Lower Free Energy Barrier Research Articles

Regulating single-atom Mn sites by precisely axial pyridinic-nitrogen coordination to stabilize the oxygen reduction

Designing single-atom catalysts for oxygen reduction reaction (ORR) are fashionable but challenging to boost the zinc-air battery performance. Significantly enhanced ORR activity by manganese (Mn) single-atom catalysts can be achieved by accurately regulating the coordination number of isolated Mn atoms. Theoretical calculations indicate that the single Mn-N5 sites possess lower free energy barrier and higher oxygen adsorption performance than single Mn-N4 sites to accelerate the ORR kinetics. Target to it, here we synthesize an atomically dispersed Mn-N5 catalyst by precisely axial coordination of pyridinic-N doped into two-dimensional (2D) porous nanocarbon sheets (∼3.56 nm thickness), which reveals outstanding catalytic activity and ultrahigh stability for the ORR in zinc-air battery owing to the inhomogeneous charge distribution of Mn-N5 sites compared to the conventional single-site Mn-N4 catalyst and Pt/C. This work gives a new strategy for in situ regulating the electronic structure of metal single-atoms and further promoting the overall ORR performance in energy systems.

Novel theoretical insight on CO2 electroreduction mechanism induced by aromatic ester-functionalized ionic liquids: A bulk-phase reaction pathway

CO2 electroreduction reaction (CO2ER) to high valued products in ionic liquid (IL) systems is a promising way for CO2 utilization, and understanding the mechanism on CO2ER will be helpful to design new electrolytes. In this study, the mechanism of CO2 to oxalate using the aromatic ester-functionalized IL ([N2222][4-MF-PhO]) was systematically investigated by the Density Functional Theory (DFT) calculations. A new reaction pathway named bulk-phase pathway was proposed in which the contribution of the electrolyte to CO2 reactions is more important than the electrode surface as shown in most reports. Our simulation results indicated that CO2 does not obtain electrons from the electrode directly, while the anion of the IL, [4-MF-PhO]−, acts as a catalyst and electron transporter in the new pathway. The reason is that the bulk-phase pathway has lower free energy barriers and higher selectivity for oxalate compared with the formation of oxalate on the electrode surface. The new insight was proved by the experimental work in our previous studies. Inspired by the bulk-phase pathway, the interactions of CO2 with the reduced esters, ketones and nitriles containing unsaturated bonds were further studied. It was found that the lower the Gibbs free energy barrier for them binding to CO2, the more charge is transferred to CO2. The new theoretical insight proposed in this work will be helpful for the design of IL electrolytes in CO2ER.

Porphyrinic MOF-derived rich N-doped porous carbon with highly active CoN4C single-atom sites for enhanced oxygen reduction reaction and Zn-air batteries performance

Development of high-performance single-atom catalysts (SACs) for oxygen reduction reaction (ORR) has received extensive attention. However, unsatisfactory loading and utilization of metal single-atom sites largely limit the catalytic efficiency. Herein, two dimensional (2D) nanoporous Co single-atom decorated nitrogen-doped carbon catalyst (NP-CoSANC) was prepared by the pyrolysis of urea-adsorbed Co-TPyP MOF precursor. The adsorbed urea was proven to effectively inhibit metal ion agglomeration and create flourishing pore structures simultaneously. Benefit from the high-density CoN4C single-atom sites and porous structure, NP-CoSANC presents superior ORR performance with high E1/2 of 0.86 V and low Tafel slope of 62.4 mV dec−1 in 0.1 M KOH solution, surpassing most of advanced transition metal ORR catalysts and even commercial 20% Pt/C. The density functional theory calculations reveal that defect-edge and N-rich doped CoN4C sites in NP-CoSANC endow it with a lower free energy barrier to direct 4e− ORR pathway compared to traditional center CoN4C model. As a proof of concept, homemade aqueous primary and flexible quasi-solid-state Zn-air battery exhibit high peak power density as well as charge/discharge stability. This work proposes a facile and effective approach for constructing high-performance SACs electrocatalysts and provides a rational guidance in next-generation energy storage and conversion devices.

Unravelling the dissolution dynamics of silicate minerals by deep learning molecular dynamics simulation: A case of dicalcium silicate

Quantitative analyses of the thermodynamics and kinetics of silicate minerals dissolution at atomic level are difficult currently, both experimentally and computationally. Here, we apply the deep learning, enhancing sampling molecular dynamics and density functional theory to build up a deep neural network potential with quantum mechanics accuracy, which can determine the free energy surfaces, minimum free-energy reaction pathways and kinetic rates for Ca dissolution from the water/dicalcium silicate interface at different temperatures. We find that the Ca dissolution is a spontaneous reaction and follows different minimum free-energy reaction pathways at different temperatures. The dissolution time of the five-coordinated Ca ion is on the order of hundreds of seconds at ambient temperature and increases to the order of nanoseconds after heating. The relatively slow dissolution kinetics comparing to the low free energy barriers of each elementary reaction is attributed to the multi-directional and multi-step nature of the dissolution reaction. This new atomistic insight promotes a better understanding of the dicalcium silicate and cement hydration.

Binding of carboxylate and water to monovalent cations.

The interactions of carboxylate anions with water and cations are important for a wide variety of systems, both biological and synthetic. To gain insight on properties of the local complexes, we apply density functional theory, to treat the complex electrostatic interactions, and investigate mixtures with varied numbers of carboxylate anions (acetate) and waters binding to monovalent cations, Li+, Na+ and K+. The optimal structure with overall lowest free energy contains two acetates and two waters such that the cation is four-fold coordinated, similar to structures found earlier for pure water or pure carboxylate ligands. More generally, the complexes with two acetates have the lowest free energy. In transitioning from the overall optimal state, exchanging an acetate for water has a lower free energy barrier than exchanging water for an acetate. In most cases, the carboxylates are monodentate and in the first solvation shell. As water is added to the system, hydrogen bonding between waters and carboxylate O atoms further stabilizes monodentate structures. These structures, which have strong electrostatic interactions that involve hydrogen bonds of varying strength, are significantly polarized, with ChelpG partial charges that vary substantially as the bonding geometry varies. Overall, these results emphasize the increasing importance of water as a component of binding sites as the number of ligands increases, thus affecting the preferential solvation of specific metal ions and clarifying Hofmeister effects. Finally, structural analysis correlated with free energy analysis supports the idea that binding to more than the preferred number of carboxylates under architectural constraints are a key to ion transport.

The treatment of dispersion terms for solution systems.

DFT calculations of reaction mechanisms in solution have always been a hot topic, especially for transition-metal-catalyzed reactions, in which the traditional DFT-D3 method has been extensively employed. The overestimation of the dispersion from the traditional DFT-D3 method leads to a quite low activation free-energy barrier, so it is worth finding a proper way to deal with the dispersion for solution systems. The solvent-solute dispersion is also important for solution systems, and thus it should be calculated together with the solute dispersion. The newly generated solute-solute dispersion energy should be shared equally with the newly formed cavity between two interacting species; therefore, only half of the solute-solute and solvent-solute dispersion terms belong to the solute molecule. The detailed treatment of dispersion correction for solution systems has been fully addressed, and this method has been confirmed with the examples of ligand exchange reactions and catalytic reactions.

Co doping promotes the alkaline overall seawater electrolysis performance over MnPSe3 nanosheets.

Herein, two-dimensional cobalt-doped MnPSe3 nanosheets (CMPS) were constructed, which served as an outstanding bifunctional catalyst for alkaline seawater splitting, i.e., offering the current density of 10 mA cm-2 with applied overpotentials of 59 and 300 mV for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. The assembled two-electrode system of CMPS//CMPS also demonstrated excellent catalytic activity (10 mA cm-2, 1.59 V) and can remain stable for more than 100 h. Moreover, the theoretical calculations showed that CMPS features a suitable H* adsorption beneficial for the HER, as well as a lower free energy barrier favorable for the OER.

Nanoparticle-induced biomembrane fusion: unraveling the effect of core size on stalk formation.

Membrane fusion in vitro is a strategy to load model or cell-derived vesicles with proteins, drugs, and genetic materials for theranostic applications. It is thus crucial to develop strategies to control the fusion process, also through synthetic fusogenic agents. Ligand-protected, membrane-penetrating gold nanoparticles (Au NPs) can facilitate membrane fusion, but the molecular mechanisms remain unresolved. Here, we tackle NP-induced stalk formation using a coarse-grained molecular dynamics approach and enhanced sampling techniques. We show that smaller (2 nm in diameter) NPs lead to a lower free energy barrier and higher stalk stability than larger NPs (4 nm). We demonstrate that this difference is due to a different ligand conformational freedom, which in turn depends on the Au core curvature. Our study provides precious insights into the mechanisms underlying NP-mediated membrane fusion, while our computational approach is general and applicable to studying stalk formation caused by other fusogenic agents.

Expanded [23]-Helicene with Exceptional Chiroptical Properties via an Iterative Ring-Fusion Strategy.

Expanded helicenes are an emerging class of helical nanocarbons composed of alternating linear and angularly fused rings, which give rise to an internal cavity and a large diameter. The latter is expected to impart exceptional chiroptical properties, but low enantiomerization free energy barriers (ΔG‡e) have largely precluded experimental interrogation of this prediction. Here, we report the syntheses of expanded helicenes containing 15, 19, and 23 rings on the inner helical circuit, using two iterations of an Ir-catalyzed, site-selective [2 + 2 + 2] reaction. This series of compounds displays a linear relationship between the number of rings and ΔG‡e. The expanded [23]-helicene, which is 7 rings longer than any known single carbohelicene and among the longest known all-carbon ladder oligomers, exhibits a ΔG‡e that is high enough (29.2 ± 0.1 kcal/mol at 100 °C in o-DCB) to halt enantiomerization at ambient temperature. This enabled the isolation of enantiopure samples displaying circular dichroism dissymmetry factors of ±0.056 at 428 nm, which are ≥1.7× larger than values for previously reported classical and expanded helicenes. Computational investigations suggest that this improved performance is the result of both the increased diameter and length of the [23]-helicene, providing guiding design principles for high dissymmetry molecular materials.

Surfactant in a Polyol-CO2 Mixture: Insights from a Classical Density Functional Theory Study.

Silicone-polyether (SPE) surfactants, made of a polydimethyl-siloxane (PDMS) backbone and polyether branches, are commonly used as additives in the production of polymeric foams with improved properties. A key step in the production of polymeric foams is the nucleation of gas bubbles in the polymer matrix upon supersaturation of dissolved gas. However, the role of SPE surfactants in the nucleation of gas bubbles is not well understood. In this study, we use classical density functional theory to investigate the effect of an SPE surfactant on the nucleation of CO2 bubbles in a polyol foam formulation. We find that the addition of an SPE surfactant leads to a ∼3-fold decrease in the polyol-CO2 interfacial tension at the surfactant's critical micelle concentration. Additionally, the surfactant is found to reduce the free energy barrier and affect the minimum free energy pathway (MFEP) associated with CO2 bubble nucleation. In the absence of a surfactant, a CO2-rich bubble nucleates from a homogeneous CO2-supersaturated polyol solution by following an MFEP characterized by a single nucleation barrier. Adding a surfactant results in a two-step nucleation process with reduced free energy barriers. The first barrier corresponds to the formation of a spherical aggregate with a liquid-like CO2 core. This spherical aggregate then grows into a CO2-rich bubble (spherical aggregate with a vapor-like CO2 core) of a critical size representing the second barrier. We hypothesize that the stronger affinity of CO2 for PDMS (than polyether) stabilizes the spherical aggregate with the liquid-like CO2 core, leading to a lower free energy barrier for CO2 bubble nucleation. Stabilization of such an aggregate during the early stages of the nucleation may lead to foams with more, smaller bubbles, which can improve their microstrustural features and insulating abilities.

The oxidation mechanism and kinetics of limononic acid by hydroxyl radical in atmospheric aqueous phase

Limononic acid (3-isopropenyl-6-oxoheptanoic acid, LA), as an important precursor of secondary organic aerosols (SOA), has attracted extensive attention in the field of atmospheric chemistry. However, its microscopic oxidation mechanism is still unclear. In this study, the density functional theory calculations were conducted to study the oxidation mechanism of LA by ·OH in aqueous phase. The results show that the reactions of hydroxylation are more likely to occur than the dehydrogenation reaction, because of the lower free energy barrier (2.3–5.4 kcal mol−1). At 298 K, the total rate constant of the reaction initiated by ·OH is 1.06 × 1010 M−1 s−1, which fits the experimental value well. Among all reactions, the hydroxylation reaction in C9 site of LA is the most favorable pathway, and the corresponding hydroxylation intermediate (IM4) can react with ·OH, H2O, dissolved O2, and HO2·. Three important tropospheric free radicals (R·, RO· and RO2·) are generated during the subsequent reaction process. Meanwhile alcohols, ketones, aldehydes, and oxidized acids can be formed in the overall reaction scheme. These products are the precursor for the formation of SOA, and this transformation process will increase the O/C ratio of aqueous phase SOA. This study has an important significance for understanding the oxidation mechanism of monoterpenoids in the atmospheric aqueous phase.

Thermodynamic Insight into Thermal Cracking of n-Hexane in the H-FAU Zeolite

To understand catalytic cracking processes of alkanes in the petrochemical industry, the monomolecular cracking mechanism of n-hexane in the H-FAU zeolite has been examined using ab initio molecular dynamics simulations combined with enhanced sampling techniques. It has been found that the adsorption energies of short n-alkanes are linearly correlated with the chain length and contact areas with the zeolite framework, while no such correlation can be applied to the short n-alkenes. High-dimensional free energy landscapes characterizing four cracking modes of n-hexane at three different temperatures have been drawn. The electron transfer in the cracking process was analyzed by quantifying the charge evolution for carbon atoms. The formation of carbonium ions in each cracking mode is found to be the rate-limiting step. At high temperature, free energy differences are nonpositive values, suggesting that the cracking of n-hexane is spontaneous, which is also coincident with the experimental observation. The cracking of the central C–C bond of n-hexane is easier than the cracking of terminal C–C bonds, which is due to the lower free energy barrier. The present work sheds new insights into the cracking process of n-alkanes in the industrially used FAU zeolite catalysts.

Fast pyrolysis of algae model compounds for bio-oil: In-depth insights into the volatile interaction mechanisms based on DFT calculations

Fast pyrolysis of algae biomass is a promising approach to produce high value-added bio-oil, while the interaction mechanisms among the pyrolytic volatiles of algae components remain unclear. To simplify the pyrolysis process and better investigate the intermediates distribution, glucose (Glc), alanine (Ala), and glycerol (Gly) were selected as the model compounds for algae biomass. The fast pyrolysis behaviors, volatile release properties, and bio-oil compositions of model compounds and their binary mixers were performed using TG-FTIR and Py-GC/MS to reveal the volatile interaction mechanisms combined with density functional theory (DFT) calculations. Experimental results showed that co-pyrolysis of Ala with Glc and Gly displayed a synergistic effect on the weight loss and CO2 release at lower temperatures, and the volatile interactions inhibited the weight loss and CO production at higher temperatures during co-pyrolysis of model compounds. Compared with individual pyrolysis, the contents of pyrazines, furans, and pyridines in the bio-oils from co-pyrolysis of Glc+Ala, Glc+Gly, and Ala+Gly increased by 14.47%, 18.59%, and 12.07%, respectively. Ethylamine generated from Ala and small molecular aldehydes and ketones derived from Glc and Gly were the main precursors. Based on the products evolution, the possible volatile interaction pathways were proposed at the molecular level and the energy profiles were assessed by DFT calculations. Calculation results manifested that the Maillard and acetal reactions with lower free energy barriers were the initial reactions driving the formation of heterocyclic compounds. These findings could offer insights into the fast pyrolysis mechanisms of algae biomass and provide theoretical support for the feedstock and pyrolysis process optimization and the bio-oil composition regulation, thereby promoting the efficient utilization of algae.

Electronic-level deciphering of the desalination mechanism of high-performance graphenylene membranes

The insufficiency of available freshwater has surged as a major global concern in recent years, and the issue is raising ever more awareness. Meanwhile, the recent experimental synthesis of graphenylene has inspired researchers to pursue its applicative merits. The related properties regarding lithium-ion storage electrodes and gas separation membranes of graphenylene have been reported. However, the suitability of graphenylene as a desalination membrane has yet to be fully established. In the present study, with a view to delineating the action mechanism of graphenylene desalination membrane, we have dissected its structure and properties through first-principles and molecular dynamics. The results illustrate that graphenylene comprising dodecagonal, hexagonal, and tetragonal carbon rings contains 0.55 nm nanopores, which should permit outstanding desalination properties. In addition, the periodic pore distribution should support stress dispersion in service to preserve membrane integrity. The low water-molecule free-energy barrier, remarkable salt ion adsorption properties, and favorable anti-agglomeration ability endow graphenylene with desirable permeability and selectivity. Furthermore, we demonstrate that graphenylene is a 2D Dirac semi-metallic material. The interlinked electronic structure of which confers it with self-cleaning behavior, facilitating the restoration of water flux. The present research has elucidated the mechanism whereby effective desalination may be achieved on graphenylene membranes. Further, it has clarified the potential applicability of such membranes. They show good prospects for the advancement of seawater desalination technology.

Role of Repeated Conformational Transitions in Substrate Binding of Adenylate Kinase.

The catalytic cycle of the enzyme adenylate kinase involves large conformational motions between open and closed states. A previous single-molecule experiment showed that substrate binding tends to accelerate both the opening and the closing rates and that a single turnover event often involves multiple rounds of conformational switching. In this work, we showed that the repeated conformational transitions of adenylate kinase are essential for the relaxation of incorrectly bound substrates into the catalytically competent conformation by combining all-atom and coarse-grained molecular simulations. In addition, free energy calculations based on all-atom and coarse-grained models demonstrated that the enzyme with incorrectly bound substrates has much a lower free energy barrier for domain opening compared to that with the correct substrate conformation, which may explain the the acceleration of the domain opening rate by substrate binding. The results of this work provide mechanistic understanding to previous experimental observations and shed light onto the interplay between conformational dynamics and enzyme catalysis.

Mechanistic insights into copper oxides catalyzed bio-based furfural hydrogenation using methanol as in-situ hydrogen donor

In the present study, CuO and Cu2O were employed in H-transfer transformation of bio-based furfural (FFR) using methanol as both the solvent and an in-situ hydrogen source, to clearly elucidate the catalytic mechanism of copper oxides (CuOx, x = 0.5, 1) during the reaction. A number of characterization techniques including XPS, H2-TPD, in-situ FTIR, etc., were undertaken to track the state of copper species. Owing to the synergistic effect of in-situ generated H2, Cu0 species and CuOx derived Brønsted/Lewis acid sites, FFR could be efficiently hydrogenated into FA and subsequently deoxygenated into MF in methanol. The combined results from in-situ characterizations and experiments show that the FFR-to-FA (FA, furfuryl alcohol) and FFR-to-MF (MF, 2-methylfuran) conversions largely depended on the hydrogen production capability from methanol reforming and FFR adsorption strength over CuOx. Density functional theory (DFT) calculations demonstrated that the CuO(200) surface revealed lower free energy and energy barriers for methanol reforming and FFR transfer hydrogenation. This study provides a promising guidance for reasonably designing copper-based materials in the hydro-upgrading of bio-based unsaturated molecules into value-added chemicals in methanol without external H2 supply.

Resolving the amine-promoted hydrolysis mechanism of N2O5 under tropospheric conditions

Hydrolysis of N2O5 under tropospheric conditions plays a critical role in assessing the fate of O3, OH, and NOx in the atmosphere. However, its removal mechanism has not been fully understood, and little is known about the role of entropy. Herein, we propose a removal path of N2O5 on the water clusters/droplet with the existence of amine, which entails a low free-energy barrier of 4.46 and 3.76 kcal/mol on a water trimer and droplet, respectively, at room temperature. The free-energy barrier exhibits strong temperature dependence; a barrierless hydrolysis process of N2O5 at low temperature (≤150 K) is observed. By coupling constrained ab initio molecular dynamics (constrained AIMD) simulations with thermodynamic integration methods, we quantitively evaluated the entropic contributions to the free energy and compared NH3-, methylamine (MA)-, and dimethylamine (DMA)-promoted hydrolysis of N2O5 on water clusters and droplet. Our results demonstrate that methylation of NH3 stabilizes the product state and promotes hydrolysis of N2O5 by reducing the free-energy barriers. Furthermore, a quantitative analysis of the internal coordinate distribution of the reaction center and the relative position of surrounding species reveals that the significant entropic contribution primarily results from the ensemble effect of configurations observed in the AIMD simulations. Such an ensemble effect becomes more significant with more water molecules included. Lowering the temperature effectively minimizes the entropic contribution, making the hydrolysis more exothermic and barrierless. This study sheds light on the importance of the promoting effect of amines and the entropic effect on gas-phase hydrolysis reactions, which may have far-reaching implications in atmospheric chemistry.

Structure and Dynamics of the Isozymes II and IX of Human Carbonic Anhydrase.

Human carbonic anhydrases (HCAs) are responsible for the pH control and sensing in our body and constitute key components in the central pH paradigm connected to cancer therapeutics. However, little or no molecular level studies are available on the pH-dependent stability and functional dynamics of the known isozymes of HCA. The main objective of this Article is to report the first bench-marking study on the structure and dynamics of the two most efficient isozymes, HCA II and IX, at neutral pH using classical molecular dynamics (MD) and constant pH MD (CpHMD) simulations combined with umbrella sampling, transition path sampling, and Markov state models. Starting from the known crystal structures of HCA II and the monomeric catalytic domain of HCA IX (labeled as HCA IX-c), we have generated classical MD and CpHMD trajectories (of length 1 μs each). In all cases, the overall stability, RMSD, and secondary structure segments of the two isozymes are found to be quite similar. Functionally important dynamics of these two enzymes have been probed in terms of active site hydration, coordination of the Zn(II) ion to a transient excess water, and the formation of putative proton transfer paths. The most important difference between the two isozymes is observed for the side-chain fluctuations of His-64 that is expected to shuttle an excess proton out of the active site as a part of the rate-determining intramolecular proton transfer reaction. The relative stability of the stable inward and outward conformations of the His-64 side-chain and the underlying free energy surfaces are found to depend strongly on the isozyme. In each case, a lower free energy barrier is detected between predominantly inward conformations from predominantly outward ones when simulated under constant pH conditions. The kinetic rate constants of interconversion between different free energy basins are found to span 107–108 s–1 with faster conformational transitions predicted at constant pH condition. The estimated rate constants and free energies are expected to validate if the fluctuation of the His-64 side-chain in HCA IX may have a significance similar to that known in the multistep catalytic cycle of HCA II.

Comprehensive theoretical study of nickel‐NHC‐catalyzed enantioselective intramolecular indole C—H cyclization: Reaction mechanism, reactivity, regioselectivity, and electronic processes

Density functional theory was used to illuminate the reaction mechanism of Ni(0)‐NHC‐catalyzed enantioselective intramolecular CH cyclization of indoles. The ωB97XD functional with 6‐31G(d,p) basis set (SDD for nickel) was employed to optimize all intermediates and transition states. The computations revealed that the reactivity of C1H bond was the best among all CH bonds of indole catalyzed by the Ni‐NHC complex, and in the C1H bond activation, the concerted oxidative addition was more dominant than the ligand‐to‐ligand H transfer reaction, due to the lower free energy barrier. The most favorable channels predicted theoretically went through the ligand‐to‐ligand H transfer reaction and reductive elimination, and two chiral products ((R)‐ and (S)‐tetrahydroindolizines) were coexisted as a result of similar transition states and little difference in free energy barrier. Hence, the model ligandL1was not good for the regioselectivity. Nevertheless, another ligandL2, a chiral N‐heterocyclic carbene ligand, gave good regioselectivity, and it changed the rate‐determining step and generally decreased the free energies of transition states and intermediates.

Unexpected role of two ortho-OH groups for the hydrogenation of CO2 to methanol catalyzed by Fe bipyridinol complexes

A density functional theory study of Fe-catalyzed hydrogenation of CO2 to methanol reveals an unexpected metal–ligand cooperation mechanism with two ortho-OH groups play key roles for the breaking of C–O and O–H bonds and formation of O–H…O hydrogen bonds. The formation of methanediol via hydride transfer from Fe to formic acid (4′ → TS6,7) is the rate-determining step with a total barrier of 30.8 kcal mol−1 in free energy. Furthermore, by replacing the two meta (R1) and two para (R2) hydrogen atoms in the bipyridinol ligand and the coordinated CN and CO with different functional groups, we built a series of stable iron complexes and computationally predicted their catalytic activities for hydrogenation of CO2 to methanol. Among all newly proposed iron complexes, 1t and 1u are promising catalyst candidates for hydrogenation of CO2 to methanol with rather low total free energy barriers of 26.6 and 28.8 kcal mol−1, respectively. The electrostatic potential map analysis indicates that stronger electron donating ligands on iron may lead to higher catalytic activities.