Change In Rate-determining Step Research Articles

Highly selective electrocatalytic CO2 reduction to multi-carbon products at CuPd synergistic sites over OD-Cu based catalysts

CO2 reduction is a potential way to kill two birds with one stone by converting greenhouse gas into fuels through clean energy. Cu-based catalysts, with a wide range of products, often have limited selectivity and low activity for specific products. In this study, Cu2O was used as the oxide-derived copper (OD-Cu) substrate, which was loaded with CuPd nanoparticles by using pulsed potential and galvanic replacement. Theoretical calculations revealed that the strong H-affinity Pd sites can reduce the selectivity of formate by stabilizing *COOH and simultaneously lowering the d-band center of neighboring Cu. Additionally, the asymmetric C–C coupling of *CHO+*CO, distributed between Cu and Pd sites, can lower the free energy change of rate-determining step to promote production of the C2+ products. Faradaic efficiency (FE) of the C2+ products over Cu2O-CuPd0.5μM was more than three times higher than that over untreated Cu2O NW, rising from 16.50 % to 54.47 % under −1 V (vs RHE) bias. Moreover, the FE for formate and CO decreased respectively from 28.57 % and 19.30 % to 3.69 % and 11.82 %, accompanied by a simultaneous increase in current density by 71.49 %.

Theoretical Insights into Single-Atom Catalysts Supported on N-Doped Defective Graphene for Fast Reaction Redox Kinetics in Lithium-Sulfur Batteries.

Single-atom catalysts are proven to be an effective strategy for suppressing shuttle effect at the source by accelerating the redox kinetics of intermediate polysulfides in lithium-sulfur (Li-S) batteries. However, only a few 3d transition metal single-atom catalysts (Ti, Fe, Co, Ni) are currently applied for sulfur reduction/oxidation reactions (SRR/SOR), which remains challenging for screening new efficient catalysts and understanding the relationship between structure-activity of catalysts. Herein, N-doped defective graphene (NG) supported 3d, 4d, and 5d transition metals are used as single-atom catalyst models to explore electrocatalytic SRR/SOR in Li-S batteries by using density functional theory calculations. The results show that M1 /NG (M1 = Ru, Rh, Ir, Os) exhibits lower free energy change of rate-determining step and Li2 S decomposition energy barrier, which significantly enhance the SRR and SOR activity compared to other single-atom catalysts. Furthermore, the study accurately predicts the by machine learning based on various descriptors and reveals the origin of the catalyst activity by analyzing the importance of the descriptors. This work provides great significance for understanding the relationships between the structure-activity of catalysts, and manifests that the employed machine learning approach is instructive for theoretical studies of single-atom catalytic reactions.

A controllable cobalt-doping improve electrocatalytic activity of ZnO basal plane for oxygen evolution reaction: A first-principles calculation study

To design highly efficient, low-cost, and good stable electrocatalysts for the oxygen evolution reaction (OER) which is the key to the water-splitting, we doped ZnO with different concentrations of Co based on density functional theory calculations. The precise Co concentrations control from 0 % to 100 % can be achieved by cation exchange reaction in the experiment. The introduction of Co can modulate the electronic structure of ZnO, as the concentration of Co increases, the band gap greatly decreases, the system tunes from the semiconductor to the metal, and the electric conductivity is significantly improved, favoring the OER. The introduction of Co induces charge arrangement, the electrons gradually concentrate on the active site, and the bindings between the oxygen-containing intermediate and the surface are greatly improved, therefore, the potential of the OER decreases, the rate-determining step changes from the first step in pure ZnO to the third step in Co exchanged ZnO. Additionally, Co0.5Zn0.5O exhibits the favorite OER activity with the overpotential of 0.4 V. These interesting findings may simulate experimental validations and facilitate the development of more efficient OER catalysts.

2LaCo0.7Fe0.3O3/N-doped carbon bifunctional electrocatalyst derived from g-C3N4 nanosheets for zinc-air battery

Perovskite oxides have been widely studied as one of the excellent bifunctional oxygen electrocatalysts (ORR: Oxygen Reduction Reaction, OER: Oxygen Evolution Reaction) due to its flexible crystal structure and low cost. Herein, 2LaCo0.7Fe0.3O3/N-doped carbon is prepared by sol-gel method using g-C3N4 nanosheets as carbon source and nitrogen source. The LaCo0.7Fe0.3O3 nanoparticles have excellent dispersibility on the N-doped carbon substrate and the composite material has higher oxygen vacancy concentration, showing enhanced bifunctional electrocatalytic activity (△E=Ej=10,OER-E1/2,ORR=1.03 V) compared with the original LaCo0.7Fe0.3O3 (△E = 1.12 V). Moreover, 2LaCo0.7Fe0.3O3/N-doped carbon-based Zn-air battery presents excellent cycling stability (over 24 h) and high peak power density of 116 mW cm−2. Density functional theory (DFT) calculations revealed the mechanism of improvement electrocatalytic activity of 2LaCo0.7Fe0.3O3/N-doped carbon, which comes from the regulated electronic structure, including enhanced conductivity and the covalence of Co-O bond, reducing the free energy change of rate-determining step. This study provides a novel strategy for the design and prepare perovskite-type bifunctional electrocatalysts.

FeCoP2 Nanoparticles Embedded in N and P Co-doped Hierarchically Porous Carbon for Efficient Electrocatalytic Water Splitting.

The design and synthesis of low-cost and efficient bifunctional electrocatalysts for water splitting are critical and challenging. Hereby, a bimetallic phosphide embedded in a N and P co-doped porous carbon (FeCoP2@NPPC) material was synthesized by using sustainable biomass-derived N- and P-containing carbohydrates and non-noble metal salts as precursors. The obtained material exhibits good catalytic activities in hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and overall water splitting. The bimetallic alloy phosphide (FeCoP2) is the active site for electrocatalysis. Theoretical calculation indicates that the sub-layer Fe atoms and top-layer Co atoms in FeCoP2 exhibit a synergistic effect for enhanced electrocatalytic performance. The carbon matrix around the FeCoP2 can prevent the corrosion during the catalytic reactions. The hierarchically porous structure of the FeCoP2@NPPC material can promote the transfer of electrons and electrolyte, and increase the contact area of the active sites and electrolytes. N- and P-containing functionalities improve the wetting and conductivity properties of the porous carbon. Due to the synergistic effects, FeCoP2@NPPC requires a low overpotential of 114 and 150 mV at the current density of 10 mA cm-2 for HER in 0.5 M H2SO4 and 1.0 M KOH, and an overpotential of 236 mV for OER in 1.0 M KOH solution, which are much lower than those of FeP@NPPC and CoP@NPPC. Based on the density functional theory calculation, FeCoP2 yields the smallest Gibbs free energy change of rate-determining step among the samples, which leads to better electrochemical performances. In addition, when FeCoP2@NPPC was used as a bifunctional catalyst in water splitting, the electrolyzer needed a low voltage of 1.60 V to deliver the current density of 10 mA cm-2. Furthermore, this work provides a strategy for preparing sustainable, stable, and highly active electrocatalysts for water splitting.

Assessing the nucleophilic character of 2-amino-4-arylthiazoles through coupling with 4,6-dinitrobenzofuroxan: Experimental and theoretical approaches based on structure-reactivity relationships

A kinetic study of the reactions of potentially bioactive 2-amino-4-arylthiazoles with highly reactive 4,6-dinitrobenzofuroxan (DNBF) is reported herein in acetonitrile solution. The complexation reaction was followed by recording the UV–vis spectra with time at λmax = 482 nm. Electronic effects of substituents influencing the rate of reaction have been studied using structure-reactivity relationships. It is shown that the Hammett plot relative to the reaction of DNBF with 2-amino-4-(4-chlorophenyl)thiazole exhibit positive deviation from the log k1 versus σ correlation, while it showed excellent linear correlation in terms of Yukawa–Tsuno equation. It has be noticed that the nonlinear Hammett plot observed for 2-amino-4-(4-chlorophenyl) thiazole is not attributed to a change in rate-determining step but is due to nature of electronic effect of substituent caused by the resonance of stabilization of substrates. The second-order rate constant (k1) relating to the bond C–C and C-N forming step of the complexation processes of DNBF with 4-substituted-aminothiazoles and 2-amino-5-methyl-4-phenylthiazole, respectively, is fit into the linear relationship log k = sN (N + E), thereby permitting the assessment of the nucleophilicity parameter (N) of the 2-amino-4-arylthiazoles of the range (4.90 < N < 6.85). 2-amino-4-arylthiazoles is subsequently ranked by positioning its reactivity on the general nucleophilicity scale developed recently by Mayr and coworkers (2003) leading an interesting and a direct comparison over a large domain of π-, σ -, and n-nucleophiles. The global electrophilicity/nucleophilicity reactivity indexes of the 2-amino-4-arylthiazoles have been investigated by means of a density functional theory (DFT) method. .

Determination of relevant factors affecting the surface oxygen exchange coefficient of solid oxide fuel cell cathode with ionic conducting oxide coating

In order to investigate the relevant factors affecting the transport properties of solid oxide fuel cell cathodes upon coating with Ce0.9Gd0.1O1.95 (GDC), the surface oxygen exchange coefficient, k*, and oxide ion diffusivity, D*, of La0.6Sr0.4CoO3-δ (LSC) and La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) were measured by a combination of isotope exchange technique and secondary ion mass spectrometry (SIMS). The k* of LSCF/GDC enhanced compared to a bare LSCF for all measured temperatures. In contrast, the k* of LSC/GDC only showed an enhancement compared to the bare LSC at relatively low temperatures (below 873 K) but not at higher temperatures. From these results, GDC has a less significant effect on the k* of LSC compared to LSCF. This suggests that the enhancement of k* is possibly induced only when the oxygen vacancy concentration, δ, of the cathode is lower than that of GDC, although another factor may affect the enhancement. The enhancement of k* is attributed to the formation of triple-phase boundary where a spill-over mechanism controlled the oxygen reduction reaction. The change of rate-determining step of LSCF due to the GDC coating suggests that the enhancement of the k* is not only with respect to the incorporation process, but it is possibly enhancing the reaction steps that involve δ as well.

Amorphous cobalt-cerium binary metal oxides as high performance electrocatalyst for oxygen evolution reaction

The amorphous Co-Ce binary metal oxides Co1-yCeyOx prepared by the photochemical metal–organic deposition (PMOD) method is developed as high performance electrocatalysts for oxygen evolution reaction (OER). The influence of Ce content on the OER activity is investigated in terms of geometric and electronic factors. Ce can remarkably enhance the OER activity of CoOx due to the synergistic effect of surface area, Co3+ content, and metal-OH bond strength when the content of Ce is less than 60%. Co0.9Ce0.1Ox supported on fluorine-doped tin oxide (FTO) coated glass substrate shows overpotential of 320 (2) mV at 10 mA cm−2 in 1 M KOH solution. The OER mechanism exploration reveals that the rate determining step changes with the Ce content due to the variation of metal-OH bond strength. This work sheds light on the design of high-performance yet cost-effective OER catalysts.

Temperature Effect on Co-Based Catalysts in Oxygen Evolution Reaction.

Oxygen evolution reaction (OER), as the critical step in splitting water, is a thermodynamically "up-hill" process and requires highly efficient catalysts to run. Arrhenius' law suggests that the higher temperature, the faster the reaction rate, so that a larger OER current density can be achieved at a lower η. Herein, we report an abnormal temperature effect on the performance of Co-based catalysts, e.g., Co3O4, Li2CoSiO4, and Fe-doped Co(OH) x, in OER in alkaline electrolytes. The OER performance reached a maximum when the temperature increased to 65 °C, and the OER performance declined when the temperature became higher. The mechanism was investigated by using Co3O4 as a model sample, and we propose that at an optimal temperature (around 55-65 °C) the main rate-determining step changes from OH- adsorption dominant to a mixed mode and both the adsorption and the cleavage of the OH group can be rate-determining, which leads to the fastest kinetics.

S- and N-Doped Graphene Nanomaterials for the Oxygen Reduction Reaction

In the current work, heteroatom-doped graphene materials containing different atomic ratios of nitrogen and sulphur were employed as electrocatalysts for the oxygen reduction reaction (ORR) in acidic and alkaline media. To this end, the hydrothermal route and different chemical reducing agents were employed to synthesize the catalytic materials. The physicochemical characterization of the catalysts was performed by several techniques, such as X-ray diffraction, Raman spectroscopy and elemental analysis; meanwhile, the electrochemical performance of the materials toward the ORR was analyzed by linear sweep voltammetry (LSV), rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) techniques. The main results indicate that the ORR using heteroatom-doped graphene is a direct four-electron pathway, for which the catalytic activity is higher in alkaline than in acidic media. Indeed, a change of the reaction mechanism was observed with the insertion of N into the graphenic network, by the rate determining step changes from the first electrochemical step (formation of adsorbed OOH) on glassy carbon to the removal of adsorbed O (Oad) from the N-graphene surface. Moreover, the addition of sulphur atoms into the N-graphene structure increases the catalytic activity toward the ORR, as the desorption of Oad is accelerated.

Mechanism of the Hydrogen Evolution Reaction in Mildly Acidic Environments on Gold

Despite numerous studies investigating the kinetics of the hydrogen evolution reaction (HER) on a gold surface in acidic solutions, the underlying mechanism of this reaction have remained controversial to date. In the present study, the existing mechanisms are reevaluated and found to be inadequate in explaining the steady state polarization behavior of the hydrogen evolution reaction in an extended cathodic potentials and mildly acidic pH range. It was shown that a mechanism including a surface diffusion step of Hads alongside the Volmer, Heyrovsky, and Tafel elementary steps, best describes the experimental data obtained in acidic perchlorate solutions up to pH 5, while the rate determining step changes both with pH and electrode potential. This overall HER mechanism was further verified using a comprehensive mathematical model based on the proposed elementary steps, where a satisfactory agreement with experimental results was obtained.

Kinetic Study of Carbophilic Lewis Acid Catalyzed Oxyboration and the Noninnocent Role of Sodium Chloride.

In the present study, the oxyboration reaction catalyzed by IPrAuTFA in the presence and absence of NaTFA has been examined with kinetic studies, mass spectrometry, and 1H NMR and 11B NMR spectroscopy. Data from monitoring the reactions over the temperature range from 30 to 70 °C, the catalyst range from 1.3 to 7.5 mol %, and the NaTFA additive range from 2.5 to 30 mol % suggests a mechanism that involves rate-determining catalyst generation. Data from additive studies that replaced NaTFA with NaBARF (BARF = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) or Bu4NTFA as an alternative additive suggest that catalyst quenching from residual NaCl remaining from a one-pot substrate synthesis/reaction method is the cause of this effect, despite the low solubility of this NaCl byproduct in toluene. Material produced through an alternative, sodium chloride free substrate synthesis exhibited faster reaction rates, consistent with a change in rate-determining step that depended on the substrate synthesis route.

Direct growth of 3D flower-like Pt nanostructures by a template-free electrochemical route as an efficient electrocatalyst for methanol oxidation reaction

In this paper, a facile electrochemical approach is reported towards the fabrication of Pt needle-like NFs (nanoflowers) at the surface of GCE (glassy carbon electrode). The morphology and composition of the Pt NFs are characterized by SEM (scanning electron microscopy) and EDS (energy dispersive spectroscopy), respectively. Taking methanol oxidation as a model reaction in an acid medium, the electrocatalytic performance of as-prepared Pt NFs has been evaluated by CV (cyclic voltammetry), chronoamperometry and EIS (electrochemical impedance spectroscopy) techniques. These 3D (three dimensional) NFs exhibit the excellent electrocatalytic activity and high level of poisoning tolerance to the carbonaceous oxidative intermediates for the electro-oxidation reaction in acidic media. In addition, EIS information discloses different impedance behaviors for methanol electrooxidation at various potentials on the Pt NFs catalyst and also the change of rate-determining step with increasing potential. The comparison of Pt NFs and Pt NPs (nanoparticles), prepared by electrodeposition in the presence of hydrogen bubbles, shows that the NFs catalyst has higher activity, better long-term stability and lowers Pt loading. The simplicity of method and quality of prepared surfaces suggest applications in catalysis where a convenient method to prepare Pt catalysts with high surface area in one step is desirable.

Concerted proton-electron transfer in electrocatalytic O2 reduction by iron porphyrin complexes: axial ligands tuning H/D isotope effect.

The electrochemical O2 reduction by thiolate- and imidazole-bound iron porphyrin complexes and H/D isotope effects on 4e(-) (determined by rotating disc electrochemistry) and 2e(-) (determined by rotating ring disc electrochemistry) O2 reduction rates are investigated. The results indicate that a thiolate axial ligand shows an H/D isotope effect greater than 18 and 47 for the 4e(-) and 2e(-) O2 reductions, respectively. Alternatively, an imidazole axial ligand results in H/D isotope effects of 1.04 and 4.7 for the 4e(-) and 2e(-) O2 reduction, respectively. The catalytic O2 reduction mechanism is investigated in situ with resonance Raman coupled with rotating disc electrochemistry. The data indicate that the rate-determining step changes from O-O bond heterolysis of Fe(III)-OOH species for a thiolate axial ligand to an O-O bond heterolysis of an Fe(II)-OOH for an imidazole axial ligand.

Kinetic Study on Aminolysis of 2-Chloro-4-nitrophenyl X-Substituted-Benzoates in Acetonitrile and in H2O Containing 20 mol % DMSO: Effects of Medium and Substituent X on Reactivity and Reaction Mechanism

Abstract Second-order rate constants for reactions of 2-chloro-4-nitrophenyl X-substituted-benzoates 1a–1j with a series of cyclic secondary amines in MeCN have been measured. Comparison of the kinetic results with those reported previously for the corresponding reactions carried out in H2O containing 20 mol % DMSO has revealed that amines are less reactive in MeCN. The Brønsted-type plot for the aminolysis of 2-chloro-4-nitrophenyl benzoate (1f) in MeCN is linear with βnuc = 0.64, which is in contrast to the curved Brønsted-type plot reported for the reaction performed in the H2O–DMSO misxture. The Hammett plot for the reactions of 1a–1j with piperidine consists of two intersecting straight lines while the Yukawa–Tsuno plot exhibits an excellent linear correlation with ρX = 1.22 and r = 0.60, indicating that the nonlinear Hammett plot is not due to a change in rate-determining step but is caused by stabilization of substrates possessing an electron-donating substituent through resonance interactions between the substituent and the carbonyl functionality. It has been concluded that medium change from the H2O–DMSO mixture to MeCN forces the aminolysis of 1a–1j to proceed through a concerted mechanism by destabilizing the zwitterionic tetrahedral intermediate (T±).

Kinetic Study on Aminolysis of 4-Chloro-2-Nitrophenyl X-Substituted-Benzoates in Acetonitrile and in 80 mol % H2O/20 mol % DMSO: Effect of Medium on Reactivity and Reaction Mechanism

A kinetic study on aminolysis of 4-chloro-2-nitrophenyl X-substituted-benzoates (6a-i) in MeCN is reported. The Hammett plot for the reactions of 6a-i with piperidine consists of two intersecting straight lines, while the Yukawa-Tsuno plot exhibits an excellent linear correlation with <TEX>${\rho}_X$</TEX> = 1.03 and r = 0.78. The nonlinear Hammett plot is not due to a change in rate-determining step (RDS) but is caused by the resonance stabilization of substrates possessing an electron-donating group in the benzoyl moiety. The Br<TEX>${\phi}$</TEX>nsted-type plot for the reactions of 4-chloro-2-nitrophenyl benzoate (6e) with a series of cyclic secondary amines is linear with <TEX>${\beta}_{nuc}$</TEX> = 0.69, an upper limit for reactions reported to proceed through a concerted mechanism. The aminolysis of 6e in aqueous medium has previously been reported to proceed through a stepwise mechanism with a change in RDS on the basis of a curved Br<TEX>${\phi}$</TEX>nsted-type plot. It has been concluded that instability of the zwitterionic tetrahedral intermediate (<TEX>$T^{\pm}$</TEX>) in MeCN forces the reaction to proceed through a concerted mechanism. This is further supported by the kinetic result that the amines used in this study are less reactive in MeCN than in <TEX>$H_2O$</TEX>, although they are more basic in MeCN over 7 <TEX>$pK_a$</TEX> units.

Kinetic Study on Nucleophilic Displacement Reactions of 2-Chloro-4-Nitrophenyl X-Substituted-Benzoates with Primary Amines: Reaction Mechanism and Origin of the α-Effect

The <TEX>${\alpha}$</TEX>-Effect; Ground state; Transition state; Intramolecular H-bonding; Yukawa-Tsuno plot; Second-order rate constants for aminolysis of 2-chloro-4-nitrophenyl X-substituted-benzoates (1a-h) have been measured spectrophotometrically in 80 mol % <TEX>$H_2O/20$</TEX> mol % DMSO at <TEX>$25.0^{\circ}C$</TEX>. The Br<TEX>${\emptyset}$</TEX>nsted-type plot for the reactions of 2-chloro-4-nitrophenyl benzoate (1d) with a series of primary amines curves downward, which has been taken as evidence for a stepwise mechanism with a change in rate-determining step (RDS). The Hammett plots for the reactions of 1a-h with hydrazine and glycylglycine are nonlinear while the Yukawa-Tsuno plots exhibit excellent linearity with <TEX>${\rho}_X=1.22-1.35$</TEX> and <TEX>${\gamma}= 0.57-0.59$</TEX>, indicating that the nonlinear Hammett plots are not due to a change in RDS but are caused by stabilization of substrates possessing an electron-donating group (EDG) through resonance interactions between the EDG and C=O bond of the substrates. The <TEX>${\alpha}$</TEX>-effect exhibited by hydrazine increases as the substituent X changes from a strong EDG to a strong electron-withdrawing group (EWG). It has been concluded that destabilization of hydrazine through the electronic repulsion between the adjacent nonbonding electrons is not solely responsible for the substituent dependent <TEX>${\alpha}$</TEX>-effect but stabilization of the transition state is also a plausible origin of the <TEX>${\alpha}$</TEX>-effect.

The anodic dissolution of pyrite (FeS2) in hydrochloric acid solutions

Pyrite is an abundant mineral, and its dissolution is important in the formation of acid mine drainage and the extraction of metals. It has also been considered as a candidate material for electrochemical solar cells. We studied the anodic processes on pyrite in hydrochloric acid. The current–voltage curve at steady state has three regions. At potentials below 0.6V, the current is low, and the Tafel slope is high; between 0.6 and 0.9V, the current is significant, and the Tafel slope is 0.082V/decade; at potentials above 0.9V, the current remains high, but the Tafel slope changes to 0.430V/decade. From careful measurements of the dissolution of pyrite by chlorine, we show that this third region is due to charge transfer and not ohmic resistance. The shape of the current–voltage curve is unaffected by the type of acid used. The effect of temperature was measured, and the activation energies for the middle and upper region are 46 and 105kJ/mol, respectively. The steady-state current–voltage curves are not affected by the concentration of HCl, but are affected by the concentration of chloride ions, with an order of reaction of −0.1, indicating that the rate only slightly dependent on chloride ions. Pretreatment tests and XPS analysis of the surface confirm that the change in Tafel slope is not due to the formation of a surface coating or oxidation product. A comprehensive model of the anodic dissolution of pyrite that describes the current–voltage behavior of pyrite over the entire anodic region is proposed. At low potentials, the behavior of pyrite is typical of an n-type semiconductor with some dissolution. However, as the potential is increased, the Fermi level overlaps with an intrinsic surface state. The rate of dissolution of the pyrite in this region is dependent on the occupancy of the surface state. This accounts for previous reports indicating that pyrite is quasi-metallic. The rate-determining step is the transfer of charge across the space-charge layer. As the potential is increased to about 0.9V, the surface state is unoccupied, and the rate-determining step changes to the transfer of ions across the Helmholtz layer. An expression based on this model fits the experimental data. An important feature of this model is that it also explains two contradictory features of pyrite electrochemistry. On the one hand, n-type and p-type pyrite samples may have similar kinetic parameters, which suggests that the electronic structure of pyrite makes little difference. On the other hand, illuminating the sample with light increases the rate of dissolution, which suggests that the electronic structure is important. The proposed model based on surface states explains these seemingly contradictory observations.

DFT Study on Mechanism of Ruthenium-catalyzedmetaSulfonation

The transition metal catalyzed direct C—H functionalization reaction is an important strategy for selective activa- tion of aromatic C—H bond and its functionalization. In the present study, DFT method (M06//B3LYP) has been used for mechanistic study on the Ruthenium-catalyzed meta-sulfonation of 2-phenylpyridines reported by Frost and co-workers. All calculations involved in this study were performed using Gaussian09 suite of program. The proposed mechanism consists of four main steps: pyridine directed ortho C—H activation, electrophilic substitution, reductive elimination and catalyst regen- eration. It is found that the ortho C-H activation is the rate-determing step of overall catalytic cycle. At 298.15 K and stan- dard pressure, the KIE of ortho C—H activation step was calculated for the model reaction. Good agreement has been gained between the calculations (KIE=4.8) and the experimental observed primary isotope effect (KIE=3.0). The regioselectivity determining step of the catalytic cycle is found to be the electrophilic substitution. With the coordination of Ru center to the ortho carbon atom, the electron density distribution on the benzene ring of cycloruthenium intermediate changes accordingly. The charge distribution and the steric hindrance around the Ru center then make the electrophilic substitution occur favorably at the para-position of Ru—C bond (the para-position of directing group) in preference to the chelating-C or the ortho of the chelating group. In one word, regioselectivity was determined by electrophilic substitution, rather than the directed C—H activation. In the process of reaction, K2CO3 played an important role in promoting the reaction. K2CO3 first participates in the ortho C—H activation, and then it stabilizes the TsCl dissociating intermediates. In the electrophilic substitution step, K2CO3 directly engages in the cleavage of C—H bonds. Finally K2CO3 takes part in the generation of ortho C—H. As to the solvent effect, we found that the rate determining step changes with the changes of solvent polarity. When the solvent changes from acetonitrile to toluene (or dioxane), the rate determining step would vary from ortho C—H activation to the TsCl dissociation and the reaction activation energy would increase accordingly. This trend is consistent with the experimen- tal yields reported by Frost et al. All these consistency verify the calculation results provided in this study. Keywords Ru catalysis; meta sulfonation; C—H functionalization; mechanism; density functional theory (DFT)

Base-Catalyzed Dehydration of 3-Substituted Benzene cis-1,2-Dihydrodiols: Stabilization of a Cyclohexadienide Anion Intermediate by Negative Aromatic Hyperconjugation

Evidence that a 1,2-dihydroxycyclohexadienide anion is stabilized by aromatic "negative hyperconjugation" is described. It complements an earlier inference of "positive" hyperconjugative aromaticity for the cyclohexadienyl cation. The anion is a reactive intermediate in the dehydration of benzene cis-1,2-dihydrodiol to phenol. Rate constants for 3-substituted benzene cis-dihydrodiols are correlated by σ(-) values with ρ = 3.2. Solvent isotope effects for the reactions are k(H(2)O)/k(D(2)O) = 1.2-1.8. These measurements are consistent with reaction via a carbanion intermediate or a concerted reaction with a "carbanion-like" transition state. These and other experimental results confirm that the reaction proceeds by a stepwise mechanism, with a change in rate-determining step from proton transfer to the loss of hydroxide ion from the intermediate. Hydrogen isotope exchange accompanying dehydration of the parent benzene cis-1,2-dihydrodiol was not found, and thus, the proton transfer step is subject to internal return. A rate constant of ~10(11) s(-1), corresponding to rotational relaxation of the aqueous solvent, is assigned to loss of hydroxide ion from the intermediate. The rate constant for internal return therefore falls in the range 10(11)-10(12) s(-1). From these limiting values and the measured rate constant for hydroxide-catalyzed dehydration, a pK(a) of 30.8 ± 0.5 was determined for formation of the anion. Although loss of hydroxide ion is hugely exothermic, a concerted reaction is not enforced by the instability of the intermediate. Stabilization by negative hyperconjugation is proposed for 1,2-dihydroxycyclohexadienide and similar anions, and this proposal is supported by additional experimental evidence and by computational results, including evidence for a diatropic ("aromatic") ring current in 3,3-difluorocyclohexadienyl anion.