P-toluenesulfonic Acid Monohydrate Research Articles

Innovative deep decontamination of radioactive solid surfaces using microemulsions of deep eutectic solvents in supercritical carbon dioxide

Radioactive solid wastes resulting from the application of nuclear energy pose a risk to the environment, yet traditional decontamination methods generate many secondary wastes. Thus, sustainability-driven innovations are receiving increasing attention to achieve eco-friendly decontamination methods characterized by minimizing secondary waste generation. Here, we proposed a sustainable and pioneering approach to decontaminate radioactive solid wastes using microemulsions of deep eutectic solvents (DESs) in supercritical carbon dioxide (SC-CO2). We also determined the optimal ratios of the additives as follows: prepared DESs with a molar ratio of choline chloride (ChCl) to p-toluenesulfonic acid monohydrate (ptsa) of 1:2, and nonyl phenol polyoxyethylene ether (NP-10) to ChCl:ptsa DESs in a molar ratio of 0.8. Remarkably, after optimization of the decontamination process in terms of pressure, temperature and cleaning time, the radioactivity on the sample surface can be reduced to a level that is reusable in the nuclear field. Detailed analyses provided insights into the intermolecular interactions within the DESs. And the decontamination mechanisms involved the spontaneous evolution of microemulsions with ChCl:ptsa as the core to facilitate the solvation and removal of radioactive contaminants. Additionally, repeated cleaning was proved to be effective for ultra-deep decontamination, with the radioactivity on the sample surface reaching a clearance level and a decontamination efficiency of 99.8%. The DESs-in-CO2 systems successfully decontaminated various materials and simulated radionuclides, even real radioactive metal wastes from the nuclear industry. It was notable that the DESs-in-CO2 systems achieved decontamination efficiencies of more than 95.0% for radioactive contaminants on these solid surfaces. Overall, this innovative approach provided a green alternative to conventional radioactive solids decontamination, meeting the need for sustainable development of nuclear energy.

Facile synthesis of functional binaphthyl-based hyperbranched polyarylenes by organo-catalyzed alkyne polycyclotrimerization

Binaphthyl-based polymers have attracted increasing interest due to their unique properties and wide applications. Nevertheless, binaphthyl-containing hyperbranched polymers are rarely reported. Herein, two binaphthyl-based hyperbranched polyarylenes with high molecular weights were facilely synthesized in satisfactory yields by the polycyclotrimerization of internal diynes containing binaphthyl units in 1,2,4-trichlorobenzene (TCB) under heating in the presence of p-toluenesulfonic acid monohydrate (p-TsOH·H2O). The resultant polymers are soluble in common organic solvents, and thermally stable with 5% weight loss temperatures higher than 380 °C. They showed electrochemical properties, and optical activities. Furthermore, the polymer-doped polyvinyl alcohol (PVA) films exhibited yellow room-temperature phosphorescence (RTP) with the lifetime longer than 630 ms.

Blue Electrophosphorescence from Iridium(III) Phosphors Bearing Asymmetric Di-N-aryl 6-(trifluoromethyl)-2H-imidazo[4,5-b]pyridin-2-ylidene Chelates.

Efficient blue phosphors remain a formidable challenge for organic light-emitting diodes (OLEDs). To circumvent this obstacle, a series of Ir(III)-based carbene complexes bearing asymmetric di-N-aryl 6-(trifluoromethyl)-2H-imidazo[4,5-b]pyridin-2-ylidene chelates, namely, f-ct6a‒c, are synthesized, and their structures and photophysical properties are comprehensively investigated. Moreover, these emitters can undergo interconversion in refluxing 1,2,4-trichlorobenzene, catalyzed by a mixture of sodium acetate (NaOAc) and p-toluenesulfonic acid monohydrate (TsOH·H2O) without decomposition. All Ir(III) complexes present good photoluminescence quantum yield (ΦPL = 83-88%) with peak maximum (max.) at 443-452nm and narrowed full width at half maximum (FWHM = 66-73nm). Among all the fabricated OLED devices, f-ct6b delivers a max. external quantum efficiency (EQE) of 23.4% and Commission Internationale de L'Eclairage CIEx , y coordinates of (0.14, 0.12), whereas the hyper-OLED device based on f-ct6a and 5H,9H,11H,15H-[1,4] benzazaborino [2,3,4-kl][1,4]benzazaborino[4',3',2':4,5][1,4]benzazaborino[3,2-b]phenazaborine-7,13-diamine, N7,N7,N13,N13,5,9,11,15-octaphenyl (ν-DABNA) exhibits max. EQE of 26.2% and CIEx , y of (0.12, 0.13). Finally, the corresponding tandem OLED with f-ct6b as dopant gives a max. luminance of over 10000 cd m-2 and max. EQE of 42.1%, confirming their candidacies for making true-blue OLEDs.

Synthesis of isoamyl hexanoate catalyzed by deep eutectic solvent based on benzyltrimethylammonium chloride: RSM and ANN optimization and kinetics

AbstractBackgroundIsoamyl hexanoate is a widely used edible spice. In this paper, the process and kinetics of the esterification of hexanoic acid and isoamyl alcohol to isoamyl hexanoate catalyzed by deep eutectic solvents (DESs) were studied. Several kinds of DESs (BTMAC‐zPTSA) were prepared from benzyltrimethylammonium chloride (BTMAC) and p‐toluenesulfonic acid monohydrate (PTSA) with a molar ratio of z. The synthesis process of isoamyl hexanoate was optimized by response surface methodology and artificial neural network. The effects of process conditions such as the kind and load of catalyst, alcohol/acid ratio and reaction temperature were studied. In the temperature range of 338.15 to 368.15 K, BTMAC‐1.2PTSA was selected as the catalyst, and then the kinetic data and chemical equilibrium composition of the esterification of hexanoic acid with isoamyl alcohol were determined.ResultsAfter optimizing the synthesis process of isoamyl hexanoate using response surface methodology and artificial neural network, the conversion rate of hexanoic acid increased to 92.15%. The esterification reaction kinetics were described using a pseudo‐homogeneous model based on activity, with an average relative deviation of 1.9% and activation energy of 25.79 kJ mol−1. The catalytic performance of catalyst BTMAC‐1.2PTSA remains good after five cycles of use.ConclusionThe established kinetic model has a good predictive effect on the kinetic data for the synthesis of isoamyl hexanoate. The catalyst BTMAC‐1.2PTSA has good stability and recyclability, and has the potential to replace traditional catalysts in the production of isoamyl hexanoate. © 2023 Society of Chemical Industry.

Synthesis and characterization of acidic deep eutectic solvents based on p-Toluenesulfonic acid

There is growing interest in identifying suitable, less hazardous deep eutectic solvent (DES) alternatives for conventional solvents and catalysts, particularly acid catalysts which can be volatile and highly corrosive. In this study, novel acidic DESs based on mixtures of either tetrabutylammonium hydrogensulfate (TBAHS) (DES1) or tetraphenylphosphonium bromide (TPPB) (DES2) with p-toluenesulfonic acid monohydrate (PTSAM) are prepared. The properties of these DESs were investigated using multiple analytical techniques, including density, viscosity, conductivity and diffusion measurements. DES1 formed a room-temperature liquid, whereas DES2 melted at 337 K at its eutectic point. Hammett acidity studies revealed that both the DESs show super acidity (H0DES1 = -2.91 and H0DES2 = -1.89) even when present in relatively dilute (40 mM) aqueous solutions. Conductivity measurements established that both DESs show good ionicity at elevated temperatures based on a Walden plot, although DES2 displayed reduced ionicity close to its melting point. Equilibrium structural arrangements for these DESs are proposed based on findings, with the formation of a complex hydrogen-bonded anion and isolated cation. These DESs are proposed as potential replacements for ionic liquids, and existing acid catalysts in acid catalyzed reactions.

Combined process of cyclohexyl cyclohexanecarboxylate synthesis from cyclohexanol and CO catalyzed by the Pd(OAc)2–PPh3–p-toluenesulfonic acid system

Objectives. To study the possibility of combining acid-catalytic cyclohexanol dehydration and alkoxycarbonylation of the formed cyclohexene with cyclohexanol and carbon(II) oxide in a single reactor in order to achieve high yields of the target cyclohexyl cyclohexanecarboxylate product under mild conditions using the Pd(OAc)2–PPh3–p-toluenesulfonic acid catalytic system.Methods. The combined process took place in a toluene medium in a periodic steel reactor designed to operate at elevated pressure, equipped with a glass insert, a magnetic stirrer, and a sampler, as well as gas input and discharge devices. The reaction mass with the components of the catalytic system was placed in a glass reactor inside a steel autoclave. The reaction mass samples obtained during the combined process were analyzed by gas–liquid chromatography with a flame ionization detector.Results. The possibility of combining cyclohexanol dehydration catalyzed by p-toluenesulfonic acid monohydrate and formed cyclohexene alkoxycarbonylation with cyclohexanol and CO during catalysis by the Pd(OAc)2–PPh3–p-toluenesulfonic acid system in a single reactor was demonstrated. Under mild conditions (temperature 110°C; CO pressure 2.1 MPa), the target product yield reached 64.8% in 5 h. However, the combined process is complicated by the formation of a cyclohexanecarboxylic acid by-product formed as a result of the cyclohexyl cyclohexanecarboxylate hydrolysis and the cyclohexene hydroxycarbonylation.Conclusions. The reactions of intramolecular acid-catalytic cyclohexanol dehydration and formed cyclohexene alkoxycarbonylation catalyzed by the Pd(OAc)2–PPh3–p-toluenesulfonic acid system can be combined in a single reactor. p-Toluenesulfonic acid can simultaneously act as a catalyst for the cyclohexanol dehydration and a co-catalyst of the palladium–phosphine system of cyclohexene alkoxycarbonylation. The involvement of cyclohexene, representing a product of reversible cyclohexanol dehydration, in the alkoxycarbonylation reaction is a factor in shifting the dehydration reaction equilibrium towards the formation of cyclohexene. Cyclohexanecarboxylic acid is a by-product of the proposed combined process. A factor in the reduction of target product yield is water formed as a result of cyclohexanol dehydration due to the involvement of the latter in the hydrolysis reaction and the course of the cyclohexene hydroxycarbonylation.

Environmentally benign synthesis of 2,4-diarylquinolines under metal- & solvent-free conditions

The synthesis of 2,4-diarylquinolines is reported by employing arylamine, aryl aldehyde, and aryl acetylene using 30 mol% p-toluenesulfonic acid monohydrate (p-TSA·H2O) as a catalyst at 110 °C temperature under solvent-free conditions via a one-pot three-component reaction. The salient features of the present protocol are the nonrequirement of a metal catalyst, additive, solvent, and inert atmospheric reaction conditions, good yield, substrate scope, and shorter reaction time. Using 30 mol% p-toluenesulfonic acid monohydrate (p-TSA·H2O) as a catalyst in solvent-free conditions has made the three-component reaction efficient enough to form 2,4-diarylquinoline without the involvement of any heavy metal catalyst or additives.

Encapsulated paracetamol-based eutectic solvents for the treatment of low-grade palm oil mixed with microalgae oil

The development of an efficient and green catalyst is a key step for the esterification of free fatty acid (FFA) in non-edible oil feedstocks. This study introduces a novel acidic deep eutectic solvent (DES) catalyst derived from an active pharmaceutical ingredient for FFA esterification. The DES was formed by paracetamol and p-toluenesulfonic acid monohydrate (PTSA), as the hydrogen bond acceptor (HBA) and the hydrogen bond donor (HBD) components, respectively. The DES was encapsulated in medical capsules for the loading and handling steps; and they utilized to esterify a high content of FFA (8.11%) in a low-grade palm oil-microalgae oil mixture to achieve an acceptable FFA limits (<2%) for transesterification reaction. The FFA content was decreased to < 2% under optimum conditions (catalyst dosage 2% wt/wt of catalyst to oil, 10:1 molar ratio of methanol to oil at 60 °C for 60 min reaction time). Paracetamol forms a eutectic mixture with PTSA and is an effective HBA component in the formation of the DES catalyst. The analysis of relative solubility screening, σ-profile and σ-potential confirm certain level of solubility and homogeneity of this DES in methanol during the pre-treatment process. The formation of the eutectic mixture endows the catalyst with advantages for the esterification reaction such as improvements in recyclability and hygroscopicity of PTSA.

Selective and Efficient Synthesis of Pine Sterol Esters Catalyzed by Deep Eutectic Solvent.

Phytosterol esters have attracted widespread academic and industrial interests due to their advantages in lowering cholesterol, as antioxidants, and in preventing or treating cancer. However, the generation of by-products limits the application of phytosterol esters in food fields. In this study, deep eutectic solvents (DESs), a series of green, nontoxic, low-cost and biodegradable solvents, were adopted as the catalyst for the synthesis of pine sterol esters. The results showed that the acidic DES which was prepared with choline chloride (ChCl) and p-toluene sulfonic acid monohydrate (PTSA) with a molar ratio of 1:3 performed best in the prescreening experiments. To further improve the efficiency of the pine sterol ester, the molar ratio of substrates, the amount of catalyst, the reaction temperature and the reaction time were optimized, and its yield was improved to 94.1%. Moreover, the by-products of the dehydration side reactions of the sterol can be efficiently inhibited. To make this strategy more universal, other fatty acids were also used as the substrate for the synthesis of pine sterol esters, and a yield of above 92.0% was obtained. In addition, the reusability of DES was also investigated in this study, and the efficiency of DES was well maintained within five recycled uses. Finally, DFT calculations suggested that the suitable H-bonds between ChCl and PTSA decreased the nucleophilic capacity and increased the steric hindrance of the latter, and further prevented the attack on βH and reduced the generation of by-products. This study developed a reliable and eco-friendly strategy for the preparation of high-quality phytosterol esters with low-dosage catalyst usage and high selectivity.

Aryl sulfonic acid catalysts: Effect of pendant group structure on activity in hydrolysis of polyethylene terephthalate

AbstractA series of aryl sulfonic acids were tested as catalysts for acid hydrolysis occurring at the surface of poly(ethylene) terephthalate (PET) particles. Specifically, p‐toluenesulfonic acid monohydrate (PTSA), 2‐naphthalenesulfonic acid (2‐NSA), and 1,5‐naphthalenedisulfonic acid tetrahydrate (1,5‐NDSA) were chosen to provide sulfonic acid active groups and varying hydrophobic functionality. The effect of catalyst concentration and reaction temperature on PET hydrolysis rate was studied. The aryl sulfonic acid catalysts exhibited much higher rates of PET hydrolysis than the mineral acid, H2SO4. At 150°C and 4 M catalyst, the time required to achieve more than 90% TPA yield was 3, 3, and 8 h, and 18 h for (PTSA), (2‐NSA), (1,5‐NDSA), and H2SO4, respectively. Ethyl acetate hydrolysis was performed as a model reaction to probe the activity of the catalysts in homogenous reactions to compare with the heterogenous hydrolysis reaction occurring at the PET surface. The higher catalytic activities for PET hydrolysis of the PTSA, 2‐NSA, and 1,5‐NDSA than H2SO4 was attributed to improved wetting by the reaction media and affinity of the aryl sulfonic acid catalysts for the PET surface.

Direct Synthesis of Coumarin Derivatives from Alkynoic Esters via Dual Organocatalysis

AbstractAn efficient synthetic method for coumarin derivatives was developed using a dual organocatalytic reaction. A combination of p-toluenesulfonic acid monohydrate and piperidine was found to efficiently catalyze the cyclization between salicylaldehydes and alkynoic esters to give various coumarin derivatives in good yield and high selectivity. Mechanistic and kinetic data suggested that the conjugate addition between piperidine and alkynoic esters played a crucial role in the reaction mechanism.

Exploration of Oxidative Ritter-Type Reaction of α-Arylketones and Its Application for the Collective Total Syntheses of Erythrina Alkaloids

Exploration of Oxidative Ritter-Type Reaction of α-Arylketones and Its Application for the Collective Total Syntheses of <i>Erythrina</i> Alkaloids

Surface decontamination of protective duplex oxide layers on stainless steel waste using deep eutectic solvents

The decontamination capabilities of deep eutectic solvents (DESs) formed from choline chloride (ChCl) and p-toluenesulfonic acid monohydrate (PtsA), ChCl:PtsA, under different conditions (hydrated, heated, and agitated) were tested with simulant oxidized stainless steel 304 specimens. Although the leaching rates were satisfactory under all conditions, hydrated and stirred ChCl:PtsA at 60 °C showed the fastest leaching rate of 0.1647 mg/min. Oxidized specimens with an average mass gain of 1.2 ± 1 mg were leached, and their masses were reduced by 558 ± 22 mg after 26 h. These results were understood by improved physical properties of ChCl:PtsA upon hydration. Metal oxide solubility of CoO and NiO increased with water, and those of Cr2O3 and Fe3O4 decreased with hydration. Importantly, the use of choline chloride-based DESs in decontamination applications may significantly reduce the cost of decontamination because these DESs can be mass-produced and their components are both easily obtainable and economical. Also, DESs are biodegradable and eco-friendly. The different speciation of Co and Ni, which bond with Cl-, compared with Fe and Cr, which bond with H2O, illustrated the potential for a metal recovery for secondary liquid waste reduction.

Physical properties of deep eutectic solvents based on p-toluene sulfonic acid and employment as catalyst

This work studied the physical properties of three deep eutectic solvents (DESs) and their catalytic activity in the esterification of palmitic acid and ethanol. Three types of deep eutectic solvents (DESs) were successfully prepared using p-toluene sulfonic acid monohydrate (PTSA) as hydrogen bond donor (HBD) with dodecyl trimethyl ammonium chloride (DTAC), tetradecyl trimethyl ammonium chloride (TTAC) and octadecyl trimethyl ammonium chloride (OTAC) as hydrogen bond acceptor (HBA). The freezing points of DESs were measured in the range of 50%–80% HBD mole percentage, and the composition of DTAC-PTSA, TTAC-PTSA and OTAC-PTSA at the eutectic point (the lowest freezing points) was determined respectively (DES1, DES2 and DES3). The physical properties (density, refractive index, dynamic viscosity, conductivity and pH value) of DES1∼DES3 were studied respectively in the temperature range of 298.15 K to 343.15 K. The results showed that dynamic viscosity, conductivity, density and refractive index all change approximately regularly with the increase of temperature, while pH value remains almost constant when the temperature exceeds 325 K. The catalytic ability of the three DESs in the esterification reaction of palmitic acid and ethanol was investigated, and DES2 was selected as the catalyst for subsequent experiments. The effects of temperature, acid-ethanol molar ratio, catalyst dosage and reaction time on esterification reaction were further studied, and DES’ recyclability was also tested. The results obtained showed the great potential of DES in catalytic applications.

Controllable conversion of shrimp shells into chitin or derived carbon material using acidic deep eutectic solvent

It is important to convert waste shrimp shells into chitin and its derivatives, but it still faces disadvantages of pollution and long process. In this research, deep eutectic solvents made from choline chloride (ChCl) and p-toluenesulfonic acid monohydrate (TsOH) were used to convert shrimp shells into chitin or carbon materials through adjustment of temperature with a dissolution-regeneration method in one step. Obtained precipitates and regenerated products gradually changed from chitin to carbon materials with the temperature adjusted to 130 °C and 110 °C, respectively. The α-chitin with the highest purity of 97.9 ± 0.1 wt% was prepared using 15 wt% ChCl/TsOH (1:2, n:n) aqueous solution at 70 °C, and the yield was 59.4 ± 1.9 wt%. Moreover, the carbon materials prepared by ChCl/TsOH (1:4) treatment with water and ethanol as anti-solvent were S/N/O-doped carbon materials, which exhibited potential in the supercapacitor with a specific capacitance of 24.8 and 19.3 F/g at 1 A/g, respectively.

Selenium Dioxide-Mediated Bromination of α,β-Unsaturated Ketones Using N-Bromosuccinimide in the Presence of p-Toluenesulfonic Acid: A Versatile Route for the Synthesis of α'-Bromo-4-arylbut-3-en-2-one and α',α'-Dibromo-4-arylbut-3-en-2-one.

An efficient method for the synthesis of α,β-unsaturated α′-bromoketones and α,β-unsaturated α′,α′-dibromoketones is described using N-bromosuccinimide (NBS) as the brominating agent mediated by selenium dioxide (SeO2) in the presence of p-toluenesulfonic acid (PTSA) monohydrate in toluene. The method is simple, employing easily available shelf reagents to afford a wide range of products in good yields. The method highlighted that simple fine-tuning of the reaction conditions and molar equivalents of the reactants easily affords either mono- or dibrominated products in excellent yields. A number of these products have not been reported in the literature. All of the reactions were carried out in gram-scale quantities.

P-Toluenesulfonic Acid-Catalyzed Reaction of Phthalaldehydic Acids with Difluoroenoxysilanes: Access to 3-Difluoroalkyl Phthalides

AbstractA convenient approach for the synthesis of 3-difluoroalkyl phthalides has been developed from phthalaldehydic acids and difluoroenoxysilanes by using relatively inexpensive p-toluenesulfonic acid monohydrate (PTSA) as a catalyst. A series of 3-difluoroalkyl phthalides and cyclic difluoroalkyl ethers were obtained in up to 99% yield. The products obtained could be readily converted into difluoroalkyl phthalide derivatives by simple modifications.

Kinetic study on the reaction of p‐tert‐butylbenzoic acid with methanol catalyzed by deep eutectic solvent based on choline chloride

AbstractThe synthesis of methyl p‐tert‐butylbenzoate using deep eutectic solvent (DES) as a green catalyst was studied in this work. Four DESs were prepared by combining choline chloride (ChCl) and p‐toluenesulfonic acid monohydrate (PTSA) with different molar ratios (1:1–3). It was found that ChCl‐1.5PTSA with a molar ratio of 1:1.5 was superior to other DESs both in physical properties and catalytic performance. Therefore, it was selected to carry out the kinetic experiments. The effects of stirring speed, temperature, molar ratio of methanol to p‐tert‐butylbenzoic acid, and catalyst loading on the conversion of p‐tert‐butylbenzoic acid were discussed. In addition, the reaction kinetics of p‐tert‐butylbenzoic acid and methanol with ChCl‐1.5PTSA as catalyst was explored in the temperature range of 332.15–349.15 K, and the experimental data were well fitted by pseudo‐homogeneous model. Moreover, the recyclability of ChCl‐1.5PTSA was evaluated. The result shows that the catalyst could be easily separated from the reaction system without any extractant, and the performance of the catalyst remains stable in multiple times recycling. Therefore, ChCl‐1.5PTSA is expected to be an efficient and environmentally friendly catalyst for the methyl esterification of p‐tert‐butylbenzoic acid with a bright future in industrial application.

Synthesis of functionalised 2,3-dihydroquinolin-4(1H)-ones vs. quinoline or N-alkenylindole derivatives through sequential reactions of 2-alkynylanilines with ketones.

This study describes diversity-oriented synthesis of 2,2,3-substituted-2,3-dihydroquinolin-4(1H)-ones vs. functionalised quinoline or N-alkenylindole derivatives through Brønsted acid mediated or Lewis acid catalyzed sequential reactions of 2-alkynylanilines with ketones. In particular, a series of challenging quinolin-4-one derivatives are prepared with good functional group tolerance in an atom-economical fashion by using p-toluenesulfonic acid monohydrate as a promoter of the reaction of ketones with 2-alkynylanilines in EtOH at reflux, while the same starting materials give the corresponding 4-substituted quinolines in toluene at 110 °C both in the presence of p-toluenesulfonic acid monohydrate as the promoter and FeCl3 as the catalyst. The divergent formation of N-alkenylindole derivatives occurs by switching to the use of ZnBr2 as the catalyst under the same reaction conditions. Conversely, only 4-methylsubstituted quinoline derivatives were isolated by reacting 2-ethynylanilines and/or 2-trimethylsylilanilines with ketones in all examined cases.

Solid-State 17O NMR Studies of Sulfonate Jump Dynamics in Crystalline Sulfonic Acids: Insights into the Hydrogen Bonding Effect.

We report variable-temperature (VT) 17O solid-state nuclear magnetic resonance (NMR) spectra for three crystalline sulfonic acids: l-cysteic acid monohydrate (CA), 3-pyridinesulfonic acid (PSA), and p-toluenesulfonic acid monohydrate (TSA). We were able to analyze the experimental VT 17O NMR spectra to obtain the activation barriers for SO3- jumps in these systems. Using the density functional-based tight-binding (DFTB) method, we performed potential energy surface scans for SO3- jumps in the crystal lattice of CA, PSA, and TSA, as well as for three related crystalline sulfonic acids (taurine, homotaurine, and 4-aminobutane-1-sulfonic acid) for which relevant 17O solid-state NMR data are available in the literature. The calculated activation barriers are in reasonable agreement with the experimental values. On the basis of the DFTB results, we hypothesized that activation barriers for SO3- jumps in the crystal lattice depend largely on the hydrogen bonding energy difference between the ground state and the transition state.