Free Enthalpy Of Activation Research Articles

Pulse Radiolysis Study on Reactions of a Hydrated Electron with Europium(III)−Aminopolycarboxylate Complexes in Aqueous Perchlorate Media

Pulse radiolysis of 1:1 complexes of europium(III)-aminopolycarboxylates was studied in aqueous perchlorate solutions to understand relations of the reduction rate of Eu(III) with hydration state and stability of the complex. The rate constant for the reaction of a hydrated electron (e aq - ) with the complex was determined in the temperature range of 278.15 to 333.15 K and the ionic strength range of 0.05 to 1.0 mol kg -1 . The rate constant was linearly related not only with the residual hydration number of the complex representing the hydration state but also with the stability constant and redox potential as equilibrium constants. The reaction of eaq - with the complex was described by diffusive encounter and successive electron-transfer processes between eaq - and the complex. Gibbs free energy, enthalpy, and entropy of activation for the reaction were obtained from the effect of temperature on the rate constant, and the entropy term was found to be a dominant factor for the electron-transfer process after the diffusion process. These indicated that the entropy term, reflecting ionic interaction of Eu(III) -carboxylate groups and dehydration of aquo Eu(III) ion on the complexation, caused the difference in the rate constant between the complexes, and linear correlation between the rate constant and hydration number. The effect of ionic strength on the rate constant was examined by

Composition-dependent dynamical structures of monohydric alcohol-water mixtures studied by microwave dielectric analysis

Dielectric relaxation measurements on methanol (MA)-water, ethanol (EA)-water, and 1-propanol (1PA)-water mixtures were carried out using TDR (Time Domain Refletometry) for the entire concentration range of 0 ≤ X (molar fraction of alcohol) ≤ 1.0 up to 25GHz at 20°C-30°C. We have calculated the excess activation free energy, enthalpy, and entropy of these mixtures, ΔG E , ΔH E , and ΔS E , and the excess partial molar quantities for alcohol, ΔG A E , ΔH A E , and ΔS A E (A = MA, EA, and 1PA) and those for water, ΔG W E , ΔH W E , and Δ S W E from the temperature-dependent relaxation times. There exists boundary above and below which behaviors of the partial molar quantities are quite different. If we call the concentration corresponding to the boundary X b, X b∼0.30, 0.18, and 0.14 for methanol-water, ethanol-water, and 1-propanol-water, respectively. In the region of X ≥ X b, Δ H A E and Δ S A E are nearly zero. This means that, in terms of the activation enthalpy and entropy, alcohol molecules in the mixtures find themselves in not a very different environment from that in pure liquid. In water-rich region, ΔH A E and ΔS A E exhibit two maxima at X 1 and X 2, where X 1∼0.045, 0.04, and 0.03, and X 20.12, 0.08, and 0.06, for methanol-water, ethanol-water, and 1-propanol-water, respectively. This is clearly attributed to hydrophobic hydration. Detailed analysis of the behavior of ΔH A E and ΔS A E (A=MA, EA, and 1PA) suggests the formation of two kinds of saturated hydration structures and their transition; the clathrate hydration shells with tetrahededral local arrangements of water molecules around X∼ X 1 and non-clathrate shells with large cavities with three-coordinated local arrangements around X∼ X 2.

Viscosities of electrolyte solutions

The effect of solute–solvent interactions on the viscosity of dilute electrolyte solutions is considered in terms of the solute contribution to the activation free energy, enthalpy or entropy, for viscous flow. Two classes of electrolyte are considered, those involving inorganic ions and those involving larger organic ions such as the tetraalkylammonium salts. The solute contributions to the activation free energies for viscous flow Δμ 2 θ‡ for inorganic electrolytes vary systematically with the charge and radius of the ion in a way that is consistent with a primarily electrostatic effect, increasing linearly with the ionic charge and with the inverse of the ionic radius. In contrast, the Δμ 2 θ‡ values for the tetraalkylammonium halides increase linearly with the cationic volume. For both types of electrolytes, Δμ 2 θ‡ in organic solvents is relatively insensitive to the solvent, while the values for aqueous systems are markedly different from those in the organic solvents.

Noncovalent Interactions between Tetrazole and anN,N‘-Diethyl-Substituted Benzamidine

Amidines have long been known to form strong noncovalent complexes with carboxylates and phosphates. However, their interaction with tetrazoles, which are acidic heterocycles and important bioisosteric replacements for carboxylic acids in medicinal chemistry, has remained unexplored so far. The binding of a tetrazole to an N,N'-diethyl-substituted benzamidine has been studied for the first time by X-ray crystallography, solution NMR methods, and electrospray mass spectrometry. The amidinium group of model complex 3 was found to prefer an E,Z configuration in the crystal. Benzamidinium and tetrazolate groups alternate along an infinite chain of hydrogen bonds and salt bridges between the amidine-NH groups and the two tetrazole-N atoms next to the ring carbon. In solution, a 1:1 complex was evident from Job's method of continuous variation, and an association constant of 4.0 x 10(3) +/- 1.6 x 10(3) M(-)(1) (in CDCl(3)/CD(3)CN, 6:1) could be determined by (1)H NMR dilution experiments. Tetrazolate was not only found to be a weaker ligand than carboxylates but, surprisingly, the binding mode also changed with concentration in neat CDCl(3). At low concentrations, the amidine group in complex 3 adapted an E,E configuration as it does in a related carboxylic acid complex 4. With increasing concentration, the E,Z isomer starts to predominate. A free activation enthalpy DeltaG(298)() of 64 +/- 1 kJ mol(-)(1) for the E,E to E,Z isomerization was determined by line shape analysis at different magnetic fields. Binding strength was further probed in a competition experiment between a bisamidine, a carboxylate, and a tetrazolate by electrospray mass spectrometry.

Application of the transition state theory to water transport across cell membranes

We have applied the transition state theory of Eyring et al. (The Theory of Rate Processes, McGraw-Hill, 1941) to water transport across cell membranes. We have then evaluated free energy (ΔF≠), enthalpy (ΔH≠) and entropy (ΔS≠) of activation for water permeation across membranes, such as Arbacia eggs, Xenopus oocytes with or without aquaporin water channels, mammalian erythrocytes, aquaporin proteoliposomes, liposomes and collodion membrane. ΔH≠ was found to be correlated with ΔS≠. This is so-called ΔH≠ and ΔS≠ compensation over the ranges of ΔH≠ and ΔS≠ from 2 to 22 kcal/mol and from −26 to 45 e.u., respectively, indicating that low ΔH≠ values correspond to negative ΔS≠. Large positive ΔS≠ and high ΔH≠ values might be accompanied by reversible breakage of secondary bonds in the membrane, presumably in membrane lipid bilayer. Largely negative ΔS≠ and low ΔH≠ values for aquaporin water channels, aquaporin proteoliposomes and porous collodion membrane could be explained by the immobilization of permeating water molecules in the membrane, i.e., the partial loss of rotational and/or translational freedoms of water molecules in water channels.

Linear Relations of Activation Energy in Non-isothermal Kinetics

The object of this work is the quantitative explanation of linear correlation between activation energy (E), initial decomposition temperature (T i) and ionic potential (V i), observed for thermal degradation of some complexes of transitional metals. The proposed model allowed the evaluation of characteristic parameter proportional to the activation free enthalpy and also the variation of effective electrical charge (ΔQ *) of ligand, in the formation process of the activated complex. These results are satisfactory, taking into account that we utilized many simple hypotheses for deduction of Arrhenius equation.

Composition-dependent dynamical structures of 1-propanol–water mixtures determined by dynamical dielectric properties

Using time domain reflectometry, we carried out dielectric relaxation measurements on 1-propanol–water mixtures for the entire concentration range in the frequency range 100 MHz–25 GHz at 20, 25, and 30 °C. We have calculated the excess partial molar activation free energy, enthalpy, and entropy for 1-propanol, ΔG1PAE, ΔH1PAE, and ΔS1PAE, and those for water, ΔGWE, ΔHWE, and ΔSWE from the relaxation times. In the region of X (molar fraction of 1-propanol) ⩾0.14, ΔH1PAE and ΔS1PAE take nearly zero. This means that 1-propanol molecules in the mixtures find themselves in not a very different environment from that in pure liquid. In the water-rich region, ΔH1PAE and ΔS1PAE exhibit two maxima at X∼0.03 and X∼0.06, corresponding roughly to 0.9 and 0.78 g/cm3 of water content, respectively. This fact, together with the results of the molecular dynamics studies of Sciortino et al. suggest the formation of two kinds of saturated hydration structures: the clathrate hydration shells with tetrahedral local arrangements of water molecules around X∼0.03 and nonclathrate shells with large cavities with three-coordinated local arrangements around X∼0.06. Hydrophobic hydration seems to partly share the same mechanism with structural enhancement in pure water by lowering local density.

Correlation between the experimental and calculated (COSMO-AM1) activation free enthalpy of the hydroxide-induced elimination reaction of small haloalkanes in water

The hydroxide-induced concerted elimination reaction of small haloalkanes was studied by the COSMO-AM1 method. Satisfactory correlations were found between the experimentally and calculated activation free enthalpy of chloro and bromo eliminations. The established quantitative structure–activity relationships (QSAR) predicts several reactivity sequences correctly. Although the statistical quality is good, a large prediction interval is still obtained, making it less suitable for predictive purposes. The established QSAR tends to an increased deviation for haloalkanes with high activation energy. This is caused by an increased occurrence of competition effects with S N-reactions. Although a correlation was found between the calculated values and derived reorganization energies from the Marcus equation, it could not be stated unequivocally that this type of reaction also obeys this equation.

Hydrophobic hydration and molecular association in methanol–water mixtures studied by microwave dielectric analysis

Dielectric relaxation measurements on the methanol–water mixtures for the entire concentration range were carried out using time domain reflectometry in the frequency range from 500 MHz to 25 GHz at 20, 25, and 30 °C. The excess partial molar activation free energy, enthalpy, and entropy for methanol, ΔGMAE, ΔHMAE, and ΔSMAE, and those for water, ΔGWE, ΔHWE, and ΔSWE, were calculated from accurately measured concentration and temperature dependence of the dielectric relaxation time of the mixtures. The behavior of the excess partial molar quantities in the regions below and above X (molar fraction of methanol) ∼0.3 are quite different from each other. In a water-rich region, ΔHMAE and ΔSMAE exhibit two maxima at X∼0.045 and X∼0.12, which is clearly attributed to structural enhancement of the hydrogen bond network of water, the so-called hydrophobic hydration. Appearance of two maxima in ΔHMAE and ΔSMAE implies that water molecules surround methanol molecules in qualitatively different manners around the two points. In the concentrated region of X⩾0.3, the values of ΔHMAE and ΔSMAE become nearly zero, which means that methanol molecules in the mixtures find themselves in not a very different environment from that in pure methanol, associated and forming chainlike clusters. Water molecules seem to exothermically attach to the hydrophilic site of methanol.

The study of chemical reactions, absorption and extraction, using apparatuses with serpentine pie for pelliclizing-bubbling, XIV. Hydroperoxidation of 1,3-diisopropylbenzene with oxygen

Experimental results obtained by thermal hydroperoxidation of 1,3-diisopropylbenzene in the temperature region of 102–105°C, using a Vodnar-1 type apparatus with discontinuous operation are presented. Based on the results, the activation energy (E=48.5±2 kJ/mol), enthalpy (ΔH+=47±2 kJ/mol), entropy (ΔS+) and the activation free enthalpy (ΔG+) were calculated.

Hydration and Hydrolysis of alpha-Oxo Carboxylic Acid Derivatives and Conjugate Addition to alpha,beta-Unsaturated Carbonyl Compounds: A Density Functional Study.

Hydration of the keto group as well as hydrolysis (ester and amide) of alpha-oxo carboxylic acid derivatives has been studied by density functional theory (B3LYP/6-31G and B3LYP/6-311G). Both uncatalyzed as well as water-assisted processes have been considered. Solvent effects were approximated by the self-consistent isodensity surface polarized continuum (SCIPCM) model. For hydrolysis reactions a concerted as well as a stepwise mechanism was calculated. In the latter one, addition of the nucleophile to a tetrahedral intermediate was found to be rate determining. In uncatalyzed processes both concerted and stepwise mechanisms are calculated to have comparable activation energies and free enthalpies. In catalyzed reactions the stepwise mechanism is predicted to be considerably more favorable. Hydration of the alpha-oxo group should be much more feasible than either ester or amide hydrolysis.

Dynamical aspects of mixing schemes in ethanol–water mixtures in terms of the excess partial molar activation free energy, enthalpy, and entropy of the dielectric relaxation process

Dielectric relaxation measurements on the ethanol–water mixture for the entire concentration range in very small increments were carried out using TDR in the frequency range from 300 MHz to 25 GHz at 20 °C, 22.5 °C, and 25 °C. The activation enthalpy ΔH and entropy ΔS for the mixtures were separated from the activation free energy ΔG, and hence the excess partial molar activation free energy, enthalpy, and entropy for ethanol, ΔGEAE, ΔHEAE, and ΔSEAE, and those for water, ΔGWE, ΔHWE, and ΔSWE were calculated. The concentration dependence of these partial molar quantities shows the existence of two regions bound at X (molar fraction of ethanol) ∼0.18. In the water-rich region of X<0.1, ΔHEAE and ΔSEAE take large positive values, exhibiting two sharp maxima at X=0.04 and X=0.08, which is clearly attributed to structural enhancement of the hydrogen bond network of water by ethanol, the so-called hydrophobic hydration. From a standpoint of dynamics, mixing schemes of ethanol and water around the two points X=0.04 and X=0.08 seem to be qualitatively different. On the other hand, in the region of X>0.18, the values of ΔHEAE and ΔSEAE take nearly zero. This means that ethanol molecules in the mixtures are in almost the same environment as those are in pure ethanol, forming chainlike clusters surrounded or exothermically attached to by water molecules.

Pulmonary surfactant-associated polypeptide C in a mixed organic solvent transforms from a monomeric alpha-helical state into insoluble beta-sheet aggregates.

In the 35-residue pulmonary surfactant-associated lipopolypeptide C (SP-C), the stability of the valyl-rich alpha-helix comprising residues 9-34 has been monitored by circular dichroism, nuclear magnetic resonance, and Fourier transform infrared spectroscopy in both a mixed organic solvent and in phospholipid micelles. The alpha-helical form of SP-C observed in freshly prepared solutions in a mixed solvent of CHCl3/CH3OH/0.1 M HCl 32:64:5 (v/v/v) at 10 degrees C undergoes within a few days an irreversible transformation to an insoluble aggregate that contains beta-sheet secondary structure. Hydrogen exchange experiments revealed that this conformational transition proceeds through a transition state with an Eyring free activation enthalpy of about 100 kJ mol(-1), in which the polypeptide segment 9-27 largely retains a helical conformation. In dodecylphosphocholine micelles, the helical form of SP-C was maintained after seven weeks at 50 degrees C. The alpha-helical form of SP-C thus seems to be the thermodynamically most stable state in this micellar environment, whereas its presence in freshly prepared samples in the aforementioned mixed solvent is due to a high kinetic barrier for unfolding. These observations support a previously proposed pathway for in vivo synthesis of SP-C through proteolytic processing from a 21-kDa precursor protein.

Ab Initio Study of the Regiochemistry of 1,3-Dipolar Cycloadditions. Reactions of Diazomethane and Formonitrile Oxide with Ethene, Propene, Acrylonitrile, and Methyl Vinyl Ether.

Structures and energetics of reactants and transition structures of the cycloadditions of diazomethane (DZM) and formonitrile oxide (FNO) with ethene (ET), propene (PR), acrylonitrile (ACN), and methyl vinyl ether (MVE) have been investigated with the use of ab initio molecular orbital calculations. The reaction of acetonitrile oxide (MNO) with acrylonitrile has been also included for comparisons. Structure optimizations were performed at the RHF/6-31G(d) and density functional B3LYP/6-31G(d) levels of approximation. Single-point electronic energies were computed up to the MP4SDTQ/6-31G(d) level. Kinetic contributions to activation enthalpies and entropies were computed at the RHF/6-31G(d) level. Transition structures of ethene cycloadditions (prototype reactions) were also checked with the MP2/6-31G(d) approximation. Solvent effects were introduced both at a semiempirical level (AMSOL) and at an ab initio level using the Pisa model (interlocking spheres) and the IPCM procedure (isodensity surface polarized continuum model). Electronic activation energies are found to be very sensitive to the treatment of electron correlation and failed to converge to values unaffected by further theoretical improvements: indeed, the inclusion of full fourth-order correlation (MP4) decreases the activation energies by 5-10 kcal/mol with respect to the preceding level of correlation (MP3). Anyway, activation free enthalpies and entropies of the reactions under study appear to be close to the experimental values available for this class of reactions. Still in agreement with experimental observations is the effect of solvent polarity on the reaction rates. Theoretical regioselectivity is less sensitive to the level of calculation, although the inclusion of electron correlation, both with the Moeller-Plesset technique and the use of the density functional theory, is able to reverse the regiochemical predictions obtained with RHF energies for the reactions of nitrile oxides with acrylonitrile. This explains why the frontier orbital theory, which is based on uncorrelated HF-wave functions, cannot arrive at the correct prediction of the regiochemistry in these cases. Calculated solvent effects appear to influence the regiochemistry of 1,3-dipolar cycloaddition, but in general, they reinforce the prediction obtained in vacuo.

DFT computational study of the epoxidation of olefins with dioxiranes

Transition structures (TSs) of the reactions of dioxirane and dimethyldioxirane with ethylene, propene, cis-2-butene and trans-2-butene were located with the B3LYP/6-31G ∗ method. The TSs of the reactions of ethylene and cis-2-butene exhibit a symmetrical spiro butterfly structure with synchronous formation of the two new CO bonds and with substantial alignment of π bond axis with the breaking OO bond. In the case of propene and trans-2-butene a slight asynchrony in CO bond formation was predicted by calculations. Theoretical activation free enthalpies (gas phase) reproduce the experimental (acetone solution) higher reactivity of cis with respect to trans disubstituted alkenes which in turn are correctly predicted more reactive than monosubstituted alkenes. Also the calculated absolute activation free enthalpies, after correction for electrostatic solvation effects by single point SCRF calculation (Tomasi model), were found to be in reasonable accord with experimental data.

Cage and Entropy Effects in the Dynamics of Dissociative Electron Transfer

Dissociative electron transfers in condensed phases occur in two steps. The fragments are first formed within a solvent cage from which they further diffuse. The formation of caged, rather than free-moving, fragments is taken into account in an improved version of the dissociative electron transfer theory where entropic aspects are emphasized. A more detailed treatment than previously available of the fragmentation and solvent reorganization factors is given in terms of both energies and free energies. The reason that the bond dissociation energy, rather than the bond dissociation free energy, represents the contribution of fragmentation to the intrinsic barrier ensues. The resulting equations that relate the activation free enthalpy and entropy, as well as the symmetry factor, to the standard free enthalpy and entropy of the reaction are given for electrochemical, bimolecular, and intramolecular reactions. Solvation radii change upon electron transfer triggered bond cleavage. An iterative procedure is proposed for adapting the estimation of the solvent reorganization factor to the ensuing coupling of the fragmentation and solvent reorganization coordinates. Experimental examples illustrating applications of the theory are discussed.

Thermal activation processes of tensile deformation in γ-TiAl alloy

Abstract Thermal activation parameters: activation volume V , activation enthalpy Δ H , activation free enthalpy Δ G and activation entropy Δ S of tensile deformation at yield point of a near-lamellar γ -titanium aluminide of Ti–47a/oAl–2a/oMn–2a/oNb–0.8v/oTiB 2 have been measured in a wide temperature range from 77 to 1273 K. From the values and their temperature dependence of the measured activation parameters, as well as the temperature dependence of the yield stress, the dislocation mechanisms of tensile deformation of γ -titanium aluminide have been speculated. It is found that there exist three temperature regions, which correspond to different possible thermal activation mechanisms of dislocation motion. In low temperature region (77–398 K), the mechanism is mainly characterized by the overcoming of Peierls–Nabarro friction. In intermediate temperature region (523–973 K), the mechanism is a weak thermal activated process although the plastic flow stress is neither sensitive to temperature nor to the strain rate. In the high temperature region (973–1273 K), the rate controlling mechanism is dislocation climbing. In addition, it is found that, activation entropy Δ S , whose variation with temperature is similar to that of activation volume V , also reflects the thermal activation mechanism of dislocation movement in some degree.

Simultaneous effects of immobilization and substrate protection on the thermodynamics of glucose isomerase activity and inactivation

The influence of enzyme immobilization on the main thermodynamic quantities of glucose isomerization to fructose and vice versa was studied in batch tests by using a commercial immobilized glucose isomerase. The values calculated for the immobilized system showed, when compared with those reported for the native enzyme, higher equilibrium constants at T > 70°C and simultaneous increases in both standard enthalpy and entropy changes of reaction. Activation enthalpy and entropy changes of both forward and reverse enzyme-catalyzed reactions were calculated from the initial glucose isomerase activity evaluated at different temperatures by the Briggs-Haldane model. The activation enthalpy of the forward reaction was about 24% less than that of the native enzyme. Enzyme inactivation tests performed at different temperatures and sugar equilibrium concentrations confirmed the existence of the so-called substrate protection phenomenon which resulted in a 29–36% reduction in the decay constant. A slight increase in the activation free enthalpy of the inactivation reaction was also shown.

Phosphaalkenes as Building Blocks in Ene Reactions: Synthesis and Reactivity of 3-Amino-1,2-dihydro-1,2-diphosphetes

Phospha−ene reactions of the type II between methylidenephosphanes as enophiles and C-aminophosphaalkenes possessing allylic hydrogen atoms proceed by P−P bond formation to furnish the corresponding functionalized diphosphanes. Thus, the reaction of the methylidenephosphane 1 with the C-amino-substituted ethylidenephosphane 3a runs smoothly at room temperature to afford the unsymmetrical 1,2-diphosphane 4 as a 60:40 mixture of two diastereomers in 64% yield. Rotation about the P−N bond in 4 is hindered at room temperature, but free enthalpies of activation of ΔG⧧(PN) = 16.3 kcal·mol-1 (major diastereomer) and 15.2 kcal·mol-1 (minor diastereomer) were determined by variable temperature 1H NMR spectroscopy. In contrast, reactions of the ethylidenephosphanes 3a−c with the chlorophosphane 2 as enophile follow an unusual course involving a regiospecific ene reaction of the type II and a subsequent intramolecular ring closure reaction to furnish the 3-amino-1,2-dihydro-1,2-diphosphetes 7a−c in good yields (53−7...

Etude par analyse enthalpique différentielle de la stabilité de peroxycetals

The thermal stability of peroxycetals R 1(Me)C(OMe)OOtBu was analyzed by DSC. Rate constants and activation parameters of their decompositions in a 1.5 M solution of triphenylmethane in diphenylmethane were determined. A significant influence of R 1 was identified by the comparison of the free activation enthalpies of their homolytic decomposition.