Silane with Hydrogen and Methyl Radicals: Rate Constants and Kinetic Isotopic Effects.

The thermal rate constants and H/D kinetic isotope effects (KIEs) of the reactions SiH4 + H SiH3 + H2 and SiH4 + CH3 SiH3 + CH4 were calculated with the variational transition state theory including multidimensional tunneling corrections (VTST-MT). The ωB97X-D and CCSD(T) methods were employed to compute the barrier height and reaction energy. The ωB97X-D/aug-cc-pVTZ methodology was used to build reaction paths, and the CCSD(T)/CBSQ-5 approach was used to improve the energetics of the stationary points in the calculations of CVT/μOMT thermal rate constants. For the SiH4 + H pathway, the CVT/μOMT rate constants and H/D KIEs are in excellent agreement with previous experimental and theoretical calculations; otherwise, for the SiH4 + CH3 pathway, which has few experimental and theoretical data available, it has provided, for the first time, rate constants and kinetic isotope effects using variational transition state theory with multidimensional tunneling corrections.

Kinetic study of the growth of PAHs from biphenyl with the assistance of phenylacetylene

Biphenyl is a crucial precursor to polycyclic aromatic hydrocarbons (PAHs), and phenylacetylene is an abundant product in aromatic hydrocarbons combustion. By exploring the reaction kinetics of phenylacetylene with biphenyl radicals, we further explore the novel hydrogen-abstraction phenylacetylene-addition (HAPaA) mechanism which is recently proposed to account for alternative mass growth pathways of PAHs. A combination of M06–2X/6–311+G(d,p) and PWPB95-D3/def2-QZVPP calculations were performed to construct the potential energy surfaces, and the rate coefficients were determined via solution of transition state theory based master equations. We demonstrate the capability of biphenyl species to grow with the assistance of phenylacetylene by unraveling the ring growth process of biphenyl radicals, establishing the main evolution routes of key intermediates, and quantifying the competition relationship between various channels. Energetic analysis and kinetic calculations demonstrate that the initial orientations of the reacting moieties do have a remarkable impact on the detailed kinetics of the entrance channels. However, the two adducts formed from initial additions achieve a rapid equilibrium because of the largest rate constants of the interconversion reactions between them, which counteracts the orientation effect on the overall kinetics. Further reaction pathways and corresponding products are related to the aryl radical position. Specifically, the aromatics of 4-phenylphenanthrene or phenanthrene, formed through cyclization reactions followed by hydrogen or phenyl eliminations, are preferred for the “armchair” type 2-biphenyl radical. In contrast, products featuring a triple bond generated through CH β-scission reactions are favored for the “free” type 3-biphenyl and 4-biphenyl radicals. The present research can serve as a good basis for further experimental and modeling studies of PAHs and soot formation.

Electrostatically Dominated Pre-Organization in Cyclodextrin Metal-Organic Frameworks.

Electrostatic interactions between oppositely charged entities play a key role in pre-organizing substrates and stabilizing transition states of reactions in enzymes. The use of electrostatic interactions to pre-organize ions in nanoconfined pores, however, has not been investigated to its full potential. Herein, we describe how carboxylate anions can be pre-organized at the behest of their electrostatic interactions with K+ cations in nanoconfined tunnels present in γ-cyclodextrin metal-organic frameworks, i.e., CD-MOFs. Several carboxylate anions, which are all much smaller than the cavities of the tunnels, were visualized by X-ray crystallography when nanoconfined in CD-MOFs, despite the large voids present in the tunnels. These anions were found to be aligned within a planar array defined by four K+ cations, positioned around the periphery of the tunnels. The strong electrostatic interactions between the carboxylate anions and the K+ cations dictate the orientation of the anions and override the influence of all other possible noncovalent bonding interactions between them and the tunnels. Consequently, the aligned pairs of γ-cyclodextrin rings constituting the tunnels become distorted, resulting in them having lower symmetry and fewer disordered carboxylate anions in the solid state. Our findings offer a transformative strategy for controlling the packing and orientation of ions in nanoconfined environments.

NO2 properties that affect its reaction with pristine and Pt-doped SnS2: a gas sensor study.

The reaction of NO2 with pristine and Pt-doped SnS2 surfaces is investigated theoretically and compared with the experiment. Transition state theory formalism for gas sensors is adopted to present NO2 gas sensing. The dissociation temperature at approximately 150 °C is found to be of great importance in NO2 reactions. The adsorption and transition states of NO2 with pristine and Pt-doped SnS2 are calculated. Pt doping includes 0.5, 1, and 1.5% in accordance with available experimental results. The variation of thermodynamic quantities such as Gibbs free energy with Pt concentration and temperature is calculated. Transition state theory parameters that are suitable for the present sensor are determined. The results include the variation in response time with temperature, Pt concentration, and NO2 concentration. Response and response time as a function of temperature are rarely investigated in theoretical calculations, which is one of the advantages of the present study. Optimum response temperature and Pt concentration are found. The results agree with available experimental results. Density functional theory at the B3LYP level optimize molecular structures. 6-311G** basis set is used for all elements except Sn and Pt treated using SDD basis set. Gaussian 09 program and its facilities are used to perform present optimizations.

A Roadmap for Simulating Chemical Dynamics on a Parametrically Driven Bosonic Quantum Device.

Chemical reactions are commonly described by the reactive flux transferring the population from reactants to products across a double-well free energy barrier. Dynamics often involves barrier recrossing and quantum effects like tunneling, zero-point energy motion, and interference, which traditional rate theories, such as transition-state theory, do not consider. In this study, we investigate the feasibility of simulating reaction dynamics using a parametrically driven bosonic superconducting Kerr-cat device. This approach provides control over parameters defining the double-well free energy profile, as well as external factors like temperature and the coupling strength between the reaction coordinate and the thermal bath of nonreactive degrees of freedom. We demonstrate the effectiveness of this protocol by showing that the dynamics of proton-transfer reactions in prototypical benchmark model systems, such as hydrogen-bonded dimers of malonaldehyde and DNA base pairs, could be accurately simulated on the currently accessible Kerr-cat devices.

A DFT-based kinetic equation for Co3O4 decomposition reaction in high-temperature thermochemical energy storage

Thermal energy storage supports stable grid integration of variable renewable energy sources by reducing power curtailment and generation costs. Thermochemical energy storage (TCES) offers high energy density and long-duration, long-distance storage advantages, making it a focus in large-scale applications such as concentrated solar power plants. Metal oxide systems like Co3O4/CoO are widely used due to their operational flexibility, yet limited understanding of their decomposition kinetics hinders optimization of TCES materials and reactor design. In this study, we developed a rate equation based on density functional theory and transition state theory to identify the reaction mechanisms and rate constants at the gas–solid interface, prior to predict Co3O4 decomposition kinetics. A microkinetic model was then constructed to couple surface reactions with oxygen ion diffusion across the bulk, which was integrated into a reactor model accounting for mass transfer steps. The model’s accuracy was validated against the data from the micro-fluidized bed thermogravimetric analyzer experiments. For particles smaller than 150 μm, the formation step of adsorbed O2 (energy barrier of 0.8 eV) controls the reaction rate, while gas diffusion dominates the reaction rate for particles larger than 500 μm. To conclude, the optimal conditions for maximizing charge–discharge kinetics in TCES applications were identified as: 850–900 °C, 0–10 vol% O2, and 150–500 μm. This model provides theoretical guidance for optimizing TCES materials and reactor design, reducing experimental costs while maintaining accuracy.

Understanding the Directed Evolution of a Natural-like Efficient Artificial Metalloenzyme.

The artificial metalloenzyme containing iridium in place of iron along with four directed evolution mutations C317G, T213G, L69V, and V254L in a natural cytochrome P450 presents an important milestone in merging the extraordinary efficiency of biocatalysts with the versatility of small molecule chemical catalysts in catalyzing a new-to-nature carbene insertion reaction. This is a show-stopper enzyme, as it exhibits a catalytic efficiency similar to that of natural enzymes. Despite this remarkable discovery, there is no mechanistic and structural understanding as to why it displays extraordinary efficiency after the incorporation of the four active site mutations by directed evolution methods, which so far has been intractable to any experimental methods. In this study, we have deciphered how directed evolution mutations gradually alter the protein conformational ensemble to populate a catalytically active conformation to boost a multistep catalysis in a natural-like artificial metalloenzyme using large-scale molecular dynamics simulations, rigorous quantum chemical (QM), and multiscale quantum chemical/molecular mechanics (QM/MM) calculations. It reveals how evolution precisely positions the cofactor-substrate in an unusual but effective orientation within a reshaped active site in the catalytically active conformation stabilized by C-H···π interactions from more ordered mutated L69V and V254L residues to achieve preferential transition state stabilization compared to the ground state. This work essentially tracks down in atomistic detail the shift in the conformational ensemble of the highly active conformation from the less efficient single mutant to the most efficient quadruple mutant and offers valuable insights for designing better enzymes. The active conformation correctly reproduces the experimental barrier height and also accounts for the catalytic effect, which is in good agreement with experimental observations. Moreover, this conformation features an unusual bonding interaction in a metal-carbene species that preferentially stabilizes the rate-determining formation of an iridium porphyrin carbene intermediate to render the observed high catalytic rate acceleration. Our study provides crucial insights into the underlying rationale for directed evolution, reports the major catalytic role of nonelectrostatic interactions in enzyme catalysis different from the electrostatic model, and suggests a crucial principle toward designing enzymes with natural efficiency.

Structure-performance relationship and molecular structure optimization design of acid phosphate ester extractants

The extraction performance of acid phosphate ester extractants (PEEs) decreases significantly as the acidity of the aqueous phase increases. Therefore, finding PEEs suitable for high-acid environments has become critical to addressing their performance attenuation. However, the unclear structure-performance relationship of PEEs has led to a lack of guidance in experimental synthesis, significantly impeding the emergence of new acid-resistant PEEs. To tackle these issues, ten novel phenyl phosphate ester extractants were designed based on the molecular formula of phosphodiester (R1R2P(=O)OH) and the conjugation effect of the benzene ring. The relationship between the intrinsic extraction performance of acid PEEs and substituent groups (phenoxy, alkoxy, and alkyl) was quantitatively established by calculating the Gibbs free energy changes of three primary reactions (dimer dissociation, monomer acid ionization, and metal coordination) during divalent cobalt ion extraction. Additionally, the impact of different substitution sites (para, meta, and ortho) of the benzene ring on the extraction performance of acid PEEs was studied. The results indicated that introducing phenoxy groups into traditional acid PEEs (P204 and P507) can effectively enhance the extractants’ acid ionization ability and thermodynamic driving force. The extraction performance of acid PEEs is positively correlated with the number of phenoxy groups, and carbon chain substitution at the meta-position of the benzene ring is the most advantageous. By screening five low-toxicity and low-cost phenolic hydroxyl reagents currently available on the market, design a phenyl phosphate ester extractant ([(CH3)3CCH2C(CH3)2C6H4O]2P(=O)OH, P-4TO) that combines cost and performance advantages. Furthermore, the transition state theory was used to confirm the feasibility of preparing P-4TO, which provided a comprehensive theoretical research method for the future design and synthesis of efficient PEEs.

High-Level Calculation for Assessing the Atmospheric Reactivity of Pentachlorophenol with Hydroxyl Radical: Mechanism and Kinetics.

This work aims to investigate the reactivity of the pentachlorophenol (PCP) compound, C6Cl5OH, with a hydroxyl (OH) radical in the gas phase. Geometry optimizations and vibrational frequency calculations were performed using DFT-M06-2X/6-311++G(d,p). Single-point energy calculations were carried out using the single-reference coupled cluster method, specifically CCSD(T), and the multiconfigurational CASPT2 level of theory to characterize the atmospheric degradation processes of PCP with OH radicals and to identify which chemical species resulting from their decomposition could remain in the gas phase. The performance of various families of basis sets was tested. The widely used augmented-correlation-consistent basis set family aug-cc-pVXZ-DK (X = D, T, and Q) was compared to the atomic natural orbital basis sets ANO-RCC-VXZP (X = D, T, and Q). The energy profile at 298 K showed that the Cl- and OH-abstractions are not energetically favorable. H-abstraction and OH-addition/Ck (k = 1-6) are characterized by the lowest Gibbs energies of activation and are strongly exothermic. The canonical transition state theory (TST) with a simple Wigner tunneling correction is used to predict the rate constants over the 220-400 K temperature range for each reaction channel. The overall rate constant at 298 K based on our calculations is about 3.25 × 10-13, 3.10 × 10-15, and 2.21 × 10-15 cm3 molecule-1 s-1 at M06-2X, CASPT2/ANO-RCC-VQZP, and CASPT2/aug-cc-pVQZ-DK levels of theory, respectively. The PCP atmospheric lifetime at 298 K is predicted to be in the range from 1 to 12-16 years at 0 km in the presence of variable OH concentration (9.00 × 105 to 1.50 × 107 molecules cm-3) based on CASPT2 data.

Understanding the Negative Apparent Activation Energy for Cu2O and CoO Oxidation Kinetics at High Temperature near Equilibrium

The pairs of Cu2O/CuO and CoO/Co3O4 as the carriers of transferring oxygen and storing heat are essential for the recently emerged high-temperature thermochemical energy storage (TCES) system. Reported research results of Cu2O and CoO oxidation kinetics show that the reaction rate near equilibrium decreases with the temperature, which leads to the negative activation energy obtained using the Arrhenius equation and apparent kinetics models. This study develops a first-principle-based theoretical model to analyze the Cu2O and CoO oxidation kinetics. In this model, the density functional theory (DFT) is adopted to determine the reaction pathways and to obtain the energy barriers of elementary reactions; then, the DFT results are introduced into the transition state theory (TST) to calculate the reaction rate constants; finally, a rate equation is developed to describe both the surface elemental reactions and the lattice oxygen concentration in a grain. The reaction mechanism obtained from DFT and kinetic rate constants obtained from TST are directly implemented into the rate equation to predict the oxidation kinetics of Cu2O without fitting experimental data. The accuracy of the developed theory is validated by experimental data obtained from the thermogravimetric analyzer (TGA). Comparing the developed theory with the traditional apparent models, the reasons why the latter cannot appropriately predict the true oxidation characteristics are explained. The reaction rate is jointly controlled by thermodynamics (reaction driving force) and kinetics (reaction rate constant). Without considering the effect of the reaction driving force, the negative apparent activation energy of Cu2O oxidation is obtained. However, for CoO oxidation, the negative apparent activation energy is still obtained although the effect of the reaction driving force is considered. According to the DFT results, the activation energy of the overall CoO oxidation reaction is negative, but the energy barriers of the elementary reactions are positive. Moreover, according to the first-principle-based rate equation theory, the pre-exponential factor in the kinetic model is dependent on the partition function ratio and decreases with the temperature for the Cu2O and CoO oxidation near equilibrium, which results in the apparent activation energy being slightly lower than the actual value.

Fluorination Affects the Force Sensitivity and Nonequilibrium Dynamics of the Mechanochemical Unzipping of Ladderanes.

When multiple reaction steps occur before thermal equilibration, kinetic energy from one reaction step can influence overall product distributions in ways that are not well predicted by transition state theory. An understanding of how the structural features of mechanophores, such as substitutions, affect reactivity, product distribution, and the extent of dynamic effects in the mechanochemical manifolds is necessary for designing chemical reactions and responsive materials. We synthesized two tetrafluorinated [4]-ladderanes with fluorination on different rungs and found that the fluorination pattern influenced the force sensitivity and stereochemical distribution of products in the mechanochemistry of these fluorinated ladderanes. The threshold forces for mechanochemical unzipping of ladderane were decreased by α-fluorination and increased by γ-fluorination; these changes correlated to the different stabilizing or destabilizing effects of fluorination patterns on the first transition state. Using ab initio steered molecular dynamics (AISMD), we compared the product distributions of synthesized and hypothetical ladderanes with different substitution patterns. These calculations suggest that fluorination on the first two bonds of ladderane gives rise to a larger fraction of dynamic trajectories and a larger fraction of E-alkene product through a mechanism resulting from larger momentum because of the greater atomic mass of fluorine. Fluorination on the third and fourth rungs instead gives a larger fraction of E-alkene product primarily due to electronic effects. These combined experimental and computational studies of the mechanochemical unzipping of fluorinated ladderanes provide an example of how relatively simple substituents can affect the extent of nonstatistical dynamics and, thus, mechanochemical outcomes.