Rate Constants Of Reverse Reactions Research Articles

Thermal decomposition of 1-ethyl-3-methylimidazolium acetate

To realize the potential applications of ionic liquids as working fluids and solvents one should have knowledge about their long-term stability, which depends on their thermally initiated decomposition. However, the possible decomposition pathways are not yet known accurately. 1-Ethyl-3-methylimidazolium acetate ([C2Mim][OAc]) is widely studied and shows potential in cellulose dissolution. In this study, we investigate possible [C2Mim][OAc] decomposition pathways by combining decomposition experiments and ab initio rate constant computations. Both approaches yield activation energies around 135 kJ·mol−1, which significantly deviate from earlier TGA measurements with open surface yielding 100–110 kJ·mol−1 and are probably affected by evaporation. Our study uses an improved TGA measurement protocol with a sealed pan with a tiny hole that fosters measurement of the actual chemical process leading to mass loss. Ab initio computations of this study comprise vapor-liquid equilibria of reactants and products as well as forward and reverse reaction rate constants in the gas and liquid phase. Calculations for the gas phase closely match a first-order analysis of the experiments, but calculations for the liquid phase strongly depend on the quality of solvation modeling. The computations indicate that the SN2 pathway with an activation energy of 142–144 kJ·mol−1 dominates thermal decomposition of [C2Mim][OAc].

Chromogenic properties of heterocyclic compounds: Barochromic effect of indoline spiropyrans in the gas phase

Spiropyrans of the indoline and benzothiazole series, which exist under normal conditions in a colored merocyanine form, have been obtained. Solutions of the synthesized compounds exhibit negative photochromism. Previously unknown chromogenic properties were found in the gas phase of the synthesized indoline spiropyrans, namely, the barochromic effect, consisting of the discoloration of a gas containing spiropyran molecules when the pressure of the air is reduced. This effect is due to the conversion of the merocyanine isomers absorbing in the visible region of the spectrum into cyclic isomers absorbing in the UV region of the spectrum. It has been established that solid-phase films of indoline series spiropyrans, which are in a stable merocyanine form under normal conditions, have the properties of negative T- and P-type photochromism. Extremely low values of the rate constants of reverse thermal reactions make it possible to consider them as reverse positive–negative media for recording and processing optical information.

Headspace study of chiral interconversion of N-acetyl-homocysteine thiolactones

The rate constants for (L)-N-acetyl homocysteine thiolactone enantiomerization have been obtained from batch-wise studies and by dynamic gas chromatography of racemic mixtures. Results from the batch-wise experiments show that the kinetics of racemization at 150 °C is the same for vials made of glass, silanized glass or Teflon-coated glass so that the vial surface exhibited no effect on the kinetics of racemization. From the temperature dependence of the rate constants the preexponential factor, activation energy, the activation Gibbs energy and activation entropy have been obtained from transition state theory. The catalytic effect of G-DP, G-BP and B-DP GC chiral stationary phases on racemization has been observed and quantified by the values of rate constants; B-DP exhibited the greatest activity. The Eyring activation parameters obtained from batch-wise experiment were compared with theoretical values acquired from quantum chemical modelling. Agreement between the experimental and calculated values of activation Gibbs energy, activation enthalpy and activation entropy is good. The dynamic gas chromatography of racemic mixture on chiral B-DP, G-DP and G-BP capillary columns indicate that the rate constants of forward and reverse reactions are different in chiral environments. The greatest accelerating effect in the process of enantiomerization has been identified for G-BP both in the batch-wise experiments and by the dynamic gas chromatography.

Prehydration and the Reversibility of Solid-State Hydrogen-Deuterium Exchange.

The reversibility of solid-state hydrogen-deuterium exchange (ssHDX) and the effects of prehydration on the rate and extent of deuterium incorporation were evaluated using poly-d,l-alanine (PDLA) peptides colyophilized with various excipients. In prehydration studies, samples were equilibrated at a controlled relative humidity (6% or 11% RH) for 12 h and then transferred to corresponding D2O humidity conditions (6% or 11% RD) for deuterium labeling. In amorphous samples, the rate and extent of deuterium incorporation were similar in prehydrated samples and controls not subjected to prehydration. In reversibility studies, PDLA samples were maximally deuterated in controlled D2O humidity conditions (6% or 11% RD) and then transferred to corresponding H2O relative humidity (0%, 6%, 11%, or 43% RH). Hysteresis in deuterium removal was observed when compared with the deuterium incorporation kinetics for all formulations and conditions, confirming that the reaction is reversible in the solid state and that the forward and reverse processes differ. The extent of deuterium loss reached a plateau that depended on the delabeling relative humidity. Reverse reaction rate constants were quantified using a first-order kinetic model, a limiting case of the reversible first-order model applicable under sink conditions. For other conditions, plateau (steady-state) deuteration levels were related to forward and reverse rate constants in a reversible first-order kinetic model. The results support a mechanistic interpretation of ssHDX kinetics as a reversible first-order process, in which the forward (deuteration) rate depends on the activity of the deuterium donor.

Observation of glucose-6-phosphate anomeric exchange in real-time using dDNP hyperpolarised NMR.

A hyperpolarised-NMR acquisition approach that is sensitive to the process of glucose-6-phosphate anomerization is presented. Using selective depolarisation of one of the anomer's signals, it is possible to observe the replenishing of this signal due to the fast anomeric exchange of this compound. The forward to reverse reaction rate constants ratio was ca. 1.6.

Molecular modeling approach to elucidate gas phase hydrodeoxygenation of guaiacol over a Pd(111) catalyst within DFT framework.

Excessive amounts of oxy-functional groups in unprocessed bio-oil vitiate its quality as fuel; therefore, it has to be channelized to upgrading processes, and catalytic hydrodeoxygenation is one of the most suitable routes for the upgrading of crude bio-oil. In this computational work, catalytic hydrodeoxygenation (HDO) of guaiacol, which is an important phenolic compound of crude bio-oil, has been carried out using density functional theory (DFT) over a Pd(111) catalyst. The Pd(111) catalyst surface does not endorse direct eliminations of functional groups of guaiacol; however, it is found to perform excellently in stepwise dehydrogenation reactions of oxy-functionals of guaiacol according to present DFT results. The catechol product, formed through dehydrogenation of the methoxy group, followed by elimination of CH2 and association of the hydrogen atom, has been identified as one of the major products. The overall reaction rate is controlled by scission of CH2 from 2-methylene-oxy-phenol with an activation energy demand of 23.06kcal mol-1. Further, the kinetic analysis of each reaction step involved in HDO of guaiacol over the Pd(111) catalyst surface has also been carried out at atmospheric pressure and at a wide range of temperatures from 473 to 673K, with temperature intervals of 50K. In the kinetic analysis part, various kineticparameters, such as forward and reverse reaction rate constants, Arrhenius constants, and equilibrium rate constants, are reported. The kinetic modeling of the dominating reaction steps has revealed that even a lower temperature of 473K provides a favorable reaction environment; and the temperature increment further improves the reaction favorability.

Calibration-free quantitative immunoassay by flow cytometry: Theoretical consideration and practical implementation for IgG antibody binding to CD14 receptors on human leukocytes.

We propose a calibration-free method to determine the number of receptors per cell, as well as the direct and the reverse reaction rate constants for a single receptor. The method is based on the analysis of the temporal evolution of the cells mean fluorescent intensity measured by a flow cytometer during the ligand-receptor (antigen-antibody) binding under the conditions of their comparable concentrations. We developed the kinetic approach accounting both for the delay between the dilution and the measurement and for the practical duration of the measurement itself. The method was applied to determine thenumber of CD14 receptors on human blood mononuclear (granulocytes, monocytes, lymphocytes) cells of several donors. We also obtained the direct ( (5.6 ± 0.2) × 107 M-1 min-1 ) and reverse ( (1.3 ± 0.2) × 10-2 min-1 ) rate constants of ligand-receptor interaction, and estimated the size of the binding site as b = 0.5 nm. The latter allows one to recalculate the rate constants for a different ligand, fluorescent label, medium viscosity, and/or temperature. The knowledge of the rate constants is essential for the calibration-free determination of the number of receptors per cell from a single kinetic curve of the cells mean fluorescence intensity.

The stability, kinetics and inter-fragment electron communication of the tautomers of twelve selected β-diketone molecules: A computational study

Keto-enol equilibrium is known to depend on the difference in the free energy between the keto and enol tautomers and is greatly influenced by the nature of the substituents, temperature, and the polarity of the solvents. New insight was gained into the series of twelve differently substituted β-diketones (A-L), showing that the keto form of each β-diketone has a lesser tendency towards resonance formation, compared to their enol forms. For molecules G and H (which contain an electron-withdrawing CF3 substituent), the experimentally reported high tendency towards the enol tautomer, was computationally traced to a high level of the alternative enolic resonance weight also in their keto structure, as well as to the highly favourable enolic inter-fragment stability of the energy of interaction. Computational results further showed that the polar solvent dioxane enhances the enol form of these twelve molecules more effectively than water and chloroform media. The abundance of either the enol or keto tautomer, was also found to be dependent on the competitive ratio of both the forward and reverse reaction rate constants (namely the computed values for Kforward/Kreverse). High similarity was observed between the experimental and computed UV spectra of the selected molecules in their enol forms, which provides further evidence supporting predictions for the most favourable position of the enolization (for unsymmetrical molecules E and G-L with two possible enol positions), as well as confirming the previously reported trend of their experimental equilibrium Ke/k values.

Kinetics of Competitive Sorption in Decontamination of Materials from Radionuclides

kinetic model of the mass transfer of a microcomponent in the simplest competitive system from the sorbed state (A) into a solution (B) and then into a sorbent (C) in accordance with the scheme A ⇄ B ⇄ C was formulated within the framework of competitive sorption statics. The kinetic equations were solved numerically. The influence exerted by the weight of competing sorbents А and С and by the degree of reversibility of linear reactions on the nonequilibrium decontamination factor Kdec(t) was determined. The time in which the equilibrium decontamination factor is attained for the model of chemical sorption kinetics was estimated from the experimental data on the rate constants of direct and reverse heterogeneous reactions and on the distribution coefficients of Cs(I) in the SiO2 (A)–CsCl solution (B)–Prussian Blue (C) system.

Mathematical simulation of the hydrogenation of borodino coal

The kinetic and thermodynamic parameters of the hydrogenation of Borodino brown coal were calculated. With the use of equilibrium-kinetic analysis, the second-order reaction rate constants of forward and reverse reactions, the equilibrium constant, the activation energy, and the thermal effects of brown coal hydrogenation reactions were determined in a temperature range of 648–698 K at a hydrogen pressure of 10 MPa. In the course of the study, it was found that the experimental data are indicative of the adequacy of the equilibrium-kinetic analysis model for second-order reactions.

Rapid Hydrogen Shift Reactions in Acyl Peroxy Radicals.

We have used quantum mechanical chemical calculations (CCSD(T)-F12a/cc-pVDZ-F12//M06-2X/aug-cc-pVTZ) to investigate the hydrogen shift (H-shift) reactions in acyl peroxy and hydroperoxy acyl peroxy radicals. We have focused on the H-shift reactions from a hydroperoxy group (OOH) (1,X-OOH H-shift with X = 6, 7, 8, or 9) in the hydroperoxy acyl peroxy radicals, this H-shift is a reversible reaction and it scrambles between two peroxides, hydroperoxy acyl peroxy and peroxy peroxoic acid radicals. The forward reaction rate constants of the 1,X-OOH H-shift reactions are estimated to be above 103 s-1 with transition state theory corrected with Eckart quantum tunnelling correction. The ratio between the forward and reverse reaction rate constant of the 1,X-OOH H-shift reactions is around ∼105. Therefore, the equilibrium is pushed toward the production of peroxy peroxoic acid radicals. These very fast 1,X-OOH H-shift reactions are much faster than the reactions with NO and HO2 under most atmospheric conditions and must be included in the atmospheric models when hydroperoxy acyl peroxy radicals are oxidized. Finally, we have observed that H-shift reactions in a pentane acyl peroxy radical (C5-AOO) is fast (>1 s-1); this can have a significant influence on the possible formation of large acyl peroxy nitrate molecules.

Determination of the Reaction Rate Constant for the Opposing Reaction by Integral Method

In order to calculate the forward reaction rate constant and the reverse reaction rate constant of the opposing reaction, a calculation program was designed with VB language. According to the law of mass action, the reaction rate equation was established and it was converted into integral expression, the experimental data are calculated by using variable step size trapezoidal quadrature and least square method. The accuracy of the program in numerical simulation was verified by an example; the results show that the calculated values are in good agreement with the literature values. Inputting different parameters, the program can calculate the reaction rate of different types of the opposing reaction. It is convenient, quick and accurate to calculate the rate constant of the opposing reaction by the program. The program can be used as a tool in studying the opposing reaction.

Kinetics and Thermodynamics of Watson-Crick Base Pairing Driven DNA Origami Dimerization.

We investigate the kinetics and thermodynamics of DNA origami dimerization using flat rectangle origami components and different architectures of Watson-Crick complementary single-stranded DNA ("sticky end") linking strategies. We systematically vary the number of linkers, the length of the sticky ends on the linker, and linker architecture and measure the corresponding yields as well as forward and reverse reaction rate constants through fluorescence quenching assays. Yields were further verified using atomic force microscopy. We calculate values of H° and ΔS° for various interface designs and find nonlinear van't Hoff behavior, best described by two linear equations, suggesting distinct regimes of dimerization between those with and those without well-formed interfaces. We find that self-assembly reactions can be tuned by manipulating the interface architecture without suffering a loss in yield, even when yield is high, ∼75-80%. We show that the second-order forward reaction rate constant (k(on)) depends on both linker architecture and number of linkers used, with typical values on the order of 10(5)-10(6) (M·s)(-1), values that are similar to those of bimolecular association of small, complementary DNA strands. The k(on) values are generally non-Arrhenius, tending to increase with decreasing temperature. Finally, we use kinetic and thermodynamic information about the optimal linking architecture to extend the system to an infinite, two-component repeating lattice system and show that we can form micron-sized lattices, with well-formed structures up to 8 μm(2).

Differentiating inflamed and normal lungs by the apparent reaction rate constants of lactate dehydrogenase probed by hyperpolarized (13)C labeled pyruvate.

Clinically translatable hyperpolarized (HP) (13)C-NMR can probe in vivo enzymatic reactions, e.g., lactate dehydrogenase (LDH)-catalyzed reaction by injecting HP (13)C-pyruvate into the subject, which is converted to (13)C labeled lactate by the enzyme. Parameters such as (13)C-lactate signals and lactate-to-pyruvate signal ratio are commonly used for analyzing the HP (13)C-NMR data. However, the biochemical/biological meaning of these parameters remains either unclear or dependent on experimental settings. It is preferable to quantify the reaction rate constants with a clearer physical meaning. Here we report the extraction of the kinetic parameters of the LDH reaction from HP (13)C-NMR data and investigate if they can be potential predictors of lung inflammation. Male Sprague-Dawley rats (12 controls, 14 treated) were used. One dose of bleomycin (2.5 U/kg) was administered intratracheally to the treatment group. The lungs were removed, perfused, and observed by the HP-NMR technique, where a HyperSense dynamic nuclear polarization system was used to generate the HP (13)C-pyruvate for injecting into the lungs. A 20 mm (1)H/(13)C dual-tuned coil in a 9.4-T Varian vertical bore NMR spectrometer was employed to acquire the (13)C spectral data every 1 s over a time period of 300 s using a non-selective, 15-degree radiofrequency pulse. The apparent rate constants of the LDH reaction and their ratio were quantified by applying ratiometric fitting analysis to the time series data of (13)C labeled pyruvate and lactate. The apparent forward rate constant kp =(3.67±3.31)×10(-4) s(-1), reverse rate constant kl =(4.95±2.90)×10(-2) s(-1), rate constant ratio kp /kl =(7.53±5.75)×10(-3) for the control lungs; kp =(11.71±4.35)×10(-4) s(-1), kl =(9.89±3.89)×10(-2) s(-1), and kp /kl =(12.39±4.18)×10(-3) for the inflamed lungs at the 7(th) day post treatment. Wilcoxon rank-sum test showed that the medians of these kinetic parameters of the 7-day cohort were significantly larger than those of the control cohort (P<0.001, P=0.001, and P=0.019, respectively). The rate constants of individual lungs correlated significantly with the histology scores of neutrophils and organizing pneumonia foci but not macrophages. Both kp and kp /kl positively correlated with lactate labeling signals. No correlation was found between kl and lactate labeling signals. The results indicate bleomycin-induced lung inflammation significantly increased both the forward and reverse reaction rate constants of LDH and their ratio at day-7 after bleomycin treatment.

Equilibrium kinetic analysis of a model mixture of anthracene and benzothiophene

The kinetic and thermodynamic parameters of the hydrogenation of an anthracene–benzothiophene model mixture were calculated. The rate constants of forward and reverse reactions, the equilibrium constant, the activation energies, and the heat effects of hydrogenation reactions were determined by equilibrium kinetic analysis for second-order reactions in a model mixture of anthracene and benzothiophene in the presence of an iron-containing catalyst over a temperature range of 648–698 K at a pressure of 6 MPa.

Influence of divalent cations adsorption on the performance of electromembrane process

In this study, the adsorption equilibrium and kinetics of divalent cations were studied to describe transport phenomena occurring in an electromembrane process. The adsorption isotherms of divalent cations over cation-exchange resins were fitted to the Langmuir and Freundlich isotherms. Adsorption equilibrium experiments of divalent cations showed the preferential adsorption between adsorbent and adsorbate and Ca2+ showed about five times higher adsorption capacity than Mg2+. Adsorption kinetics was studied using the reversible first-order model and the forward and reverse reaction rate constants were estimated for divalent cations adsorption. The forward reaction rate constants for the removal of divalent cations were much higher than the reverse reaction rate constants, indicating that the rate of adsorption was dominant in ion-exchange process. In this study, the higher removal efficiency of Ca2+ than Mg2+ in an electromembrane process could be understood by the adsorption equilibrium and kinetic study.

Coordination Dynamics and Coordination Mechanism of a New Type of Anticoagulant Diethyl Citrate with Ca2+Ions

Diethyl citrate (Et2Cit) is a new potential anticoagulant. The coordination dynamics and coordination mechanism of Et2Cit with Ca2+ ions and the effect of pH on the complex were examined. The result was compared with that for the conventional anticoagulant sodium citrate (Na3Cit). The reaction order (n) of Et2Cit and Na3Cit with Ca2+ was 2.46 and 2.44, respectively. The reaction rate constant (k) was 120 and 289 L·mol−1 ·s−1. The reverse reaction rate constant (k re) was 0.52 and 0.15 L·mol−1 ·s−1, respectively. It is indicated that the coordination ability of Et2Cit with Ca2+ was weaker than that of Na3Cit. However, the dissociation rate of the calcium complex of Et2Cit was faster than that of Na3Cit. Increased pH accelerated the dissociation rate of the complex and improved its anticoagulant effect. The Et2Cit complex with calcium was synthesized and characterized by elemental analysis, XRD, FT-IR, 1H NMR, and ICP. These characteristics indicated that O in –COOH and C–O–C of Et2Cit was coordinated with Ca2+ in a bidentate manner with 1 : 1 coordination proportion; that is, complex CaEt2Cit was formed. Given that CaEt2Cit released Ca2+ more easily than Na3Cit, a calcium solution was not needed in intravenous infusions using Et2Cit as anticoagulant unlike using Na3Cit. Consequently, hypocalcemia and hypercalcemia were avoided.

Comparative study on the direct decomposition of nitrous oxide over M (Fe, Co, Cu)–BEA zeolites

Catalytic activities of M (Fe, Cu, Co)–beta (BEA) zeolites for N2O direct decomposition were systematically investigated by employing both experimental and theoretical [density functional theory (DFT)] approaches. The activities of M–BEA determined by intrinsic kinetic evaluations are in good agreement with the DFT calculations and the microkinetic analyses, revealing that the distinguishing activities of M–BEA were mainly attributed to their distinct energy barriers for the O2 desorption (Part 3). During the DFT calculations, a unique intermediate (IM) was generated only over Co–BEA, which was verified by the activity evaluation and N2O-TPD experiments, showing that the formation of IM reduced N2O decomposition rate of Co–BEA. The negative effect of the IM was thereafter investigated by the microkinetic analyses through which it was known that the second forward reaction rate constant of Co–BEA was lower than its reverse reaction rate constant, resulting in a final decline of the N2O decomposition rate.

Optimum process and kinetic study of Jatropha curcas oil pre-esterification in ultrasonical field

H2SO4 catalyzed pre-esterification of Jatropha curcas oil was investigated in this study. Ultrasound was used to enhance the mixing extent of methanol and oil. The experimental results showed that 18% methanol (wt% of oil), 1.2% H2SO4 catalyst (wt% of oil) and 900W/L ultrasonic power density were the optimum conditions for the pre-esterification of J. curcas oil with a 20mg KOH/g initial acid value (AV). The kinetics of the pre-esterification shows that the apparent forward reaction rate constant k+A increased with the rise of the amounts of methanol, catalyst and ultrasonic power density to some extent, while the reverse reaction rate constant was almost independent on these factors. Compared with mechanical stirring, ultrasonic radiation could greatly enhance the pre-esterification. Finally, a function of ultrasonic power density U was embedded into the kinetic equation, and under the optimum conditions, k+A could be expressed as: k+A=0.034023(−10−6U2+0.002U+0.0531). The relative deviations between the experimental data and the results calculated by the equation are less than 5%.

Multi-structural variational transition state theory: kinetics of the 1,5-hydrogen shift isomerization of the 1-butoxyl radical including all structures and torsional anharmonicity

We investigate the statistical thermodynamics and kinetics of the 1,5-hydrogen shift isomerization reaction of the 1-butoxyl radical and its reverse isomerization. The partition functions and thermodynamic functions (entropy, enthalpy, heat capacity, and Gibbs free energy) are calculated using the multi-structural torsional (MS-T) anharmonicity method including all structures for three species (reactant, product, and transition state) involved in the reaction. The calculated thermodynamic quantities have been compared to those estimated by the empirical group additivity (GA) method. The kinetics of the unimolecular isomerization reaction was investigated using multi-structural canonical variational transition state theory (MS-CVT) including both multiple-structure and torsional (MS-T) anharmonicity effects. In these calculations, multidimensional tunneling (MT) probabilities were evaluated by the small-curvature tunneling (SCT) approximation and compared to results obtained with the zero-curvature tunneling (ZCT) approximation. The high-pressure-limit rate constants for both the forward and reverse reactions are reported as calculated by MS-CVT/MT, where MT can be ZCT or SCT. Comparison with the rate constants obtained by the single-structural harmonic oscillator (SS-HO) approximation shows the importance of anharmonicity in the rate constants of these reactions, and the effect of multi-structural anharmonicity is found to be very large. Whereas the tunneling effect increases the rate constants, the MS-T anharmonicity decreases them at all temperatures. The two effects counteract each other at temperatures 385 K and 264 K for forward and reverse reactions, respectively, and tunneling dominates at lower temperatures while MS-T anharmonicity has a larger effect at higher temperatures. The multi-structural torsional anharmonicity effect reduces the final reverse reaction rate constants by a much larger factor than it does to the forward ones as a result of the existence of more low-energy structures of the product 4-hydroxy-1-butyl radical than the reactant 1-butoxyl radical. As a consequence there is also a very large effect on the equilibrium constant. The neglect of multi-structural anharmonicity will lead to large errors in the estimation of reverse reaction rate constants.