Temperature Dependence Of Rate Constants Research Articles

Determination of the Temperature-Dependent OH− (H2O) + CH3I Rate Constant by Experiment and Simulation

Abstract Experimental and simulation studies of the OH – (H2O) + CH3I reaction give temperature dependent rate constants which are in excellent agreement. Though there are statistical uncertainties, there is an apparent small decrease in the rate constant as the temperature is increased from − 60 to 125 ℃, and for this temperature range the rate constant is ∼ 1.6 times smaller than that for the unsolvated reactants OH – + CH3I. Previous work [J. Phys. Chem. A 117 (2013) 14019] for the unsolvated reaction found that the S N 2 and proton transfer pathways, forming CH3OH + I – and CH2I – + H2O, have nearly equal probabilities. However, for the microsolvated OH – (H2O) + CH3I reaction the S N 2 pathways dominate. An important contributor to this effect is the stronger binding of H2O to the OH – reactant than to the proton transfer product CH2I – , increasing the barrier for the proton transfer pathway. The effect of microsolvation on the rate constant for the OH – (H2O)0,1 + CH3I reactions agrees with previous experimental studies for X – (H2O)0,1 + CH3Y reactions. The simulations show that there are important non-statistical attributes to the entrance- and exit-channel dynamics for the OH – (H2O) + CH3I reaction.

Simulating shelf life determination by two simultaneous criteria

The shelf life of food and pharmaceutical products is frequently determined by a marker's concentration or quality index falling below or surpassing an assigned threshold level. Naturally, different chosen markers would indicate different shelf life for the same storage temperature history. We demonstrate that if there are two markers, such as two labile vitamins, the order in which their concentrations cross their respective thresholds may depend not only on their degradation kinetic parameters but also on the particular storage temperature profile, be it isothermal or non-isothermal. Thus, at least theoretically, the order observed in accelerated storage need not be always indicative of the actual order at colder temperatures, except where the two degradation reactions follow the same kinetic order and their temperature-dependence rate parameter is also the same. This is shown with simulated hypothetical degradation reactions that follow first or zero order kinetics and whose rate constant's temperature-dependence obeys the exponential model. It is also demonstrated with simulated hypothetical Maillard reaction's products whose synthesis rather than their degradation follows pseudo zero order kinetics. The software developed to do the simulations and calculate the thresholds crossing points has been posted on the Internet as a freely downloadable interactive Wolfram Demonstration, which can be used as a tool in storage studies and shelf life prediction. In principle, the methodology can be extended from two to any number of markers.

The Kinetics of the Reaction C2H5 • + HI → C2H6 + I• over an Extended Temperature Range (213–623 K): Experiment and Modeling

Abstract The present study reports temperature dependent rate constants k 1 for the title reaction across the temperature range 213 to 293 K obtained in a Knudsen flow reactor equipped with an external free radical source based on the reaction C2H5I + H• → C2H5 • + HI and single VUV-photon ionization mass spectrometry using Lyman-α radiation of 10.2 eV. Combined with previously obtained high-temperature data of k 1 in the range 298–623 K using the identical experimental equipment and based on the kinetics of C2H5 • disappearance with increasing HI concentration we arrive at the following temperature dependence best described by a three-parameter fit to the combined data set: k 1 = (1.89 ± 1.19)10−13(T/298)2.92±0.51 exp ((3570 ± 1500)/RT), R = 8.314 J mol –1 K –1 in the range 213–623 K. The present results confirm the general properties of kinetic data obtained either in static or Knudsen flow reactors and do nothing to reconcile the significant body of data obtained in laminar flow reactors using photolytic free radical generation and suitable free radical precursors. The resulting rate constant for wall-catalyzed disappearance of ethyl radical across the full temperature range is discussed. Highly correlated ab initio quantum chemistry methods and canonical transition state theory were applied for the reaction energy profiles and rate constants. Geometry optimizations of reactants, products, molecular complexes, and transition states are determined at the CCSD/cc-pVDZ level of theory. Subsequent single-point energy calculations employed the DK-CCSD(T)/ANO-RCC level. Further improvement of electronic energies has been achieved by applying spin-orbit coupling corrections towards full configuration interaction and hindered rotation analysis of vibrational contributions. The resulting theoretical rate constants in the temperature range 213–623 K lie in the range E-11–E-12 cm3 molecule –1 s –1, however experiments and theoretical modelling seem at great odds with each other.

Direct dynamics study on hydrogen abstraction reaction of morpholine with hydroxyl radical

A dual-level direct dynamic study has been employed to investigate the H-abstraction reaction of morpholine (C4H9NO) with OH. The chair conformer of C4H9NO molecule is performed as the reactant because of the lower energy. The potential energy surfaces are obtained at the MC-QCISD/3//M06-2X/6-311+G(d, p) level. The rate constants are estimated using the improved canonical variational transition state theory with the small-curvature tunneling correction over a wide temperature range of 200 to 1000 K, and a fitted four-parameter rate constant expression is obtained. The calculated rate constants are in good agreement with the available experimental values, and the negative temperature dependence of rate constant is found within 200–600 K. Moreover, the contribution of each reaction site to the title reaction is discussed with respect to the temperature. The result shows that the H-abstraction reaction mainly takes place at the –CH2 group attached to the N atom at low temperatures, while as the temperature increases the contribution of the H-abstraction to the title reaction from the –NH group becomes larger rapidly, and the –NH group turns into the most important H-abstraction reaction site when the temperature is higher than 800 K.

Predicting chemical degradation during storage from two successive concentration ratios: Theoretical investigation

When a vitamin's, pigment's or other food component's chemical degradation follows a known fixed order kinetics, and its rate constant's temperature-dependence follows a two parameter model, then, at least theoretically, it is possible to extract these two parameters from two successive experimental concentration ratios determined during the food's non-isothermal storage. This requires numerical solution of two simultaneous equations, themselves the numerical solutions of two differential rate equations, with a program especially developed for the purpose. Once calculated, these parameters can be used to reconstruct the entire degradation curve for the particular temperature history and predict the degradation curves for other temperature histories. The concept and computation method were tested with simulated degradation under rising and/or falling oscillating temperature conditions, employing the exponential model to characterize the rate constant's temperature-dependence. In computer simulations, the method's predictions were robust against minor errors in the two concentration ratios. The program to do the calculations was posted as freeware on the Internet. The temperature profile can be entered as an algebraic expression that can include ‘If’ statements, or as an imported digitized time–temperature data file, to be converted into an Interpolating Function by the program. The numerical solution of the two simultaneous equations requires close initial guesses of the exponential model's parameters. Programs were devised to obtain these initial values by matching the two experimental concentration ratios with a generated degradation curve whose parameters can be varied manually with sliders on the screen. These programs too were made available as freeware on the Internet and were tested with published data on vitamin A.

Mechanism and kinetics of low-temperature oxidation of a biodiesel surrogate: methyl propanoate radicals with oxygen molecule.

This paper presents a computational study on the low-temperature mechanism and kinetics of the reaction between molecular oxygen and alkyl radicals of methyl propanoate (MP), which plays an important role in low-temperature oxidation and/or autoignition processes of the title fuel. Their multiple reaction pathways either accelerate the oxidation process via chain branching or inhibit it by forming relatively stable products. The potential energy surfaces of the reactions between three primary MP radicals and molecular oxygen, namely, C(•)H2CH2COOCH3 + O2, CH3C(•)HCOOCH3 + O2, and CH3CH2COOC(•)H2 + O2, were constructed using the accurate composite CBS-QB3 method. Thermodynamic properties of all species as well as high-pressure rate constants of all reaction channels were derived with explicit corrections for tunneling and hindered internal rotations. Our calculation results are in good agreement with a limited number of scattered data in the literature. Furthermore, pressure- and temperature-dependent rate constants for all reaction channels on the multiwell-multichannel potential energy surfaces were computed with the quantum Rice-Ramsperger-Kassel (QRRK) and the modified strong collision (MSC) theories. This procedure resulted in a thermodynamically consistent detailed kinetic submechanism for low-temperature oxidation governed by the title process. A simplified mechanism, which consists of important reactions, is also suggested for low-temperature combustion at engine-like conditions.

Degradation Kinetics of Gamma Amino Butyric Acid in Monascus‐Fermented Rice

AbstractThe present study investigated the effect of heat on the reaction kinetics of γ‐amino butyric acid (GABA) degradation in Monascus‐fermented rice (MFR). At pH 8.0 and 121C temperature, the GABA content decreased rapidly (from 57.1 ± 0.2 to 6.9 ± 0.2) as the time interval was increased from 15 to 120 min. The thermal degradation of GABA follows the first‐order reaction kinetics. The temperature dependence of rate constants follows the Arrhenius relationship, with an activation energy value of 101.5 kJ/mol (80–121C, r2 = 0.96). GABA is easily degraded when MFR is heated to a temperature of 121C. More than 70% of the GABA content can be salvaged by limiting the heat treatment of MFR to below 80C.Practical ApplicationsMonascus‐fermented rice is being used as a nutraceutical due to the presence of different heart healthy ingredients like monacolin K or lovastatin, GABA and dimerumic acid. However, during processing of Monascus food, there may be degradation of bioactive ingredients. The present research data are given information about how different processing parameters affect the ingredient quality and will find applications in industries and researchers involved in formulating and processing Monascus nutraceuticals.

English

The liquid phase hydrogenation of 2-((1-benzyl-1, 2, 3, 6-tetrahydropyridin-4-yl)methylene)-5, 6-dimethoxy-2, 3-dihydroinden-1-one hydrochloride (1) over a 5 % Pt/C industrial catalyst was studied experimentally in a batch slurry reactor using methanol as a solvent. The catalyst was characterized by the adsorption techniques for specific surface area and pore volume, and by XRD for crystallinity. To investigate the intrinsic kinetics of the reaction, the effect of temperature, catalyst loading, hydrogen partial pressure and (1) concentration on the initial rate of hydrogenation was studied. The analysis of initial rate data showed that the gas-liquid, liquid-solid, and intraparticle mass-transfer resistances were not significant. The reaction scheme of (1) hydrogenation was proposed for the kinetic modelling. Apparent rate constants for all hydrogenation steps were calculated using a first order kinetic approach resulting in good agreement between the experimentally obtained and predicted concentrations. From the temperature dependence of rate constants, the activation energies of various reaction steps were calculated. The averaged activation energy of these steps was found to be 31.1 kJ mol-1.

Ultra-fast electron capture by electrosterically-stabilized gold nanoparticles.

Ultra-fast pre-solvated electron capture has been observed for aqueous solutions of room-temperature ionic liquid (RTIL) surface-stabilized gold nanoparticles (AuNPs; ∼9 nm). The extraordinarily large inverse temperature dependent rate constants (k(e)∼ 5 × 10(14) M(-1) s(-1)) measured for the capture of electrons in solution suggest electron capture by the AuNP surface that is on the timescale of, and therefore in competition with, electron solvation and electron-cation recombination reactions. The observed electron transfer rates challenge the conventional notion that radiation induced biological damage would be enhanced in the presence of AuNPs. On the contrary, AuNPs stabilized by non-covalently bonded ligands demonstrate the potential to quench radiation-induced electrons, indicating potential applications in fields ranging from radiation therapy to heterogeneous catalysis.

H + LiH+ collision dynamics at ultracold temperature conditions

Dynamics of H+LiH+ (v=0, j) collisions is investigated at cold and ultracold temperature conditions by a time-independent quantum mechanical method. The ab initio potential energy surface of the electronic ground state of the collisional system developed by Martinazzo et al. (2003) is used in the dynamics study. The collision dynamics at energy as low as 10−9eV has been examined. The Wigner threshold law holds in the limit of vanishing collision energy. Extremely large rotational and vibrational excitation of the product H2, formed via the LiH+ depletion channel, is found from the theoretical results. Integral cross sections and temperature dependent rate constants are reported for the H2 formation path. While elastic collision remains the dominant process, the depletion and inelastic processes also contribute to the dynamics with rotational excitation of the reagent LiH+ at ultracold temperature conditions and for s-wave scattering.

Estimating thermal degradation kinetics parameters from the endpoints of non-isothermal heat processes or storage

Where specimen retrieval for chemical analysis is not an option, as during a UHT process, or impeded by logistic considerations as in storage studies, a thermal degradation reaction's kinetic parameters can be estimated from the nutrient's or other compound's final concentrations after three or more known but different non-isothermal temperature histories. Provided the reaction follows fixed yet unknown order kinetics and its rate constant's temperature dependence follows a two-parameter algebraic model, the kinetic parameter estimation can be done by solving three simultaneous equations, themselves being the numerical solutions of three corresponding differential rate equations. In computer simulations of heat sterilization processes and storage under oscillating temperatures that include small experimental scatter, the reaction's kinetic order, its rate at a reference temperature, and the rate's temperature dependence were estimated from the final concentrations and correctly predicted dynamic degradation curves not used in their determination. The method's predictive ability was also tested with published experimental isothermal degradation data on ascorbic acid, patulin, sulforaphane, and thiamine, where the final concentrations at temperatures not used in the parameters determination were estimated from three endpoints. The described method only works where the above mentioned conditions are fully satisfied. Where applicable, it eliminates the need to experimentally determine sets of complete isothermal degradation curves, which might be impractical or too costly.

Effect of non-thermal product energy distributions on ketohydroperoxide decomposition kinetics

The decomposition of ketohydroperoxides (OQ′OOH) to two radicals is commonly predicted to be the key chain branching step in low-temperature combustion. The possibility of a direct decomposition of the OQ′OOH from its initially produced energy distribution is studied with a combination of master equation (ME) and direct trajectory simulations. The temperature and pressure dependent rate constants for the thermal decomposition of a ketohydroperoxide, HOOCH2CH2CHO, to four product channels were computed using RRKM/ME methods. Direct dynamics calculations were initiated from a transition state in the O2+QOOH reaction network to understand the fraction of energy in that transition state that is converted into the internal energy of the OQ′OOH. A novel approach to solving the master equation is used to determine the probability that a vibrationally hot OQ′OOH either will be stabilized to a thermal distribution or will react to form new products. Under most conditions, the majority of vibrationally excited OQ′OOH will be quenched into a thermal distribution. At higher internal energies and lower pressures, however, a significant fraction of the hot OQ′OOH will decompose rather than thermalize. Proper interpretation of low-pressure experiments may require inclusion of vibrationally hot intermediates, particularly if a chemical kinetic mechanism is optimized against the low-pressure data.

Mechanism and kinetics of low-temperature oxidation of a biodiesel surrogate−methyl acetate radicals with molecular oxygen

Accurate description of reactions between methyl acetate (MA) radicals and molecular oxygen is an essential prerequisite for understanding as well as modeling low-temperature oxidation and/or ignition of MA, a small biodiesel surrogate, because their multiple reaction pathways either accelerate the oxidation process via chain branching or inhibit it by forming relatively stable products. The accurate composite CBS-QB3 level of theory was used to explore potential energy surfaces for MA radicals + O2 system. Using the electronic structure calculation results under the framework of canonical statistical mechanics and transition state theory, thermodynamic properties of all species as well as high-pressure rate constants of all reaction channels were derived with explicit corrections for tunneling and hindered internal rotations. Our calculated results are in good agreement with a limited number of scattered data in the literature. Furthermore, pressure- and temperature-dependent rate constants were then computed using the Quantum Rice–Ramsperger–Kassel and the modified strong collision theories. This procedure resulted in a thermodynamically consistent detailed kinetic mechanism for low-temperature oxidation of the title fuel. We also demonstrated that even the detailed mechanism consists of several reactions of different reaction types, only the addition of the reactants and the re-dissociation of the initially formed adducts are important for low-temperature combustion at engine-liked conditions.

Further insight into the reaction FeO(+) + H2 → Fe(+) + H2O: temperature dependent kinetics, isotope effects, and statistical modeling.

The reactions of FeO(+) with H2, D2, and HD were studied in detail from 170 to 670 K by employing a variable temperature selected ion flow tube apparatus. High level electronic structure calculations were performed and compared to previous theoretical treatments. Statistical modeling of the temperature and isotope dependent rate constants was found to reproduce all data, suggesting the reaction could be well explained by efficient crossing from the sextet to quartet surface, with a rigid near thermoneutral barrier accounting for both the inefficiency and strong negative temperature dependence of the reactions over the measured range of thermal energies. The modeling equally well reproduced earlier guided ion beam results up to translational temperatures of about 4000 K.

Kinetic modeling of pseudoliving free-radical styrene polymerization occurring via reversible addition-fragmentation chain transfer

A kinetic model has been constructed for 2,2′-azobisisobutyronitrile-initiated pseudoliving free-radical styrene polymerization occurring via reversible addition-fragmentation chain transfer in the presence of dibenzyl trithiocarbonate. The inverse problem of determining the unknown temperature-dependent rate constants of the elementary reactions of the kinetic scheme has been solved. The model has been validated by comparing the theoretical and experimental molecular weight characteristics of polystyrene. The model has been employed to investigate the effects of control factors of the process on the molecular weight characteristics of polystyrene.

Sequence-dependent theory of oligonucleotide hybridization kinetics.

A theoretical approach to the prediction of the sequence and temperature-dependent rate constants for oligonucleotide hybridization reactions has been developed based on the theory of relaxation kinetics. One-sided and two-sided melting reaction mechanisms for oligonucleotide hybridization reactions have been considered, analyzed, modified, and compared to select a physically consistent as well as robust model for prediction of the relaxation times of DNA hybridization reactions that agrees with the experimental evidence. The temperature- and sequence-dependent parameters of the proposed model have been estimated using available experimental data. The relaxation time model that we developed has been combined with the nearest neighbor model of hybridization thermodynamics to estimate the temperature- and sequence-dependent rate constants of an oligonucleotide hybridization reaction. The model-predicted rate constants are compared to experimentally determined rate constants for the same oligonucleotide hybridization reactions. Finally, we consider a few important applications of kinetically controlled DNA hybridization reactions.

Reactions of n-butyl acrylate and ethyl methacrylate with ozone in the gas phase

Detailed mechanisms and reaction products for ozonolysis of n-butyl acrylate and ethyl methacrylate in the atmosphere are studied using quantum chemistry calculations. The profile of the potential energy surfaces (PESs) are constructed at the CCSD(T)/6-31G(d)+CF//B3LYP/6-31+G(d,p) level, and based on that, temperature-dependent rate constants are determined using RRKM theory. The computed total rate constants (in cm3molecule−1s−1) can be expressed in Arrhenius expressions as: k(NBA+O3)=1.42×10−13exp (−3217.68/T) and k(EMA+O3)=4.94×10−14exp (−2578.07/T). The branching ratios and yields of primary products have been presented and discussed in this paper.

Reaction Kinetics of Catalytic Esterification of Nonanoic Acid with Ethanol over Amberlyst 15

AbstractThe kinetic behaviour of heterogeneous esterification of nonanoic acid with ethanol over an acidic cation exchange resin, Amberlyst 15, was investigated in a batch reactor and effect of various parameters such as catalyst loading, molar ratio and reaction temperature on degree of fractional conversion has been studied. Internal and external diffusions were found to be negligible in this study. Nonideality of the liquid phase was taken into account by using activities instead of concentrations. The activity coefficients were estimated using UNIFAC group contribution method. Eley–Rideal (ER) kinetic model was used to interpret the obtained kinetic data. The temperature-dependent initial reaction rate constants and the adsorption coefficients for ethanol and water were determined from the observed experimental data obtained at different initial concentration of acid, alcohol and water. Activation energy and pre-exponential factor of the reaction were found to be 53.7 kJ mol−1and 1.51×105l2g−1mol−1h−1respectively.

Reactions of Fe+ and FeO+ with C2H2, C2H4, and C2H6: Temperature-Dependent Kinetics

We present the first temperature-dependent rate constants and branching ratios for the reactions of Fe(+) and FeO(+) with C2H2, C2H4, and C2H6 from 170 to 700 K. Fe(+) is observed to react only by association with the three hydrocarbons, with temperature dependencies of T(-2) to T(-3). FeO(+) reacts with C2H2 and C2H4 at the collision rate over the temperature range, and their respective product branchings show similar temperature dependences. In contrast, the reaction with ethane is collisional at 170 K but varies as T(-0.5), while the product branching remains essentially flat with temperature. These variations in reactivity are discussed in terms of the published reactive potential surfaces. The effectiveness of Fe(+) as an oxygen-transfer catalyst toward the three hydrocarbons is also discussed.

Mutarotation in biologically important pure L-fucose and its enantiomer

The sugar specific mutarotation reaction in biologically important L-fucose and its enantiomer in the pure, anhydrous, supercooled liquid state has been studied. Kinetics measurements in the temperature range 313–328 K at ambient pressure have been performed by means of dielectric spectroscopy, a method widely used for studying the molecular dynamics of glass-forming liquids. The kinetic curves have been obtained by tracking the equilibration process in sugar melted and quenched to the desired temperature. Thereafter, an activation energy equal to Ea = 140 kJ mol−1 for D-fucose and Ea = 123 kJ mol−1 for L-fucose has been derived from the Arrhenius fit of temperature dependent rate constants. It was also shown that the kinetics curves at the lowest temperatures studied have sigmoidal shape, which was connected to the high concentration of furanosidic forms.