Activation Energy For Exchange Research Articles

Adaptable Polyurethane Networks Containing Tertiary Amines as Intrinsic Bond Exchange Catalyst

AbstractVitrimers exhibit unique properties, such as thermal recyclability akin to thermoplastics, while structurally mirroring thermosets in terms of strength, durability, and chemical resistance. However, a significant limitation of these materials is their dependence on an external catalyst. Consequently, this research aims to develop vitrimer materials that incorporate an intrinsic catalyst, thus maintaining excellent thermomechanical properties and recyclability. Polyaddition polymerization is employed to synthesize the desired polymer, incorporating a self‐synthesized tertiary amine unit, (bis(2‐hydroxyethyl)‐3,3′‐((2‐(dimethylamino)ethyl)azanediyl)dipropanoate) (N‐diol), as an internal catalyst for transcarbamoylation and potential transesterification reactions. The resulting polymer, with a gel content of 97% and a glass transition temperature of 29 °C, is fabricated into test samples for comprehensive thermal and mechanical evaluations. The material demonstrates an initial Young's modulus of 555 MPa, retaining 81% of this value after two recycling processes. Additionally, using stress relaxation analysis (SRA), a topology freezing temperature of 82 °C, indicative of the characteristic Arrhenius‐like relaxation behavior, is identified with a bond exchange activation energy of 163 kJ mol−1.

High strength, self-healing and hydrophobic fully bio-based polybenzoxazine reinforced pine oleoresin-based vitrimer and its application in carbon fiber reinforced polymers

The traditional carbon fiber reinforced polymers (CFRPs) matrix is a permanent 3D cross-linked network that is difficult to degrade and reprocess. In order to solve the closed-loop recycling of carbon fibers (CFs) and develop green economy. We synthesized aldehyde-containing bio-based benzoxazine (VD) from vanillin and 1,10-diaminodecane, the epoxy resin vitrimer matrix (P-AE-MV) was prepared by curing acrylpimaric acid diglycidyl ester (AE) with a Schiff base (MV) formed by 1,8-menthane diamine (MDA) and VD. Under the double dynamic covalent bond of Schiff base and β-hydroxy ester, P-AE-MV has a very low dynamic bond exchange activation energy Ea = 48.1 kJ mol−1, which can realize the rapid topological rearrangement of polymer network, and give P-AE-MV excellent self-healing, shape memory, reprocessability and degradability. In addition, the introduction of polybenzoxazine (PBz) structure greatly improved the mechanical properties, thermal decomposition temperature, carbon yield and hydrophobicity of epoxy resin vitrimer, which is very suitable for CFRPs matrix. The CFRP (P-AE-MV-CF) prepared with P-AE-MV as matrix resin have good mechanical properties, reprocessability, shape memory and self-adhesion. Especially, it can be rapidly ammonolysis with n-butylamine under mild conditions (60 °C), achieving efficient and non-destructive recycling of CFs. This work provides an effective solution to promote the closed-loop recycling of CFs and the sustainable development of CFRPs using fully bio-based resources as raw materials.

A structure–dynamics relationship enables prediction of the water hydrogen bond exchange activation energy from experimental data

Relationships between water dynamics and structure are exploited to determine the hydrogen-bond exchange activation energy from experimental data.

A dual dynamic network self-healing bio-based vitrimer and its application in multiply recyclable carbon fiber reinforced polymers

The preparation of carbon fiber reinforced polymers (CFRP) using vitrimer with hydroxyl ester bonds as matrix requires harsh degradation conditions. To solve this problem, tung oil-based triglycidyl ester (TOTGE) and 4-aminophenyl disulfide (APD) are cured to fabricate a TOTGE−APD vitrimer with a dual dynamic network of hydroxyl ester and disulfide bonds. The TOTGE−APD vitrimer exhibits a low dynamic bond exchange activation energy (66.51 kJ/mol) due to the introduction of disulfide bonds and can achieve stress relaxation at 100 °C, thus exhibiting excellent self-healing and recycling properties. After crushing/remodeling, tensile strength of the remodeling sample can achieve 97.5% the original sample. Due to the disulfide and ester group in the vitrimer, CFRP prepared with TOTGE−APD vitrimer can be degraded in mercaptoehtanol, ethanolamine, and ethylene glycol, achieving multiple recycling.

QCT calculations of O2 + O collisions: Comparison to molecular beam experiments

We present quasiclassical trajectory simulations of O2 + O collisions under conditions representative of a crossed molecular beam experiment [Lahankar et al., J. Phys. Chem. A 120, 5348-5359 (2016)]. These calculations are compared to experimental data in order to further validate Potential Energy Surfaces (PESs) recently developed at the University of Minnesota [Z. Varga, Y. Paukku, and D. G. Truhlar, J. Chem. Phys. 147, 154312 (2017)]. Spin and spatial symmetries allow nine adiabatic PESs to represent the ground state interactions of O2 + O. We simulate trajectories adiabatically on all nine surfaces and perform analysis on data from each surface separately and the combined set of nine. It is shown that aggregated adiabatic calculations from nine surfaces agree better with the experiment for both inelastic and exchange collisions than a previous study that used a single surface and lie within the reported experimental uncertainty at almost all points. Distributions for exchange reactions using all nine PESs are interpreted using each surface's opacity function and activation energy for exchange. Rovibrationally resolved product distributions are then studied for a better understanding of energy relaxation in exchange collisions and may prove useful for further quasiclassical trajectory study and new experiments that use rovibrational spectroscopy to resolve the internal energy of the scattered products.

Modelling of bismuth segregation in InAsBi/InAs superlattices: Determination of the exchange energies

InAsBi dilute alloys are potential new candidates for the improvement of infrared optoelectronic devices such as photodetectors or lasers. In this work, InAsBi/InAs superlattices (SLs) with Bi contents ranging between 1 and 3% were grown by molecular beam epitaxy with different Bi fluxes and growth temperatures to analyze Bi segregation by cross sectional transmission electron microscopy techniques. Bi segregation profiles have been described layer-by-layer using a three-layer fluid exchange mechanism, extracting the values of the As/Bi exchange energies (E1, 1.26 ± 0.01 eV and E2, 1.36 ± 0.02 eV). A relationship to calculate the activation energies for exchange from the binding energies in III–V alloys is proposed, which would allow predicting them for other hitherto unknown compounds.

Preparation, characterization and solution behavior of α-hydroxyphosphonate complexes with tin tetrachloride

The reaction of tin tetrachloride with a series of variously substituted α-hydroxyphosphonates (L) in anhydrous dichloromethane produces new octahedral tin complexes of the type [SnCl4L2]. These adducts were characterized by multinuclear (1H, 13C, 31P and 119Sn) NMR, IR spectroscopy, and elemental analysis. The NMR data show that these complexes exist in solution as mixtures of cis and trans isomers. Moreover, the solution behavior of these complexes in the presence of excess ligand was investigated with variable temperature NMR using the coalescence temperature method. The resulting metal-ligand exchange activation energies were determined, showing values in the range 50–52 kJ/mol. The remote substituent effect on the metal-ligand interaction was also studied and compared with closely related phosphoryl-tin complexes.

Reconciling Differences between Lipid Transfer in Free-Standing and Solid Supported Membranes: A Time-Resolved Small-Angle Neutron Scattering Study.

Maintaining compositional lipid gradients across membranes in animal cells is essential to biological function, but what is the energetic cost to maintain these differences? It has long been recognized that studying the passive movement of lipids in membranes can provide insight into this toll. Confusingly the reported values of inter- and, particularly, intra-lipid transport rates of lipids in membranes show significant differences. To overcome this difficulty, biases introduced by experimental approaches have to be identified. The present study addresses the difference in the reported intramembrane transport rates of dimyristoylphosphatidylcholine (DMPC) on flat solid supports (fast flipping) and in curved free-standing membranes (slow flipping). Two possible scenarios are potentially at play: one is the difference in curvature of the membranes studied and the other the presence (or not) of the support. Using DMPC vesicles and DMPC supported membranes on silica nanoparticles of different radii, we found that an increase in curvature (from a diameter of 30 nm to a diameter of 100 nm) does not change the rates significantly, differing only by factors of order ∼1. Additionally, we found that the exchange rates of DMPC in supported membranes are similar to the ones in vesicles. And as previously reported, we found that the activation energies for exchange on free-standing and supported membranes are similar (84 and 78 kJ/mol, respectively). However, DMPC's flip-flop rates increase significantly when in a supported membrane, surpassing the exchange rates and no longer limiting the exchange process. Although the presence of holes or cracks in supported membranes explains the occurrence of fast lipid flip-flop in many studies, in defect-free supported membranes we find that fast flip-flop is driven by the surface's induced disorder of the bilayer's acyl chain packing as evidenced from their broad melting temperature behavior.

Synthesis, spectroscopic characterization and solution behavior of new tin tetrachloride adducts with γ-keto allyl phosphonates

Four new octahedral complexes of the type [SnCl4L2] (L = γ-keto allyl phosphonate) (1–4) were prepared and characterized by multinuclear (1H, 13C, 31P and 119Sn) NMR, IR spectroscopy and elemental analysis. The NMR data show, as expected, that these complexes exist in solution as mixtures of cis and trans isomers. More importantly, the solution structure was confirmed by 119Sn NMR spectra which show two triplets corresponding to the two isomers. In addition, the solution behavior of these complexes in the presence of excess ligand was studied by variable temperature NMR using the coalescence temperature method. The metal-ligand exchange activation energies were therefore determined and found to be in the range 57–60 kJ/mol. The effect of remote substituents on the metal-ligand interaction was studied and compared with closely related tin-phosphoryl complexes.

Study on behavior of alkalescent fiber FFA-1 adsorbing glyphosate from production wastewater of glyphosate

The results of thermodynamics properties researched previously show that the isothermal line of glyphosate adsorption on fiber FFA-1 meets with Freundlich equation, and of glycine is suit to Langmuir equation. The thermodynamic values of ΔHθ, ΔGθ, ΔSθ obtained provide the basic data for large scale recycling effective components from the wastewater of manufacturing glyphosate with FFA-1 fiber. Based on thermodynamics, kinetics of adsorption glyphosate on alkalescent fiber FFA-1 was studied by the static adsorption experiments in the range of 303.15–318.15K. Some models, such as kinetics equations of pseudo first-order and pseudo second-order, as well as Elovich model and the diffusion model in particle, were used for investigating the adsorbing process. The results obtained show that the process of adsorption glyphosate on alkalescent fiber FFA-1 was found to be satisfied with kinetic equation of pseudo second-order. The exchange activation energies were calculated to be 30.964kJmol−1 using the equation of Arrhenius.

Relaxation Exchange in Nanoporous Silica by Low-Field NMR

Abstract Investigating the 2D-T 2-T 2-relaxation exchange of inter-stitial water in a packing of sedimented Stöber-silicate spheres, we come the conclusion that contrary to its behaviour in macro-pores, water confined in nano-pores of silica exhibits enhanced diffusivity. The 2D-experiments, performed at different temperatures, reveal a temperature-dependent bimodal relaxation distribution and two-site relaxation exchange. Our recently introduced kinetic multi-site exchange model is applied to derive the according exchange rates. The resulting Arrhenius plot produces an exchange activation energy of 7 kJ/mol, which is well below the hydrogen bond energy or the activation energy for self-diffusion of water in the bulk. A possible hopping-mechanism as the source of enhanced proton-diffusion in nanoporous silica is discussed, as well as its significance to mass transfer in porous media.

NMR and Computational Studies of the Configurational Properties of Spirodioxyselenuranes. Are Dynamic Exchange Processes or Temperature-Dependent Chemical Shifts Involved?

Spirodioxyselenurane 4a and several substituted analogs revealed unexpected (1)H NMR behavior. The diastereotopic methylene hydrogens of 4a appeared as an AB quartet at low temperature that coalesced to a singlet upon warming to 267 K, suggesting a dynamic exchange process with a relatively low activation energy. However, DFT computational investigations indicated high activation energies for exchange via inversion through the selenium center and for various pseudorotation processes. Moreover, the NMR behavior was unaffected by the presence of water or acid catalysts, thereby ruling out reversible Se-O or benzylic C-O cleavage as possible stereomutation pathways. Remarkably, when 4a was heated beyond 342 K, the singlet was transformed into a new AB quartet. Further computations indicated that a temperature dependence of the chemical shifts of the diastereotopic protons results in convergence upon heating, followed by crossover and divergence at still higher temperatures. The NMR behavior is therefore not due to dynamic exchange processes, but rather to temperature dependence of the chemical shifts of the diastereotopic hydrogens, which are coincidentally equivalent at intermediate temperatures. These results suggest the general need for caution in ascribing the coalescence of variable-temperature NMR signals of diastereotopic protons to dynamic exchange processes that could instead be due to temperature-dependent chemical shifts and highlight the importance of corroborating postulated exchange processes through additional computations or experiments wherever possible.

Ligand exchange processes on solvated beryllium cations VII – water exchange on cationic [Be(H2O)3(Ln)]2+n (Ln: 4-O-Py−, Py, 4-(Py)-Py+, 3,5-(Py)2-Py2+, 3,4,5-(Py)3-Py3+)

The water exchange reactions on [Be(H2O)3(Ln)]2+n (Ln: 4-O-Py−, Py, 4-(Py)-Py+, 3,5-(Py)2-Py2+, 3,4,5-(Py)3-Py3+) were investigated through quantum chemical calculations (B3LYP/6-311+G**). While in all cases an associative interchange mechanism was found, the associative character was found to increase with increasing charge on the overall system, which in turn is governed by the charge on the ligand Ln. Including the CPCM solvent model into the system decreases the amplitude of the energetic effects, but leaves the general trend intact. Furthermore, the binding energy of a fourth water molecule and the overall exchange activation energy were found to increase with the overall charge on the system. The structures are also influenced by the charge, but to a smaller extent. The Be–O distances to the second coordination sphere water molecule and the distances to the entering/leaving water molecules in the transition state are significantly shortened in the higher charged systems. These effects can be attributed to electrostatic interactions.

Ligand Exchange Processes on Solvated Beryllium Cations, VI. Hydrogen Chalcogenide Exchange on [Be(Chelate)Sol2]2+ (Chelate: CH3–O–C2H4–O–CH3, CH3–S–C2H4–S–CH3, CH3–Se–C2H4–Se–CH3; Sol: H2O, H2S, H2Se)

AbstractFollowing work from Dehnicke et al. on the binding of Be2+ to crown ethers, solvent addition, and solvent exchange energies were calculated (B3LYP/6‐311+G**) for solvated beryllium complexes with bidentate ligands CH3–O–C2H4–O–CH3 [1,2‐bis(methoxy)ethane], CH3–S–C2H4–S–CH3 [1,2‐bis(methio)ethane], and CH3–Se–C2H4–Se–CH3 [1,2‐bis(methylselanyl)ethane], and H2O, H2S, and H2Se as solvents. All water exchange reactions for the different bidentate ligands show no variation, with energies of addition to the second coordination sphere of around –25 kcal·mol–1 and exchange activation energies of around 19 kcal·mol–1. Substituting water by H2S and H2Se in [(CH3–O–C2H4–O–CH3)Be(Solvent)2]2+ lowered the energy for addition of a solvent molecule in the second coordination sphere to –12 kcal·mol–1 and the exchange activation energy to 12 kcal·mol–1. All reaction mechanisms were found to be of the associative interchange (Ia) type.

ChemInform Abstract: Stable β-Phosphorylated Cyclic Aminoxyl Radicals in SDS Micelles.

The behaviour of a series of stable cyclic aminoxyl radicals in the presence of SDS micelles was studied by EPR spectroscopy. Six different compounds, i.e. two non-phosphorylated and four β-phosphorylated, were investigated. Except in the case of the strongly hydrophilic 3-CP 1, which always remained in the bulk aqueous phase, all the radicals were found to exchange between water and micelles. Their partition coefficients were evaluated from computer simulations of the EPR spectra and in the case of two aminoxyls TOMER-Et 5 and TOMER-Pri 6, the variation of the rate constant with temperature allowed us to estimate the exchange activation energy.

Interphase exchange and diffusion of oxygen in lanthanum-strontium cobaltites doped with iron

Oxides La0.4Sr0.6Co1 − xFexO3 − δ (x = 0.2 and 0.4) were synthesized by the ceramic technology. The kinetics of exchange and diffusion of oxygen in these oxides were studied by the isotopic exchange method over the temperature range 600–900°C at an oxygen pressure of 10 torr. The temperature dependences of interphase exchange rate and oxygen diffusion coefficient in the bulk and near-surface region were obtained, and the effective activation energies of exchange and oxygen diffusion were calculated. The concentration of iron ions was shown to have no substantial influence on the exchange rate and oxygen diffusion. Ratio between the values of exchange rate and diffusion coefficient were shown to influent on the precision of the values estimation.

Thermodynamic and Kinetic Characterization of Hydrogen−Deuterium Exchange in β-Phase Palladium

A Sieverts' apparatus coupled with a residual gas analyzer (RGA) is an effective method to detect composition variations during isotopic exchange. This experimental setup provides a tool for the thermodynamic and kinetic characterization of H-D isotope exchange on Pd. The H or D concentrations in the gas and solid phases during the exchanges starting from (H(2) + Pd(x)D) and (D(2) + Pd(x)H) in beta-phase Pd were monitored over a temperature range from 173 to 298 K. The equilibrium properties, i.e., the H-D separation factors alpha and equilibrium constants K(HD), were obtained and found to be very close to those in the literature. The values of equilibrium constant reported here are the only experimental K(HD) data for H-D-Pd system. The H-D exchange rates on beta-Pd were measured for both exchange directions. A comprehensive kinetic model is proposed that correlates the exchange rate and the driving force composed of the reactant concentrations and the extent of deviation from equilibrium. The rate constants were obtained using this model for two exchange directions. The rates for the two exchange directions were found to be close to each other at 173 K, but they differ with temperature increase in such a way that the (D(2) + Pd(x)H) has a higher rate than (H(2) + Pd(x)D). The exchange activation energies obtained are 2.0 and 3.5 kJ/mol for the (H(2) + Pd(x)D) and (D(2) + Pd(x)H) directions, respectively. The difference in activation energies results from the difference in the energy states of (H(2) + Pd(x)D) and (D(2) + Pd(x)D). The calculated exchange profiles using this model agree with the experimental values reasonably well.

Temperature-Programmed Elimination of Tritium in Coal Labeled by Tritiated Gaseous Hydrogen

The hydrogen exchange of an Argonne Pocahontas #3 (POC) coal with tritiated gaseous hydrogen in the presence of a Pt/Al2O3 catalyst was carried out using a fixed-bed reactor at a temperature range of 200−250 °C. Then, the tritiated coal sample was re-exchanged with hydrogen in a gaseous hydrogen atmosphere at different raising temperature rates. The changes in radioactivity of tritium releasing from the tritium-labeled coal sample in the temperature-programmed elimination (TPE) of tritium was monitored by a radioanalyzer in situ. The release profile of tritium was fit using three Gaussian distribution functions in TPE. On the basis of the waveform analysis, the amounts and activation energies of the hydrogen exchange for every type of exchangeable hydrogen were estimated. The values of exchange activation energies are 8.8 ± 0.5, 10.7 ± 0.5, and 16.3 ± 0.5 kcal/mol for three types of exchangeable hydrogen, respectively. The amount of type-I hydrogen was independent of the exchange reaction temperature. In contrast to this, the amount of types II and III of exchangeable hydrogen increased with an increasing exchange temperature, suggesting that the amount of types II and III of hydrogen in the coal is dependent upon the exchange temperature.

Time-Dependent Photoluminescence Spectroscopy as a Tool to Measure the Ligand Exchange Kinetics on a Quantum Dot Surface

The exchange kinetics of native ligands that passivate CdSe quantum dots (hexadecylamine (HDA), trioctylphosphine oxide (TOPO), and trioctylphosphine (TOP)) by thiols is followed in situ. This is realized by measuring, in real-time, the decrease in emission intensity of the QDs upon addition of hexanethiol (HT) which quenches the emission. The effect of adding an excess of native ligands prior to thiol addition on the capping exchange is studied to provide insight in the bond strength and exchange kinetics of the individual surfactants. Temperature-dependent measurements reveal faster kinetics with increasing temperature. A kinetic model to describe the time-dependent measurements is introduced, taking into account the equilibrium between native ligands before thiol addition and describing the evolution of surface coverage by all ligands over time. The model allows us to extract the quenching rate for a single thiol ligand (0.004 ns(-1)) as well as exchange rates, equilibrium constants, activation energies, and changes in Gibbs free energy for replacement of the different native surfactants by HT. The analysis reveals that the substitution half-time of HDA by HT (72 s) is much shorter than for TOP (5 h) or TOPO (2.5 h) under the same conditions. The temperature dependence of the kinetics shows that the activation energy for exchange of HDA/TOPO by hexanethiol (1.6 kJ/mol) is much smaller than for TOP (20.9 kJ/mol).

A Kinetic and Mechanistic Study of the Cl/F Exchange Reaction of CCl3F, CCl2F2, and CClF3 with Prefluorided Chromia

Chromium oxide has been fluorided to several monolayers depth by interaction with HF at 625 K. The interaction of CCl3F and CCl2F2 with these surface chromium fluoride layers has been studied by temperature-programmed reaction. In the case of the reaction of CCl3F, mono- and bi-exchange occurs with the surface monolayer fluoride, forming CCl2F2 and CClF3 simultaneously at a peak maximum temperature of 520 K. Exchange with the subsurface fluoride layers occurs at a peak maximum temperature of 680 K, as the fluoride ions from the bulk migrate to the surface and chloride ions on the surface migrate inward. In both temperature regions, mono-exchange predominates by a factor of about 102. Line-shape analysis of the temperature dependence of the rates of production of CCl2F2 and CClF3 gave activation energies of 53 and 54 kJ mol-1 for exchange with the surface fluoride, so the difference in rates must be the need for special dimer-type surface sites for the bi-exchange. The activation energy for exchange of F i...