Gibbs Free Energy Of Activation Research Articles

Studies on excess molar volumes and viscosity deviations of binary mixtures of household kerosene and premium motor spirit at 303K

Density and viscosity of binary liquid mixtures of household kerosene (HHK) and premium motor spirit (PMS) were determined over the entire range of mole fraction at 303K. Excess molar volume (VE), molar volume (Vm), Excess Viscosity (ηE), Viscosity deviations (Δη) and excess Gibbs free energies of activation of viscous flow (ΔGE) were calculated. The results obtained shows that excess free energy of activation were all positive over the entire mole fraction. The viscosity deviation shows both negative and positive values. The positive VE obtained in this study shows increase in volume in the binary mixture. This work will help in effective monitoring, detection of adulterated kerosene and subsequent enforcement of severe penalty on such adulteration.

Densities and viscosities of (ethylene glycol - water) and (1,3-propanediol - water) binary mixtures at high temperatures of 373.2 and 473.2 K and high pressures up to 40 MPa

Densities and viscosities of ethylene glycol - water and 1,3-propanediol - water binary mixtures have been measured over the whole composition range at 373.2 K and 473.2 K and at pressures up to 40 MPa. The excess molar volumes combined with partial molar volumes, and viscosity deviations have been calculated from the measurement data. The activation Gibbs free energy of viscous flow has been also evaluated from the density and viscosity data. These results were compared with the previously reported results of the monohydric alcohol - water mixtures with the same carbon numbers to discuss the effect of hydroxyl group and alkyl chain length on these properties in the wide range of temperature and pressure. The physical properties of the alcohol - water mixtures were strongly influenced by the molecular structure of the alcohol molecules under the studied conditions. In particular, the excess molar volumes and viscosity deviations showed very different behaviors for monohydric and dihydric alcohol - water mixtures.

The Participation of 3,3,3-Trichloro-1-nitroprop-1-ene in the [3 + 2] Cycloaddition Reaction with Selected Nitrile N-Oxides in the Light of the Experimental and MEDT Quantum Chemical Study.

The regioselective zw-type [3 + 2] cycloaddition (32CA) reactions of a series of aryl-substituted nitrile N-oxides (NOs) with trichloronitropropene (TNP) have been both experimentally and theoretically studied within the Molecular Electron Density Theory (MEDT). Zwitterionic NOs behave as moderate nucleophiles while TNP acts as a very strong electrophile in these polar 32CA reactions of forward electron density flux, which present moderate activation Gibbs free energies of 22.8–25.6 kcal·mol−1 and an exergonic character of 28.4 kcal·mol−1 that makes them irreversible and kinetically controlled. The most favorable reaction is that involving the most nucleophilic MeO-substituted NO. Despite Parr functions correctly predicting the experimental regioselectivity with the most favorable O-CCCl3 interaction, these reactions follow a two-stage one-step mechanism in which formation of the O-C(CCl3) bond takes place once the C-C(NO2) bond is already formed. The present MEDT concludes that the reactivity differences in the series of NOs come from their different nucleophilic activation and polar character of the reactions, rather than any mechanistic feature.

Enhancement of SNAr reactions by CH3SO3− ionic liquid and organic solvent dimethylformamide as bifunctional organocatalysts: A mechanistic study

AbstractQuantum chemical analysis is presented for the mechanism of SNAr reaction of p‐fluoronitrobenzene and p‐anisidine promoted by [C4C1im][CH3SO3] and by dimethylformamide (DMF), experimentally observed by Welton and co‐workers (Org. Lett. 2007, 9, 5247–5250). We find that [C4C1im]+ assists the detachment of the fluoride leaving group, while [CH3SO3]− supplies partial negative charge to the nucleophile NH2 to enhance the reactivity. We also show the moderate yield (22.1%) in DMF can also be understood by considering the solvent molecule as a bifunctional organocatalyst. The calculated Gibbs free energies of activation are in excellent agreement with the experimental observations.

Kinetic analysis of nitrophenol reduction and colourimetric detection of hydrogen peroxide based on gold nanoparticles catalyst biosynthesised from Cynomorium songaricum

The pseudo-first-order reaction kinetics of metallic nanocatalyst for the model reaction of nitrophenols/NaBH4 were early reported on low reaction temperatures. Here, we investigated reduction of nitrophenols in a wide range of reaction temperature with gold nanoparticles (AuNPs) catalyst that was fabricated via a low-cost and effective method using aqueous extract of Cynomorium songaricum (CS). The biosynthesised CS-AuNPs were well characterised by the analytic techniques such as X-ray diffraction (XRD), transmission electron microscope (TEM), scanning transmission electron microscopy (STEM). The TEM images showed multiple shapes and size in a range of 3–30 nm with an average size of 16 nm. The colloidal solutions were stored at the room temperature for 30 days with a stability of 63%. The kinetic study of the NiPs reduction showed that the reaction occurred via Langmuir–Hinshelwood mechanism with a half-order kinetic model for all tested reaction temperatures (30–70 °C). The thermodynamic parameters including activation energy, enthalpy, entropy and Gibbs free energy were calculated. The activation energies in the reduction of 2-nitrophenol and 3-nitrophenol were found to be as low as 4.88 kJ mol−1 and 8.66 kJ mol−1, respectively. Moreover, CS-AuNPs were used as a nanozyme for the detection of hydrogen peroxide (H2O2) via peroxidase-like reaction with 3,3′,5,5′-tetramethylbenzidine (red-TMB) that found the lowest limit of detection (LOD) of 2.25 µM.

Quantum Chemical Investigations on the Hydrolysis of Gold(III)-Based Anticancer Drugs and Their Interaction with Amino Acid Residues.

A comprehensive hydrolysis mechanism of the promising class of Au(III) anticancer drugs [Au(DMDT)Cl2] (DMDT = N,N-dimethyldithiocarbamate) (R) and [Au(damp)Cl2] (damp = 2-[(dimethylamino)methyl]phenyl) (R′) was done by means of density functional theory (DFT) in combination with the CPCM solvation model to explore the solution behavior and stability under physiological conditions. The activation free energies (ΔG) for the second hydrolysis, R (13.7 kcal/mol) and R′ (10.0 kcal/mol) are found to be relatively lower in comparison to the first hydrolysis, and their rate constant values are computed to be 5.62 × 102 and 2.90 × 105 s–1, respectively. Besides these, the interaction mechanisms of aquated R and R′ with the potential protein-binding sites cysteine (Cys) and selenocysteine (Sec) were also investigated in detail. The kinetic study and activation Gibbs free energy profiles reveal that the aquated complexes of R and R′ bind more effectively to the Se site of Sec than to the S site of Cys. Intra- and intermolecular hydrogen bonding play a pivotal role in stabilizing the intermediates and transition states involved in the ligand substitution reactions of R and R′. Natural population analysis (NPA) was done to determine the charge distributions on important atoms during the hydrolysis and ligand substitution reactions.

Esterification of Khaya senegalensis seed oil with a solid heterogeneous acid catalyst: Modeling, optimization, kinetic and thermodynamic studies

The current study investigated the esterification of Khaya senegalensis seed oil (KSSO) with a high free fatty acid (FFA) using ferric sulfate (Fe 2 (SO 4 ) 3 ) as a solid heterogeneous acid catalyst. Taguchi design approach was applied in modeling and optimizing the input variables to obtain the minimum %FFA for the KSSO esterification process. Besides modeling and optimization, the kinetic and thermodynamic studies of the process were carried out to obtain pertinent data required for reactor design. The optimum values with the minimum %FFA (0.49) for the esterification process are a temperature of 60 °C, methanol-to-KSSO ratio of 15:1, ferric sulfate dosage of 9 wt%, and time of 60 min. The reaction of the triglycerides in the KSSO with methanol showed that it followed first-order kinetics. The pre-exponential factor ( A ), activation energy ( E a ), and reaction rate constant ( k ) for the reaction are 37.73 min -1 , 18.11 kJ/mol, and 0.0554 min -1 , respectively, at a temperature of 60 °C, indicating that the esterification process proceeded at a fast rate. The thermodynamic properties determined for the process are Δ S § (entropy), Δ H § (enthalpy), and Δ G § (Gibbs free energy) of activation with respective values of – 0.205 kJ/mol.K, 15.34 kJ/mol, and 83.54 kJ/mol, confirming the endergonic, endothermic, and non-spontaneous nature of the reaction. Thus, the determination of key process conditions with the kinetic and thermodynamic properties of the KSSO esterification process could aid the use of the oil as a viable feedstock for biodiesel synthesis and reactor design for industrial applications. • High free fatty acid Khaya oil was esterified with methanol to < 1%. • Ferric sulfate served as a solid heterogeneous acid catalyst for the esterification. • Minimum %FFA of 0.49 was obtained with the optimum values predicted by the model. • The esterification process followed pseudo first-order kinetics. • The nature of the reaction was endergonic, endothermic, and non-spontaneous.

Origin of Salt Effects in SN2 Fluorination Using KF Promoted by Ionic Liquids: Quantum Chemical Analysis.

Quantum chemical analysis is presented, motivated by Grée and co-workers’ observation of salt effects [Adv. Synth. Catal. 2006, 348, 1149–1153] for SN2 fluorination of KF in ionic liquids (ILs). We examine the relative promoting capacity of KF in [bmim]PF6 vs. [bmim]Cl by comparing the activation barriers of the reaction in the two ILs. We also elucidate the origin of the experimentally observed additional rate acceleration in IL [bmim]PF6 achieved by adding KPF6. We find that the anion PF6− in the added salt acts as an extra Lewis base binding to the counter-cation K+ to alleviate the strong Coulomb attractive force on the nucleophile F−, decreasing the Gibbs free energy of activation as compared with that in its absence, which is in good agreement with experimental observations of rate enhancement. We also predict that using 2 eq. KF together with an eq. KPF6 would further activate SN2 fluorination

VOLUMETRIC AND VISCOMETRIC DETERMINATION OF BINARY LIQUID MIXTURES OF 2-ETHOXYETHANOL AND AMYL ACETATE AT TEMPERATURES OF (298.15 AND 303.15) K

Densities and viscosities have been determined at temperatures of (298.15 and 303.15) K for binary liquid mixtures of 2-ethoxyethanol with amyl acetate over the entire range of solvent composition. From the experimentally determined values, excess molar volume, VE, excess viscosity, ηE and excess Gibbs free energy of activation of viscous flow, G*E have been calculated. These calculated excess thermodynamic functions were fitted to Redlich-Kister polynomial equation to obtain the fitting coefficients and standard deviations. The values of excess molar volume, VE were negative and that of the excess viscosities, ηE were found to be positive, while excess Gibbs free energy of activation of viscous flow, ΔG*E were negative over the whole range of solvent composition for the binary systems at the studied temperatures. The behavior of these excess functions with composition of the mixtures have been discussed in respect of molecular interactions between the components of liquid systems. The mixture viscosities were correlated using empirical relations like Grunberg-Nissan, Frenkel, Hind, Wijk and Kendall-Monroe model equations. Comparison of these interaction parameters have been engaged to explain the intermolecular interactions between the alkoxy alcohol and the ester.

Volumetric and thermodynamic properties of binary mixtures of p-cymene with α-pinene, limonene and citral at atmospheric pressure and temperatures up to 323.15 K

An experimental analysis of density, refractive index, speed of sound and the viscosity at atmospheric pressure and temperatures T = (288.15 to 323.15) K has been carried out for the three binary systems, i.e. (p-cymene + α-pinene), (p-cymene + limonene) and (p-cymene + citral). Excess molar volumes, viscosity deviations, deviations in isentropic compressibility and excess Gibbs free energy of activation of viscous flow were calculated from experimental data. The results were fitted employing Redlich-Kister polynomial equation. Also, partial molar volumes, excess partial molar volumes and partial molar volumes at infinite dilutions were calculated to elucidate the non-ideal behavior of investigated mixtures. Furthermore, Prigogine-Flory-Patterson (PFP) model was successfully applied for the correlation of excess molar volumes of examined binary mixtures. The results of excess molar volume correlations show very good agreement with experimental measurement.

Efficient synergistic chemical fixation of CO2 by simple metal-free organocatalysts: Mechanistic and kinetic insights

BackgroundCarbon dioxide is a versatile, non-toxic, and renewable C1 building block source to produce valuable chemicals such as cyclic carbonates. The purpose of this research is development of efficient, metal-free and affordable catalysts for cycloaddition reaction of CO2 with epoxides at mild conditions. MethodsIn the present work, a group of novel bifunctional organocatalysts consisting of different hydrogen-bond donors (HBDs) and halogen ion is simply synthesized. Under the optimized conditions, the reaction is kinetically and thermodynamically studied. Additionally, the catalytic active species are determined, and a mechanism is proposed with the help of electrospray‐ionization time‐of‐flight mass spectrometry (ESI-TOF MS). Significant findingsThe most active catalytic system is obtained by chemically interaction of 3-bromopropionic acid (BPA) with DMF at 80 °C (BPA-DMF). An excellent product yield of 98% and a high TOF value are obtained with 0.99 mol% of BPA-DMF, which is attributed to the synergistic effects of carboxyl functional groups and halide ions. The activation energy was found to be 50.75 kJ/mol. The positive value of Gibbs free activation energy (ΔG‡) and negative value of activation entropy (ΔS‡) confirm a non-spontaneous, endergonic and kinetically controlled reaction.

Kinetic, Thermodynamic, Physicochemical, and Economical Characterization of Pectin from Mangifera indica L. cv. Haden Residues.

The effect of temperature (60, 70, 80, and 90 °C) and time (30, 45, 60, 75, and 90 min) on citric acid extraction of Haden mango (Mangifera indica L. cv. Haden) peel pectin was evaluated in the present study. In order to obtain a better understanding of both the extraction process and the characteristics of the pectin (obtained from an agro-industrial waste) for a future scaling process, the following characterizations were performed: (1) Kinetic, with the maximum extraction times and yields at all evaluated temperatures; (2) thermodynamic, obtaining activation energies, enthalpies, entropies, and Gibbs free energies for each stage of the process; (3) physicochemical (chemical analysis, monosaccharide composition, degree of esterification, galacturonic acid content, free acidity, Fourier-transform infrared spectroscopy, thermogravimetric and derivative thermogravimetric analyses); and (4) economical, of the pectin with the highest yield. The Haden mango peel pectin was found to be characterized by a high-esterified degree (81.81 ± 0.00%), regular galacturonic acid content (71.57 ± 1.26%), low protein (0.83 ± 0.05%) and high ash (3.53 ± 0.02%) content, low mean viscometric molecular weight (55.91 kDa), and high equivalent weight (3657.55 ± 8.41), which makes it potentially useful for food applications.

Fabrication and Characteristics of Methyl-Modified TiO2-SiO2 Membrane by Sol-Gel Method: Effect of Titania Content

Titania-doped methyl-modified silica membrane was prepared via an acid-catalyzed sol-gel method using tetraethylorthosilicate and methyltriethoxysilane as the silicon source and tetraethylorthosilicate as the titanium source. The influence of titanium/silicon molar ratio (nTi ) on the properties of TiO2-SiO2 sol and microstructure of unsupported TiO2-SiO2 membrane was systematically studied. The gas permeance and vapor stability of TiO2-SiO2 membrane were also tested. The results showed that, as nTi increased, the density, viscosity, refractive index and viscous activation Gibbs free energy of TiO2-SiO2 sol gradually decreased, while the particle size increased. The existence of Si-CH3 and Ti-O-Si bonds in TiO2-SiO2 membrane proved that the hydrophobic modification was successful and that SiO2 was chemically bonded to TiO2, respectively. The titanium element exists in form of Ti-O-Si structure and anatase phase TiO2 in TiO2-SiO2 membrane. When operated at 150 °C and a pressure difference of 0.3 MPa, the H2 permeance and H2/CO2 permselectivity of TiO2-SiO2 membrane with nTi =0.1 is 6.84 × 10−6 mol·m −2·Pa−1·s −1 and 6.96, respectively. After aging in the moist circumstance for 9 d, the H2 permeance of TiO2-SiO2 membrane reduced by 6.86%, while the H2/CO2 permselectivity increased by 2.71%.

Surface, textural and catalytic properties of pyridinium hydrogen sulfate ionic liquid heterogenized on activated carbon carrier

Pyridinium hydrogen sulfate ionic liquid (PHS) heterogenized on activated carbon (AC) is investigated for the first time in the current paper. For that purpose, the xPHS/AC (x = 8, 17, 33 and 66 wt%) samples are analyzed by a number of physicochemical methods such as N2 adsorption–desorption measurements, X-ray diffraction, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. Catalytic activity and reusability of xPHS/AC are evaluated in the reaction of butyl acetate synthesis. Effect of the reaction temperature and catalyst loading on the butyl acetate yield are examined in this work as well. Important thermodynamic parameters such as equilibrium constant, activation energy, Gibbs free energy and activation energy barrier for the reaction of butyl acetate synthesis in the presence of xPHS/AC are determined.Textural characterization revealed the that xPHS/AC are mixed micro-mesoporous materials. The PHS heterogenization on AC affects both ionic liquid and support phases due to a surface PHS–AC interaction. FT-IR and XPS showed that this interaction is: (i) manly expressed as a close contact between the pyridinium cation and the functional groups on the AC surface and (ii) electrostatic in nature. The existence of a PHS–AC interaction is found to be responsible for the formation of surface PHS particles of various size. However, the catalytic activity of xPHS/AC is established to be affected by the surface active phase amount, but not the PHS particle size.

Unveiling the Intramolecular Ionic Diels–Alder Reactions within Molecular Electron Density Theory

The intramolecular ionic Diels–Alder (IIDA) reactions of two dieniminiums were studied within the Molecular Electron Density Theory (MEDT) at the ωB97XD/6-311G(d,p) computational level. Topological analysis of the electron localization function (ELF) of dieniminiums showed that their electronic structures can been seen as the sum of those of butadiene and ethaniminium. The superelectrophilic character of dieniminiums accounts for the high intramolecular global electron density transfer taking place from the diene framework to the iminium one at the transition state structures (TSs) of these IIDA reactions, which are classified as the forward electro density flux. The activation enthalpy associated with the IIDA reaction of the experimental dieniminium, 8.7 kcal·mol−1, was closer to that of the ionic Diels–Alder (I-DA) reaction between butadiene and ethaniminium, 9.3 kcal·mol−1. However, the activation Gibbs free energy of the IIDA reaction was 12.7 kcal·mol−1 lower than that of the intermolecular I-DA reaction. The strong exergonic character of the IIDA reaction, higher than 20.5 kcal·mol−1, makes the reaction irreversible. These IIDA reactions present a total re/exo and si/endo diastereoselectivity, which is controlled by the most favorable chair conformation of the tetramethylene chain. ELF topological analysis of the single bond formation indicated that these IIDA reactions take place through a non-concerted two-stage one-step mechanism. Finally, ELF and atoms-in-molecules (AIM) topological analyses of the TS associated with the inter and intramolecular processes showed the great similarity between them.

Microwave-assisted heterogeneous esterification of dibutyl maleate: Optimization using response surface methodology

Microwave-assisted synthesis of dibutyl maleate was carried out with heterogeneous catalyst, Amberlyst-15. Effect of process parameters such as mole ratio, reaction temperature, catalyst loading, and molecular sieves on the conversion was reported using Response Surface Methodology (RSM). Reaction conversion of 62.6% was achieved at optimum parameters of 1:3 acid to alcohol mole ratio, 5.5% (w/w) catalyst loading, 70°C temperature, and 6% (w/w) molecular sieves for reaction time of 60 min. The optimum parameters based on RSM were 1:3.14 acid to alcohol mole ratio, catalyst loading 5.35% (w/w), 77.19°C, and 64.77% predicted conversion. Pseudo- Homogeneous (PH), Eley Rideal (ER), and Langmuir Hinshelwood Hougen Watson (LHHW) models were used to fit experimental kinetic data. Studies revealed that the Eley-Rideal model suited the experimental data. The activation energy of the esterification reaction was calculated as 10.17 kJ/mol. Gibbs free activation energy (ΔG‡), enthalpy of activation (ΔH‡), and entropy of activation (ΔS‡) were evaluated.

Selective transformation of aqueous methanol to value-added formic acid and hydrogen on bifunctional Mo2P monolayers in fuel cells

Direct methanol fuel cell (DMFC) running on hydrogen is an attractive alternative power source for a wide range of applications. Importantly, the required hydrogen can be in situ released from the stable liquid, which ensures its safe transformation and storage. However, the catalyst in DMFC is usually poisoned by the generated CO species, thus it is challenging to find an efficient and durable catalyst for practical applications. Here, the mechanism of methanol oxidation reaction (MOR) on the Mo2P monolayer is first studied using density functional theory. Compared with the initial C–H and C–O bond scissions, the initial O–H bond scission is found to be the most favorable. The reason is systematically analyzed based on steric effect, activation Gibbs free energy (Gact) decomposition and the Brønsted − Evans − Polanyi (BEP) relationships. The further dehydrogenation of CH2OH via the O–H bond scission has a significantly lower Gact than that of CH3O, thus the methanol decomposition on the Mo2P monolayer may proceed via two competitive pathways, i.e., CH3OH→CH3O→CH2O→CHO→CO and CH3OH→CH2OH→CH2O→CHO→CO. Then, through overcoming a relatively low barrier, the generated CO species can be combined with hydroxyl group from water dissociation on the Mo2P monolayer to produce formic acid, which can be easily desorbed from the catalyst surface to avoid the block of active sites. This study provides a better understanding of the MOR mechanism and paves the way for further advancing the catalyst in DMFCs.

Enthalpy–Entropy Compensation Effect for Triplet Pair Dissociation of Intramolecular Singlet Fission in Phenylene Spacer-Bridged Hexacene Dimers

Hexacene (Hc) is highly promising for singlet fission (SF). However, the number of SFs in Hc is extremely limited. As far as Hc dimers in solution are concerned, there is no report on the observation of the dissociation process from a correlated triplet pair (TT) to an individual one. The emphasis in this study is on the first observation of the quantitative TT generation together with the orientation-dependent photophysical discussions for TT dissociation using para- and meta-phenyl-bridged Hc dimers. Moreover, the activation enthalpies of Hc dimers in TT dissociation are smaller than those of pentacene (Pc) dimers, whereas the relative entropic contributions for Gibbs free energy of activation are much larger than the enthalpic ones in both Hc and Pc dimers. This implies that the vibrational motions are responsible for the intramolecular conformation changes associated with the TT dissociation. Consequently, "enthalpy-entropy compensation" has a large impact on the rate constants and quantum yields.

Transition state application to simulate CO gas sensor of pristine and Pt doped tin dioxide clusters

Carbon monoxide sensitivity of pristine and Pt doped tin dioxide (SnO2) is investigated in the present work using transition state theory. The use of transition state theory leads to a double exponent function formula for the concentration and sensitivity of the material. The method uses Gibbs free energy, enthalpy, and entropy of activation to formulate sensitivity, response time, and recovery time. The results showed that the activation energy of Pt doped SnO2 nanocluster is lower than the pristine SnO2 nanocluster. The Pt doped clusters promote higher reaction rates than pristine clusters. However, the activation energy of recovery with oxygen reaction is lower for the pristine cluster. The results also showed that the activation energy and reaction rates increase with temperature. The concentration of oxygen-deficient molecules in pristine and doped tin oxide as a function of temperature that represents the sensitivity of the sensor has the highest value at 225 °C. The theoretical results also show that response time decreases while recovery time increases with the increase of CO concentration. The results agree with experimental results.

A QM/MM Evaluation of the Missing Step in the Reduction Mechanism of HMG-CoA by Human HMG-CoA Reductase

Statins are important drugs in the regulation of cholesterol levels in the human body that have as a primary target the enzyme β-hydroxy-β-methylglutaryl-CoA reductase (HMGR). This enzyme plays a crucial role in the mevalonate pathway, catalyzing the four-electron reduction of HMG-CoA to mevalonate. A second reduction step of this reaction mechanism has been the subject of much speculation in the literature, with different conflicting theories persisting to the present day. In this study, the different mechanistic hypotheses were evaluated with atomic-level detail through a combination of molecular dynamics simulations (MD) and quantum mechanics/molecular mechanics (QM/MM) calculations. The obtained Gibbs free activation and Gibbs free reaction energy (15 kcal mol−1 and −40 kcal mol−1) show that this hydride step takes place with the involvement of a cationic His405 and Lys639, and a neutral Glu98, while Asp715 remains in an anionic state. The results provide an atomic-level portrait of this step, clearly demonstrating the nature and protonation state of the amino acid residues involved, the energetics associated, and the structure and charge of the key participating atoms in the several intermediate states, finally elucidating this missing step.