Temperature-dependent Reaction Kinetics Research Articles

Galactose isomerization to tagatose over MgBr2 follows a temperature-dependent reaction rate kinetics as predicted by first principles-based theories

Tagatose, a classified low-calorie sugar, possesses anti-hyperglycaemic and prebiotic characteristics. Therefore, it is considered a potential food additive supplement (sweetener) for lifestyle health problems in humans. While many reports on its production via biological and chemical methods have been published, the thermodynamic behavior of its synthesis using galactose is rarely focused on. Herein, we report the MgBr2-mediated galactose interconversion to tagatose (single-step) and its kinetic mechanism. The oppositely charged counterions (Mg2+ and Br‒) can induce the reaction via different pathways in a parallel fashion, i.e., the Mg2+ behaving as a Lewis acid can enable via a 1,2-hydride shift pathway, whereas the Br‒ offering the Brønsted basic sites via a proton transfer pathway. This resulted in a tagatose production as high as 22% and 73% selectivity within 60 min at 120 °C in water. Furthermore, the reaction has yielded a single co-product formation (talose up to 7%). From the inverse-gated quantitative NMR analysis, the contribution of the counterions in the reaction is roughly 52%:48% by Mg2+:Br‒. The determination of the temperature-dependent reaction kinetics (elementary steps) was made by employing the first-principles-based theories (i.e., transition state-Eyring theory for the proton transfer pathway that involves no hydride shift and Marcus theory for the hydride shift pathway). The energy calculations of the pathways made via DFT modeling suggested the higher feasibility of a hydride shift mechanism of galactose to tagatose.

Quantum Tunneling in Peroxide O-O Bond Breaking Reaction.

The importance of quantum-mechanical tunneling becomes increasingly recognized in chemical reactions involving hydrogen as well as heavier atoms. Here we report concerted heavy-atom tunneling in an oxygen-oxygen bond breaking reaction from cyclic beryllium peroxide to linear dioxide in cryogenic Ne matrix, as evidenced by subtle temperature-dependent reaction kinetics and unusually large kinetic isotope effects. Furthermore, we demonstrate that the tunneling rate can be tuned through noble gas atom coordination on the electrophilic beryllium center of Be(O2), as the half-life dramatically increased from 0.1 h for NeBe(O2) at 3 K to 12.8 h for ArBe(O2). Quantum chemistry and instanton theory calculations reveal that noble gas coordination notably stabilizes the reactants and transition states, increases the barrier heights and widths, and consequently reduces the reaction rate drastically. The calculated rates and in particular kinetic isotope effects are in good agreement with experiment.

Solid-State Reaction Heterogeneity During Calcination of Lithium-Ion Battery Cathode.

During solid-state calcination, with increasing temperature, materials undergo complex phase transitions with heterogeneous solid-state reactions and mass transport. Precise control of the calcination chemistry is therefore crucial for synthesizing state-of-the-art Ni-rich layered oxides (LiNi1-x-y Cox Mny O2 , NRNCM) as cathode materials for lithium-ion batteries. Although the battery performance depends on the chemical heterogeneity during NRNCM calcination, it has not yet been elucidated. Herein, through synchrotron-based X-ray, mass spectrometry microscopy, and structural analyses, it is revealed that the temperature-dependent reaction kinetics, the diffusivity of solid-state lithium sources, and the ambient oxygen control the local chemical compositions of the reaction intermediates within a calcined particle. Additionally, it is found that the variations in the reducing power of the transition metals (i.e., Ni, Co, and Mn) determine the local structures at the nanoscale. The investigation of the reaction mechanism via imaging analysis provides valuable information for tuning the calcination chemistry and developing high-energy/power density lithium-ion batteries.

Practicality assessment: Temperature-governed performance of CO2-containing Li–O2 batteries

Practical lithium–oxygen batteries require a shift from pure O2 to air. CO2 traces, however, fundamentally alter the O2 electrochemistry towards Li2CO3 formation via peroxocarbonate intermediates in highly-solvating electrolytes e.g., tetraethylene glycol dimethyl ether (G4). Here, we reveal that operating temperatures (0∼70 °C) critically dictate Li2CO3-associated cell overcharges (4.60∼3.77 V), by determining temperature-dependent reaction kinetics and evolving species mobility, as analogously witnessed under pure O2-conditions. Against what is observed for Li2O2 formation in the Li–O2 cell, however, cell temperatures do not govern the crystallinity of the Li2CO3 discharge product. In agreement with experimental observations, comprehensive density functional theory calculations also uncover the effect of the temperature on the Li2CO3 precipitation mechanism and shed light on the dwindling stabilization of metastable peroxocarbonate intermediates during discharge at increasing cycling temperatures. On the other hand, during extended operation, the temperature-dependent reactants mobility in the viscous G4 electrolyte and remnant Li-deficient carbonate surfaces from precedent recharge steps play equally prominent roles on the Li2CO3 precipitation mechanism and the resulting cell capacity. Our study highlights the complexity of practical Li–O2 cells with temperature-dependent performances.

Elucidating molecular mechanisms of two-dimensional chemical reactions

Elucidating molecular mechanisms of two-dimensional chemical reactions

Investigation of β-alkynol inhibition mechanism and Ru/Pt dual catalysis in Karstedt catalyzed hydrosilylation cure systems

β-Alkynol inhibitors play an important role in Karstedt catalyzed hydrosilylation cure silicone chemistry by providing a long working time at low temperatures, while allowing for rapid cure at elevated temperatures. Using 13C-labeled 1-ethynyl-1-cyclohexanol (13C-1) as a representative of the β-alkynol inhibitor, it was possible to monitor the change in this component in an actual silicone composition by 1H and 13C NMR during cure. The data showed that 13C-1 was selectively consumed by Pt-catalyzed hydrosilylation with the Si—H siloxane in the formulation in spite of the presence of large excess of vinyl groups from the vinyl polysiloxane component. The reaction rate was highly dependent on the Si—H siloxane used, the reaction temperature and presence of the tertiary -OH group in 13C-1. These results suggest that the high selectivity of the terminal alkyne over vinyl for hydrosilylation and the temperature dependent reaction kinetics are keys to understanding the inhibition mechanism for β-alkynols. These studies also inspired a Ru/Pt dual catalyst approach to lower the cure temperature of the silicone composition while maintaining a suitable working time. This approach is based on the hypothesis that the Ru co-catalyst can selectively catalyze the hydrosilylation of the β-alkynol in a temperature dependent manner, and its consumption would subsequently release the Pt-catalyst to rapidly cure the composition. Evaluation of a range of Ru-complexes led to the identification the complex [Cp*Ru(MeCN)3]+OTf- and its dodecanenitrile derivatives as a prototype from which the effectiveness of the dual catalysis approach was validated in a silicone release coating application with a cure temperature of 85oC and cure time of 4 seconds.

Cl-Atom-Initiated Atmospheric Degradation of Saturated Cyclic Hydrocarbons: Kinetic and Mechanistic Investigation.

The temperature-dependent reaction kinetics of methyl cyclohexane (MCH) and methyl cyclopentane (MCP) with Cl atoms were experimentally explored via the relative rate technique. Gas chromatography coupled with a flame ionization detector was employed to follow the reactant as well as the reference compound concentrations during the course of reaction. Gas chromatography coupled with mass spectrometry and gas chromatography coupled with infrared spectroscopy were used as the diagnostic tools for the detection and identification of the products in the title reaction. The rate coefficients for the reaction of Cl atoms with methyl cyclohexane and methyl cyclopentane were measured in the temperature range of 283-363 K at 760 Torr using isoprene and propylene as reference compounds. To support the experimental results, computational calculations were performed over the temperature range of 200-400 K using canonical variational transition state theory coupled with small curvature tunneling (CVT/SCT), conventional transition state theory (CTST) coupled with Wigner tunneling corrections, and CVT coupled with Wigner tunneling methods using the MP2 level of theory with 6-31G(d,p) as the basis set. The temperature-dependent rate coefficients were measured for the reaction of methyl cyclohexane and methyl cyclopentane with Cl atoms to be k(MCH) = [(4.48 ± 0.75) × 10-11]exp[(604 ± 25)/T] cm3 molecule-1 s-1 and k(MCP) = [(5.71 ± 0.66) × 10-12]exp[(1819 ± 669)/T] cm3 molecule-1 s-1, respectively. The measured rate coefficients at 298 K for the reactions of Cl atoms with methyl cyclohexane and methyl cyclopentane are k298KMCH = (3.36 ± 0.34) × 10-10 cm3 molecule-1 s-1 and k298KMCP = (2.25 ± 0.24) × 10-10 cm3 molecule-1 s-1, respectively. The conceivable atmospheric degradation mechanism for the reaction of methyl cyclohexane as well as methyl cyclopentane with Cl atoms was projected based on the products obtained during the reaction. Atmospheric implications, cumulative atmospheric lifetimes, thermochemistry, ozone formation potentials, and branching ratios of these molecules were also calculated and have been reported in this article.

Versatile 2-Methoxyethylaminobis(phenolate)yttrium Catalysts: Catalytic Precision Polymerization of Polar Monomers via Rare Earth Metal-Mediated Group Transfer Polymerization

The present study is one of the first examples for rare earth metal-mediated group transfer polymerization (REM-GTP) with non-metallocene catalyst systems. 2-Methoxyethylaminobis(phenolate)yttrium trimethylsilylmethyl complexes were synthesized and showed moderate to high activities in the rare earth metal-mediated group transfer polymerizations of 2-vinylpyridine, 2-isopropenyl-2-oxazoline, diethyl vinylphosphonate, diisopropyl vinylphosphonate, and N,N-dimethylacrylamide as well as in the ring-opening polymerization of β-butyrolactone. Reaction orders in catalyst and monomer were determined for the REM-GTP of 2-vinylpyridine. The mechanistic studies revealed that the catalyst systems follow a living monometallic group transfer polymerization mechanism allowing a precise molecular-weight control of the homopolymers and the block copolymers with very narrow molecular weight distributions. Temperature-dependent reaction kinetics were conducted and allowed conclusions about the influence of the bulky substituents around the metal center on the polymerization activity. Additional polymerization experiments concerning the combination of REM-GTP and ROP to obtain block copolymers were performed.

Binding Energies of O2 and CO to Small Gold, Silver, and Binary Silver−Gold Cluster Anions from Temperature Dependent Reaction Kinetics Measurements

A detailed analysis of experimentally obtained temperature-dependent gas-phase kinetic data for the oxygen and carbon monoxide adsorption on small anionic gold (Au(n)(-), n = 1-3), silver (Ag(n)(-), n = 1-5), and binary silver-gold (Ag(n)Au(m)(-), n + m = 2, 3) clusters is presented. The Lindemann energy transfer model in conjunction with statistical unimolecular reaction rate theory is employed to determine the bond dissociation energies E(0) of the observed metal cluster complexes with O(2) and CO. The accuracy limits of the obtained binding energies are evaluated by applying different transition-state models. The assumptions involved in the data evaluation procedure as well as possible sources of error are discussed. The thus-derived binding energies of O(2) to pure silver and binary silver-gold cluster anions are generally in excellent agreement with previously reported theoretical values. In marked contrast, the binding energies of O(2) and CO to Au(2)(-) and Au(3)(-) determined via temperature-dependent reaction kinetics are consistently lower than the theoretically predicted values.

Mechanism of Hydrolysis of Isopropenyl Glucopyranosides. Spectroscopic Evidence Concerning the Greater Reactivity of the alpha Anomer.

The mechanism of hydrolysis of isopropenyl alpha- and beta-glucopyranosides (1 and 2, respectively) has been studied by temperature-dependent reaction kinetics, stereochemical analysis of products by (1)H NMR, solvent (18)O- and (2)H-labeling experiments, kinetic solvent deuterium isotope effects, and alpha-values for general acid catalysis. Compounds 1 and 2 are the first vinyl acetals to be shown to undergo hydrolysis exclusively by cleavage of the vinyl ether and not the acetal C-O bond. While both glucopyranosides undergo irreversible, rate-limiting C-protonation, 1 hydrolyzes approximately four times faster than 2 and has an enthalpy of activation that is 5.8 kcal mol(-)(1) lower than that of 2, suggesting that 1 is kinetically more basic than 2. Spectroscopic evidence indicates that conjugation of the glycosidic oxygen with the alkenyl double bond is greater in 1 than in 2, probably because of steric or conformational constraints.

Temperature-dependent reaction kinetics of NH (a 1Δ)

The gas-phase reactions of NH(a 1Δ) with H 2 and selected saturated and unsaturated hydrocarbons have been studied over the 250-600 K temperature range. Olefin reactions proceed at near the gas kinetic collision rate and show no temperature dependence. H 2 and saturated hydrocarbons show temperature-dependent reactions rates, with activation energies of = 0.8-2 kcai/mole. No evidence of electronic quenching of NH(a 1Δ) to the ground state was observed with any of the hydrocarbons studied. First-order reactions rates, Arrhenius A factors and activation energies for the reactions are reported. We discuss a mechanistic interpretation of the kinetics in view of earlier kinetic and reaction-product studies and ab initio SCF Cl calculations.