Experimental Activation Enthalpy Research Articles

Computed versus experimental energy barriers in solution: Influence of the type of the density functional approximation.

Mechanistic investigations at the density functional theory level of organic and organometallic reactions in solution are now broadly accessible and routinely implemented to complement experimental investigations. The selection of an appropriate functional among the plethora of developed ones is the first challenge on the way to reliable energy barrier calculations. To provide guidelines for the choice of an initial and reliable computational level, the performances of commonly used non-empirical (PBE, PBE0, PBE0-DH) and empirical density functionals (BLYP, B3LYP, B2PLYP) were evaluated relative to experimental activation enthalpies. Most reactivity databases to assess density functional performances are primarily based on high level calculations, here a set of experimental activation enthalpies of organic and organometallic reactions performed in solution were selected from the literature. As a general trend, the non-empirical functionals outperform the empirical ones. The most accurate energy barriers are obtained with hybrid PBE0 and double-hybrid PBE0-DH density functionals, both providing similar performance. Regardless of the functional under consideration, the addition of the GD3-BJ empirical dispersion correction does not enhance the accuracy of computed energy barriers.

Theoretical calculations of homogeneous catalysis in the gas phase: elimination kinetics of 2,2-dimethoxypropane in the presence of HCl, F3CCOOH and CH3COOH

ABSTRACTThe mechanisms of gas-phase elimination kinetics of 2,2-dimethoxypropane in the presence of hydrogen chloride, trifluoroacetic acid and acetic acid were studied using Moller Plesset, ab initio combined method Complete Basis Set (CBS)-QB3 and various density functional theory methods with 6-311G(d,p) and 6-311++G(d,p) basis sets. The M06-2X/6-311++G(d,p) method provided reasonable agreement with the experimental enthalpy and energy of activation. Formation of 2-methoxypropene and methanol products occurs through six-membered cyclic ring transition state (TS) structure. The TS was characterised by single imaginary frequency, and confirmed through intrinsic reaction coordinate (IRC) calculations. The IRC calculations suggest the development of a van der Waal complex between the 2, 2-dimethoxy propane and the acid catalyst, leading to the TS formation. The process of decomposition in the absence of the acid catalyst requires much higher temperature with an energy of activation above 200 kJ/mol. This fact appears to be a consequence of a four-membered cyclic TS-type of mechanism in the non-catalysed reaction. Structural parameters, analyses of natural bond orbital charges and bond orders of the acid-catalysed elimination reactions in this study suggest that the polarisation of the C–O bond, in the direction Cδ+—Oδ−, is rate-determining in the TS. These reactions are non-synchronous concerted polar in nature.

Atropisomerization of 8-Membered Dibenzolactam: Experimental NMR and Theoretical DFT Study.

Detailed experimental and theoretical quantum mechanical analysis of the atropisomerization mechanism of a complex, bridged biaryl molecule with imbedded biphenyl, amine, and lactam moieties, 7,8-diallyl-5-benzyl-7,8-dihydrodibenzo[e,g][1,4]diazocin-6(5H)-one (1), was undertaken. Experimental Gibbs free activation energy, activation enthalpy, and activation entropy were established by temperature-dependent kinetic NMR experiments. Theoretical analysis utilized density functional theory (DFT) calculations at the B3LYP/6-31G(d) level of theory. Twelve energy minima and 17 transition states associated with five different atropisomer interconversion pathways were found by the combination of DFT calculated two-dimensional potential energy surfaces (2D PES) and the quadratic synchronous transit-guided (QST2) method. Among the five possible atropisomerization pathways, the lowest Gibbs free activation energy 25.8 kcal/mol was in close agreement with the experimentally determined value of 26.8 kcal/mol. Theoretical activation entropies and enthalpies were also consistent with experimental data. Geometrical and vibrational analysis of transition states and metastable intermediates suggested the mechanism of atropisomer interconversion of 1 as a rotation of the eclipsed endocyclic coordinate in a clockwise or counterclockwise direction along the ring. Puckering ability at least in one of the segments of the ring appears to be one of the most critical factors defining the height of atropisomerization barrier.

A Combined Computational–Experimental Study of the Kinetics of Intramolecular Diels–Alder Reactions in a Series of 1,3,8-Nonatrienes

Activation enthalpies for a series of five 1,3,8-nonatriene intramolecular Diels–Alder (IMDA) reactions involving substrates 1–5 have been determined experimentally and Singleton’s natural abundance method has been employed to determine kinetic isotope effects in the IMDA reaction of fumarate 3. The activation enthalpies for the IMDA reactions of the systems possessing a –CH2OCH2– diene/dienophile tether are significantly smaller than their counterparts possessing the –CH2OC(=O)– tether. The experimental activation enthalpies have been used to benchmark computed values from four model chemistries, namely two density functional theory functionals, B3LYP and M06-2X, and two generally very accurate composite ab initio wave function methods, CBS-QB3 and G4(MP2). G4(MP2) outperformed the computationally more expensive CBS-QB3 method, but the vastly cheaper M06-2X/6-31G(d)//B3LYP/6-31G(d) method was sufficiently accurate to be the recommended method of choice for calculating activation parameters. Experimental 2H kinetic isotope effects for the IMDA reaction of fumarate 3 confirmed the computational predictions that this Diels–Alder reaction is concerted but asynchronous.

O2 Binding to Heme is Strongly Facilitated by Near‐Degeneracy of Electronic States

This paper reports the computed O2 binding to heme, which for the first time explains experimental enthalpies for this process of central importance to bioinorganic chemistry. All four spin states along the relaxed Fe-O2-binding curves were optimized using the full heme system with dispersion, thermodynamic, and scalar-relativistic corrections, applying several density functionals. When including all these physical terms, the experimental enthalpy of O2 binding (-59 kJ mol(-1)) is closely reproduced by TPSSh-D3 (-66 kJ mol(-1)). Dispersion changes the potential energy surfaces and leads to the correct electronic singlet and heptet states for bound and dissociated O2. The experimental activation enthalpy of dissociation (~82 kJ mol(-1)) was also accurately computed (~75 kJ mol(-1)) with an actual barrier height of ~60 kJ mol(-1) plus a vibrational component of ~10 and ~5 kJ mol(-1) due to the spin-forbidden nature of the process, explaining the experimentally observed difference of ~20 kJ mol(-1) in enthalpies of binding and activation. Most importantly, the work shows how the nearly degenerate singlet and triplet states increase crossover probability up to ~0.5 and accelerate binding by ~100 times, explaining why the spin-forbidden binding of O2 to heme, so fundamental to higher life forms, is fast and reversible.

Mechanism elucidation of the cis-trans isomerization of an azole ruthenium-nitrosyl complex and its osmium counterpart.

Synthesis and X-ray diffraction structures of cis and trans isomers of ruthenium and osmium metal complexes of general formulas (nBu4N)[cis-MCl4(NO)(Hind)], where M = Ru (1) and Os (3), and (nBu4N)[trans-MCl4(NO)(Hind)], where M = Ru (2) and Os (4) and Hind = 1H-indazole are reported. Interconversion between cis and trans isomers at high temperatures (80–130 °C) has been observed and studied by NMR spectroscopy. Kinetic data indicate that isomerizations correspond to reversible first order reactions. The rates of isomerization reactions even at 110 °C are very low with rate constants of 10–5 s–1 and 10–6 s–1 for ruthenium and osmium complexes, respectively, and the estimated rate constants of isomerization at room temperature are of ca. 10–10 s–1. The activation parameters, which have been obtained from fitting the reaction rates at different temperatures to the Eyring equation for ruthenium [ΔHcis-trans‡= 122.8 ± 1.3; ΔHtrans-cis‡= 138.8 ± 1.0 kJ/mol; ΔScis-trans‡= −18.7 ± 3.6; ΔStrans-cis‡= 31.8 ± 2.7 J/(mol·K)] and osmium [ΔHcis-trans‡= 200.7 ± 0.7; ΔHtrans-cis‡= 168.2 ± 0.6 kJ/mol; ΔScis-trans‡= 142.7 ± 8.9; ΔStrans-cis‡= 85.9 ± 3.9 J/(mol·K)] reflect the inertness of these systems. The entropy of activation for the osmium complexes is highly positive and suggests the dissociative mechanism of isomerization. In the case of ruthenium, the activation entropy for the cis to trans isomerization is negative [−18.6 J/(mol·K)], while being positive [31.0 J/(mol·K)] for the trans to cis conversion. The thermodynamic parameters for cis to trans isomerization of [RuCl4(NO)(Hind)]−, viz. ΔH° = 13.5 ± 1.5 kJ/mol and ΔS° = −5.2 ± 3.4 J/(mol·K) indicate the low difference between the energies of cis and trans isomers. The theoretical calculation has been carried out on isomerization of ruthenium complexes with DFT methods. The dissociative, associative, and intramolecular twist isomerization mechanisms have been considered. The value for the activation energy found for the dissociative mechanism is in good agreement with experimental activation enthalpy. Electrochemical investigation provides further evidence for higher reactivity of ruthenium complexes compared to that of osmium counterparts and shows that intramolecular electron transfer reactions do not affect the isomerization process. A dissociative mechanism of cis↔trans isomerization has been proposed for both ruthenium and osmium complexes.

Reactions of Methyl Perfluoroalkyl Ethers with Isopropyl Alcohol: Experimental and Theoretical Studies

The reaction of an isomeric mixture of the methyl perfluoroalkyl ether, C4F9OCH3 (Novec-7100), in the presence of isopropyl alcohol (IPA) and/or water has been studied by measuring the rate of product formation using an ion-selective electrode (ISE) for fluoride ion, Karl Fisher coulometric titrations for water, and 1H and 19F NMR spectroscopy for product identification and rate studies. The results showed the methyl perfluoroalkyl ether to be very stable with products forming at the rate of ∼1 ppm per year at a laboratory temperature of 20 °C. Measurements over the temperature range of 6° to 100 °C were made on samples aged for periods up to 1.8 years. Density functional theory calculations (DFT, B3LYP/6-31+G(d)) were employed to investigate different reaction pathways and formulate the probable reaction mechanism. The experimental enthalpy (ΔH⧧) and entropy (ΔS⧧) of activation were determined based on several different kinetic measurements. The ΔH⧧ values are in the range of 20–25 kcal/mol and the corre...

Structure and dynamics of binary and ternary lanthanide(iii) and actinide(iii) tris[4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione] (TTA) – tributylphosphate (TBP) complexes. Part 3, the structure, thermodynamics and reaction mechanisms of 8- and 9-coordinated binary and ternary Y-TTA-TBP complexes studied by quantum chemical methods

Possible mechanisms for intermolecular exchange between coordinated and solvent water in the complexes Y(TTA)(3)(OH(2))(2) and Y(TTA)(3)(TBP)(OH(2)) and intermolecular exchange between free and coordinated HTTA in Y(TTA)(3)(OH(2))(HTTA) and Y(TTA)(3)(TBP)(HTTA) have been investigated using ab initio quantum chemical methods. The calculations comprise both structures and energies of isomers, intermediates and transition states. Based on these data and experimental NMR data (Part 2) we have suggested intimate reaction mechanisms for water exchange, intramolecular exchange between structure isomers and intermolecular exchange between free HTTA and coordinated TTA. A large number of isomers are possible for the complexes investigated, but only some of them have been investigated, in all of them the most stable geometry is a more or less distorted square anti-prism or bicapped trigonal prism; the energy differences between the various isomers are in general small, less than 10 kJ mol(-1). 9-coordinated intermediates play an important role in all reactions. Y(TTA)(3)(OH(2))(3) has three non-equivalent water ligands that can participate in ligand exchange reactions. The fastest of these exchanging sites has a QM activation energy of 18.1 kJ mol(-1), in good agreement with the experimental activation enthalpy of 19.6 kJ mol(-1). The mechanism for the intramolecular exchange between structure isomers in Y(TTA)(3)(OH(2))(2) involves the opening of a TTA-ring as the rate determining step as suggested by the good agreement between the QM activation energy and the experimental activation enthalpy 47.8 and 58.3 J mol(-1), respectively. The mechanism for the intermolecular exchange between free and coordinated HTTA in Y(TTA)(3)(HTTA) and Y(TTA)(3)(TBP)(HTTA) involves the opening of the intramolecular hydrogen bond in coordinated HTTA followed by proton transfer to coordinated TTA. This mechanism is supported by the good agreement between experimental activation enthalpies (within parenthesis) and calculated activation energies 68.7 (71.8) and 35.3 (38.8) kJ mol(-1). The main reason for the difference between the two systems is the much lower energy required to open the intramolecular hydrogen bond in the latter. The accuracy of the QM methods and chemical models used is discussed.

DFT study on the mechanism for the substitution of F−into Al(iii) complexes in aqueous solution

The mechanisms for the substitution of an aqua ligand with F(-) in monomeric Al complexes were studied with density functional theory (DFT). Typical mechanisms are modeled to determine the preferred substitution pathway according to the activation energy barriers. The present computational results are in favor of interchange associative (I(a)) mechanism for the substitution of F(-) into Al(H(2)O)(6)(3+), whereas interchange dissociative (I(d)) mechanism is preferred for the substitution into Al(H(2)O)(5)(OH)(2+), which is in agreement with the previous experimental findings. This implies the mechanistic changeover from I(a) to I(d) induced by the spectator hydroxyl ligand. Like the water-exchange reaction, the substitution rate is accelerated by OH(-) ligand. The difference of the computational and experimental activation enthalpy values is interpreted as the DFT errors in energy and the deviation of transmission coefficient from unity.

Comment on “Temperature and Solvent Effects on Radical Scavenging Ability of Phenols”

Comment on “Temperature and Solvent Effects on Radical Scavenging Ability of Phenols”

Characterization of Manganese(V)−Oxo Polyoxometalate Intermediates and Their Properties in Oxygen-Transfer Reactions

A manganese(III)-substituted polyoxometalate, [alpha2-P2MnIII(L)W17O61]7- (P2W17MnIII), was studied as an oxidation catalyst using iodopentafluorobenzene bis(tifluoroacetate) (F5PhI(TFAc)2) as a monooxygen donor. Pink P2W17MnIII turns green upon addition of F5PhI(TFAc)2. The 19F NMR spectrum of F5PhI(TFAc)2 with excess P2W17MnIII at -50 degrees C showed the formation of an intermediate attributed to P2W17MnIII-F5PhI(TFAc)2 that disappeared upon warming. The 31P NMR spectra of P2W17MnIII with excess F5PhI(TFAc)2 at -50 and -20 degrees C showed a pair of narrow peaks attributed to a diamagnetic, singlet manganese(V)-oxo species, P2W17MnV=O. An additional broad peak at -10.6 ppm was attributed to both the P2W17MnIII-F5PhI(TFAc)2 complex and a paramagnetic, triplet manganese(V)-oxo species. The electronic structure and reactivity of P2W17MnV=O were modeled by DFT calculations using the analogous Keggin compound, [PMnV=OW11O39]4-. Calculations with a pure functional, UBLYP, showed singlet and triplet ground states of similar energy. Further calculations using both the UBLYP and UB3LYP functionals for epoxidation and hydroxylation of propene showed lowest lying triplet transition states for both transformations, while singlet and quintet transition states were of higher energy. The calculations especially after corrections for the solvent effect indicate that [PMnV=OW11O39]4- should be highly reactive, even more reactive than analogous MnV=O porphyrin species. Kinetic measurements of the reaction of P2W17MnV=O with 1-octene indicated, however, that P2W17MnV=O was less reactive than a MnV=O porphyrin. The experimental enthalpy of activation confirmed that the energy barrier for epoxidation is low, but the highly negative entropy of activation leads to a high free energy of activation. This result originates in our view from the strong solvation of the highly charged polyoxometalate by the polar solvent used and adventitious water. The higher negative charge of the polyoxometalate in the transition versus ground state leads to electrostriction of the solvent molecules and to a loss of degrees of freedom, resulting in a highly negative entropy of activation and slower reactions.

Activation Energies of Pericyclic Reactions: Performance of DFT, MP2, and CBS-QB3 Methods for the Prediction of Activation Barriers and Reaction Energetics of 1,3-Dipolar Cycloadditions, and Revised Activation Enthalpies for a Standard Set of Hydrocarbon Pericyclic Reactions

Activation barriers and reaction energetics for the three main classes of 1,3-dipolar cycloadditions, including nine different reactions, were evaluated with the MPW1K and B3LYP density functional methods, MP2, and the multicomponent CBS-QB3 method. The CBS-QB3 values were used as standards for 1,3-dipolar cycloaddition activation barriers and reaction energetics, and the density functional theory (DFT) and MP2 methods were benchmarked against these values. The MPW1K/6-31G* method and basis set performs best for activation barriers, with a mean absolute deviation (MAD) value of 1.1 kcal/mol. The B3LYP/6-31G* method and basis set performs best for reaction enthalpies, with a MAD value of 2.4 kcal/mol, while the MPW1K method shows large errors for reaction energetics. The MP2 method gives the expected systematic underestimation of barriers. Concerted and nearly synchronous transition structures are predicted by all DFT and MP2 methods. Also reported are revised estimated 0 K experimental activation enthalpies for a standard set of hydrocarbon pericyclic reactions and updated comparisons to experiment for DFT, ab initio, and multicomponent methods. B3LYP and MPW1K methods with MAD values of 1.5 and 2.1 kcal/mol, respectively, fortuitously outperform the multicomponent CBS-QB3 method, which has a MAD value of 2.3. The MAD value of the O3LYP functional improves to 2.4 kcal/mol from the previously reported 3.0 kcal/mol.

Ab Initio Molecular Orbital and Density Functional Studies on the Solvolysis of Sarin and O,S-Dimethyl Methylphosphonothiolate, a VX-like Compound

[reaction: see text] Potential energy surfaces for the alkaline hydrolysis of sarin and O,S-dimethyl methylphosphonothiolate, a VX model compound, and the perhydrolysis of the latter have been computed at the MP2/6-31+G(d)//mPW1K/MIDI! level of theory. The effect of aqueous solvation was accounted for via the integral equation formalism polarizable continuum model (IEF-PCM) at the HF/6-31+G(d) level. Excellent agreement with the experimental enthalpy of activation for alkaline hydrolysis of sarin was found. For the alkaline hydrolysis of O,S-dimethyl methylphosphonothiolate, it was found that the P-O and P-S bond cleavage processes are kinetically competitive but that the products of P-S bond cleavage are thermodynamically favored. For the perhydrolysis of O,S-dimethyl methylphosphonothiolate, it was found that P-O bond cleavage is not kinetically competitive with P-S bond cleavage. In both cases, the data support initial formation of trigonal bipyramidal intermediates and demonstrate kinetic selectivity for nucleophilic attack on the face opposite the more apicophilic methoxide ligand.

Intramolecular Coupling of η2-Iminoacyls on Zirconium Bis(aryloxides) and Calix[4]arenes: Revised Mechanism by DFT Calculations and Car−Parrinello Molecular Dynamics Simulations

The intramolecular coupling of the two iminoacyl units in the bis(η2-iminoacyl) complexes [calix[4](OMe)2(O)2−Zr(η2-MeCNBut)2] and [(2,6-But2C6H3O)2−Zr(η2-MeCNPh)2] to generate the corresponding enediamido species has been investigated by means of both static and dynamic density functional calculations. For both systems we have characterized the stationary points of the potential energy surfaces for the coupling reaction and evaluated the overall energy profile. Car−Parrinello molecular dynamics simulations have been performed to gain insight into the detailed mechanistic features of the iminoacyl coupling and surprisingly show that this reaction occurs through an asynchronous reaction mechanism in which a decoordinated iminoacyl attacks the residual coordinated iminoacyl C−N double bond. A transition state search, performed without any symmetry constraint, led to structures consistent with the proposed mechanism 23.8 and 21.2 kcal mol-1 above the starting bis(η2-iminoacyl) reagents, in good agreement with experimental activation enthalpies. On the basis of the proposed reaction mechanism we are also able to reproduce the activation energy of the coupling reaction with electron-withdrawing substituents on the iminoacyl moiety. Inspection of the electronic structure changes along the proposed reaction pathway suggests that the iminoacyl coupling process can be described as an intramolecular attack of the decoordinated iminoacyl carbon lone-pair at the coordinated iminoacyl π* orbital, thus reconciling the iminoacyl coupling reaction mechanism with the generally accepted pattern of acyl and iminoacyl reactivity.

Correlation between ionic radius and cation diffusion in stabilised zirconia

Abstract To get an insight into the mechanism of cation diffusion in stabilized zirconia, the diffusion of all stable lanthanides was measured simultaneously in zirconia stabilised with scandia (ScSZ), yttria (YSZ) and calcia (CSZ). It was possible to identify all lanthanides with SIMS and to obtain the bulk diffusion coefficients between 1400 and 2000 K from SIMS depth profiles. For CSZ and YSZ, the lanthanide bulk diffusivities increase with increasing lanthanide ionic radius, while for ScSZ, they are nearly independent of the radius. The experimental activation enthalpies and pre-exponential factors are highest for two-valent CSZ and lower for the three-valent systems ScSZ and YSZ. They are not strongly affected by the type of the lanthanide for all systems investigated.

Lanthanide transport in stabilized zirconias: interrelation between ionic radius and diffusion coefficient.

The diffusion of all stable lanthanides was measured both in calcia stabilized zirconia (CSZ) and in yttria stabilized zirconia (YSZ) in the temperature range between 1,286 and 1,600 degrees C. The lanthanide diffusion coefficients obtained increase with increasing ionic radius. The experimental activation enthalpy of diffusion is near 6 eV for CSZ and between 4 and 5 eV for YSZ and is not strongly affected by the type of lanthanide. The results were correlated with defect energy calculations of the lanthanide diffusion enthalpy using the Mott-Littleton approach. An association enthalpy of cation vacancies with oxygen vacancies of about 1 eV (96 kJ/mol) was deduced in the case of CSZ, while there is no association in the case of YSZ. Furthermore, the change in diffusion coefficients can be correlated to the interaction parameter for the interaction between the lanthanide oxide with zirconia: The higher the interaction parameter, the higher the lanthanide diffusion coefficient.

Exploring the quantum mechanical/molecular mechanical replica path method: a pathway optimization of the chorismate to prephenate Claisen rearrangement catalyzed by chorismate mutase

A replica path method has been developed and extended for use in complex systems involving hybrid quantum/classical (quantum mechanical/molecular mechanical) coupled potentials. This method involves the definition of a reaction path via replication of a set of macromolecular atoms. An “important” subset of these replicated atoms is restrained with a penalty function based on weighted root-mean-square rotation/translation best-fit distances between adjacent (i±1) and next adjacent (i±2) pathway steps. An independent subset of the replicated atoms may be treated quantum mechanically using the computational engine Gamess-UK. This treatment can be performed in a highly parallel manner in which many dozens of processors can be efficiently employed. Computed forces may be projected onto a reference pathway and integrated to yield a potential of mean force (PMF). This PMF, which does not suffer from large errors associated with calculated potential-energy differences, is extremely advantageous. As an example, the QM/MM replica path method is applied to the study of the Claisen rearrangement of chorismate to prephenate which is catalyzed by the Bacillus subtilis isolated, chorismate mutase. Results of the QM/MM pathway minimizations yielded an activation enthalpy ΔH †† of 14.9 kcal/mol and a reaction enthalpy of −19.5 kcal/mol at the B3LYP/6-31G(d) level of theory. The resultant pathway was compared and contrasted with one obtained using a forced transition approach based on a reaction coordinate constrained repeated walk procedure (ΔH †† =20.1 kcal/mol, ΔH rxn = −20.1 kcal/mol, RHF/4-31G). The optimized replica path results compare favorably to the experimental activation enthalpy of 12.7±0.4 kcal/mol.

Structure and bonding in solution of dioxouranium(VI) oxalate complexes: isomers and intramolecular ligand exchange.

Structural isomers of [UO(2)(oxalate)(3)](4-), [UO(2)(oxalate)F(3)](3-), [UO(2)(oxalate)(2)F](3-), and [UO(2)(oxalate)(2)(H(2)O)](2-) have been studied by using EXAFS and quantum chemical ab initio methods. Theoretical structures and their relative energies were determined in the gas phase and in water using the CPCM model. The most stable isomers according to the quantum chemical calculations have geometries consistent with the EXAFS data, and the difference between measured and calculated bond distances is generally less than 0.05 A. The complex [UO(2)(oxalate)(3)](4-) contains two oxalate ligands forming five-membered chelate rings, while the third is bonded end-on to a single carboxylate oxygen. The most stable isomer of the other two complexes also contains the same type of chelate-bonded oxalate ligands. The activation energy for ring opening in [UO(2)(oxalate)F(3)](3-), deltaU++ = 63 kJ/mol, is in fair agreement with the experimental activation enthalpy, deltaH++ = 45 +/- 5 kJ/mol, for different [UO(2)(picolinate)F(3)](2-) complexes, indicating similar ring-opening mechanisms. No direct experimental information is available on intramolecular exchange in [UO(3)(oxalate)(3)](4-). The theoretical results indicate that it takes place via the tris-chelated intermediate with an activation energy of deltaU++ = 38 kJ/mol; the other pathways involve multiple steps and have much higher activation energies. The geometries and energies of dioxouranium(VI) complexes in the gas phase and solvent models differ slightly, with differences in bond distance and energy of typically less than 0.06 A and 10 kJ/mol, respectively. However, there might be a significant difference in the distance between uranium and the leaving/entering group in the transition state, resulting in a systematic error when the gas-phase geometry is used to estimate the activation energy in solution. This systematic error is about 10 kJ/mol and tends to cancel when comparing different pathways.

Lanthanide Diffusion in Calcia Stabilized Zirconia: Experimental and Theoretical Study

Multiple diffusion experiments of all stable lanthanides were performed simultaneously in calcia-stabilized zirconia (CSZ) in the temperature range between 1286° and 1463°C. The bulk diffusion coefficient obtained increases with increasing ionic radius. The experimental activation enthalpy is near 6 eV and is not strongly affected by the type of lanthanide. These results were correlated with calculations of the cation diffusion using the Mott-Littleton-approach. An association energy of cation vacancies with oxygen vacancies of about 1 eV was deduced.

Characterization of hot deformation behaviour of Zr–2.5Nb in β phase

Hot deformation characteristics of β Zr–2.5wt%Nb in the temperature range 1225–1425 K and in the constant true strain rate range of 0.002–10 s−1 were studied by uniaxial compression testing in vacuum. A processing map using the strain rate sensitivity parameter was developed for β Zr–2.5Nb on the basis of flow stress data as a function of temperature and strain rate for a given strain. A domain of high strain rate sensitivity of 0.5 at 1375 K and 0.002 s−1, was obtained and the grain size in this domain was large and equiaxed, suggesting the occurrence of large grain superplasticity. Thermal activation analysis was employed to determine the experimental activation volume and enthalpy, from which it was suggested that the rate controlling mechanism involved the movement of dislocation jogs. A comparison of Zr–2.5Nb with Zr in β phase suggested that Nb shifts the domain peak to higher temperatures.