Rate Retardations Research Articles

Influence of microstructural deformation mechanisms and shear strain localisations on small fatigue crack growth in ferritic stainless steel

Microstructurally small fatigue crack growth (FCG) rate in body-centred cubic (BCC) ferritic stainless steel is investigated by using a novel domain misorientation approach for EBSD microstructural deformation analyses, in conjunction with in situ digital imaging correlation (DIC). The DIC analyses revealed that shear strain localisations occur ahead of the crack tip during propagation and correlate well with the FCG rate retardations. Grain boundaries can be found at both peaks and valleys of the FCG rate curve and alter the interaction between crack growth and shear strain localisations. At the microstructural level, the deformation is associated with the dislocation-mediated plastic deformation process, showing increased formation of grain sub-structures in the regions of the strain localisation. Consequently, material experiences local hardening causing the FCG retardation events. If the crack avoids the hardened material region through a macroscopic cross-slip mechanism, retardation is minor. On the contrary, if the crack penetrates the hardened region, retardation is significant.

Transmission of dipolar substituent effects: ionization of a series 3-(7-substituted-1-naphthyl) propynoic acid (E)-3-(7-substituted-1-naphthyl) propenoic acids and their Esterification with Diazodiphenylmethane

Alkaline hydrolysis rates coefficients for the series of methyl 3-(7-substituted-1-naphthyl) propynoate was calculated in 70%v/v dimethylsulphoxide-water at various temperatures (25,30,40, and 50̊ C). The pKa values of 3-(7-substituted-1-naphthyl) propynoic acid and (E)- 3-(7-substituted-1-naphthyl) propenoic acid calculated in 80%w/w 2-methoxyethanol-water at room temperature (25.0̊ C). logk2 of esterification rate coefficients for 3-(7-substituted-1-naphthyl) propynioc acid and (E)-3(7-substituted-1-naphthyl) propenioc acid with DDM have been measured at 30.0̊ C. Reversed substituent dipolar effects were found in the ionization reaction. In the esterification reaction with DDM the result show similar but reduced substituted effects. Rate retardations was found in the alkaline hydrolysis. It could be result from steric effect or reversal of substituent dipolar effect with a combination of steric effect.

"Dual Layer" Self-Sorting with Cucurbiturils.

Platinum(II) complexes bearing terpyridyl (tpy) and thiolate ligands were used to test the design of a "dual layer" self-sorting system in the presence of Cucurbit[8]uril (CB[8]). Pt(II) thiolates and CB[8] form 2:1 assemblies, with both metallic centers sitting on top of one another at one of the macrocycle portals. We showed that any pair of these CB[8]-secured Pt(II) complex dimers bearing different tpy "heads" and thiolate "tails" scrambles to afford up to 10 ternary assemblies via two processes: (1) supramolecular exchanges (i.e., the egression and ingression of Pt complexes from and into CB[8]) and (2) ligand exchanges between the Pt thiolates. The mixtures of 10 assemblies were fully characterized by nuclear magnetic resonance spectroscopy. While the thiolate tails do not significantly affect the rate of the supramolecular exchanges, they were found to control (1) the kinetics of ligand exchange, with bulkier thiolates causing dramatic rate retardations, as well as (2) the thermodynamics of the self-sorting process, i.e., the distribution of assemblies at equilibrium, via intra-CB[8] assembly interactions between pairs of thiolates. Ligand exchanges are consistently slower than supramolecular exchanges. An associative pathway that involves the formation of dimers of CB[8]-secured Pt dimers (a total of 4 Pt complexes) during the ligand exchange process was invoked to rationalize the observed kinetics.

Experimental study of the kinetics of ligand-promoted dissolution of stibnite (Sb2S3)

Batch reactor experiments were carried out in order to investigate the influence of organic ligands in aqueous solution on the rate of dissolution of stibnite (Sb2S3). Ligands that may be regarded as representative of those occurring naturally in the soil solution were chosen, namely acetic, oxalic, citric, and salicylic acids, EDTA, glycine, cysteine, glucose, catechol, desferrioxamine-B, and standardized natural chestnut leaf litter extract. Millimolar concentrations of the ligands dissolved in inert buffers at pH=4, 6, and 8 were added to powdered stibnite in a stirred, thermostatted reactor (25°C) and the rate of increase in antimony concentration over time was monitored by ICP-OES. Dissolution rates were initially parabolic and settled into a linear regime. Comparison of initial and steady state rates achieved with different ligands with the blank experiments gives a very complex picture. Both rate enhancements and retardations are observed, and in some cases the net effect of a ligand at a given pH may be either an increased or a decreased mobilization with respect to the blank experiment, depending on the time of contact of the solution with the mineral. Rationalisations in terms of known principles of surface complexation are attempted. The extent of solution phase complexation of Sb(III) with the ligand under investigation is not found to be a decisive factor in the cases of those systems for which complexation constants are known. There is evidence suggesting that a linear free energy relationship may exist between the rates of ligand-promoted dissolution and pKa values of the ligands under investigation.

Molecular Dynamics Simulation of the Effect of Hydrophobic Cosolutes on the Neutral Hydrolysis of an Activated Ester

We have studied, by means of molecular dynamics (MD) simulations, the effect of two hydrophobic cosolutes, tert-butanol (t-BuOH) and ethanol (EtOH), on the neutral hydrolysis of p-methoxyphenyl dichloroacetate (MPDA) in water. Shifts were calculated in a pre-equilibrium, defined as a spatial distribution of the reacting molecules satisfying specific geometric constraints. The criteria applied represent the configurations from which the reaction can take place (reactive conformation, RC). The shifts induced by the presence of cosolute, as determined from the simulations, correspond well to the experimentally found rate retardations. In accord with experiments, an almost linear correlation between the concentration of added cosolute and the logarithm of the rate constant was found. This is in agreement with a simple mechanistic model. The percentage of RCs in pure water was calculated to be similar to15%, complementing earlier work performed in this group (Lensink, M. F.; Mavri, J.; Berendsen, H. J. C. J. Comput. Chem. 1999, 20, 886), leading to a calculated rate constant of k(calc) = 3.9 x 10(-3) s(-1) (exptl: k(obs) = 2.78 x 10(-3) s(-1)). The molecular structure of the RC was examined more extensively, investigating correlations between the positions of participating molecules.

Pairwise Gibbs Energy Interaction Parameters for α-Amino Acid−Amide Interactions in Aqueous Solution: A Kinetic Study

In order to establish the way in which the hydration characteristics of alpha-amino acids influence their noncovalent interactions with other solutes in aqueous solution, we measured medium effects for the water-catalyzed hydrolysis of an activated amide in aqueous solution in the presence of alpha-amino acids and some of their derivatives at 25 degrees C and pH 4. With the exception of phenylalanine and 3-phenylserine, all common alpha-amino acids induce significant rate enhancements. The kinetic results are analysed using the thermodynamic description of solute-solute interactions as formulated by Savage and Wood. This procedure allows the evaluation of pairwise Gibbs energy interaction parameters. These parameters neither reflect the hydrophobicity of the alpha-amino acid side chain nor the additivity of functional group interactions in the cosolute. The carboxylate group in the zwitterionic alpha-amino acid dominates the rate enhancing kinetic medium effect and reduces the availability of nearby methylene groups for interaction with the apolar initial state of the reaction. Some alpha-amino acid side chains clearly influence the interaction of the carboxylate group with the reactant, most probably due to destructive hydration shell overlap. Only for phenylalanine and 3-phenylserine are marked rate retardations observed, and presumably the hydrophobic effect of the side chains now dominates over the rate enhancing effect of the carboxylate moiety. The results provide quantitative insights into intermolecular noncovalent interactions between alpha-amino acids and the activated amide. It is shown that destructive hydration shell overlap effects play a significant role in noncovalent interactions in aqueous solution.

Vinyl ether hydrolysis. 30. Effect of β‐carboxy and β‐carbomethoxy substitution

AbstractRates of hydrolysis of the vinyl ether functional groups of (Z)‐ and (E)‐β‐methoxyacrylic acid and (Z)‐and (E)‐β‐methoxymethacrylic acid and their methyl esters were measured in aqueous perchloric acid solution. Additional rate measurements were also made for one substrate, (Z)‐β‐methoxymethacrylic acid, in buffer solutions down to pH 7, and a rate profile was constructed. The results show that the β‐carboxy and β‐carbomethoxy substituents produce strong rate retardations, ranging from 2000‐ to 25 000‐fold, for both Z‐ and E‐isomers in both the acrylic and methacrylic acid series. The rate profile for (Z)‐β‐methoxymethacrylic acid indicates that ionization of this substrate to the carboxylate ion form rises the rate of hydrolysis by a factor of 240. It is argued that this difference in reactivity of ionized and non‐ionized forms of the substrate is due to conjugative and inductive effects of the substituents, rather than β‐lactone formation as suggested in an earlier observation of the same phenomenon in a different system.

Reversibility of the ring-opening step in the acid hydrolysis of cyclic acetophenone acetals

The acid-catalysed hydrolysis of the vinyl ethers α-(2-hydroxyethoxy)styrene 1a or α-(3-hydroxypropoxy)styrene 1b yields a mixture of acetophenone and a cyclic acetophenone acetal, 2- methyl-2-phenyl-1,3-dioxolane 3a or 2-methyl-2-phenyl-1,3-dioxane 3b. Ratios [acetophenone]: [3] have been determined by HPLC and by UV spectroscopy as 15 (from 1a) and 10 (from 1b). The acetals undergo further hydrolysis, but this is considerably slower than that of the vinyl ethers, and the above numbers represent products of kinetic control. These product ratios are equal to the rate constant ratios k2:k–1 for the partitioning of the oxocarbocations 1-(2-hydroxyethoxy)-1-phenylethyl 2a or 1-(3-hydroxypropoxy)-1-phenylethyl 2b between reaction with solvent water molecules and with the internal hydroxy group. These cations are also formed as intermediates in the hydrolysis of the cyclic acetals 3. That k2 is an order of magnitude greater than k–1 signifies that the rate-limiting step in this hydrolysis is the ring-opening forming the oxocarbocation, and that there is little reversibility in aqueous solutions. The hydrolysis reactions of these cyclic acetals are considerably slower than that of an acyclic analogue, the dimethyl acetal of acetophenone, by factors of 1.0 × 103 for 3a and 1.8 × 102 for 3b. This study demonstrates that these rate retardations cannot be accounted for by reversibility of the ring-opening step. The presence of the ring results in an inherent decrease in the rate constant for the H+-catalysed formation of the oxocarbocation.

ChemInform Abstract: The Kinetics of Oxidation of Formaldehyde at a Platinum Electrode in an Acid Solution.

ChemInformVolume 21, Issue 23 Article ChemInform Abstract: The Kinetics of Oxidation of Formaldehyde at a Platinum Electrode in an Acid Solution. H. MATSUI, H. MATSUI Dep. Chem. Sci. Technol., Fac. Eng., Univ. Tokushima, Minamijosanjima, Tokushima 770Search for more papers by this authorA. KUNUGI, A. KUNUGI Dep. Chem. Sci. Technol., Fac. Eng., Univ. Tokushima, Minamijosanjima, Tokushima 770Search for more papers by this author H. MATSUI, H. MATSUI Dep. Chem. Sci. Technol., Fac. Eng., Univ. Tokushima, Minamijosanjima, Tokushima 770Search for more papers by this authorA. KUNUGI, A. KUNUGI Dep. Chem. Sci. Technol., Fac. Eng., Univ. Tokushima, Minamijosanjima, Tokushima 770Search for more papers by this author First published: June 5, 1990 https://doi.org/10.1002/chin.199023090Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume21, Issue23June 5, 1990 RelatedInformation

The Kinetics of Oxidation of Formaldehyde at a Platinum Electrode in an Acid Solution

Abstract Three types of the retardation were found for the rate of oxidation of formaldehyde at a platinum electrode in 0.5 mol dm−3 sulfuric acid. These rate retardations are attributed to the blocking of individual reaction sites by strongly chemisorbed species, the inhibition of the pairing of adjacent sites indispensable for the oxidation of formaldehyde, and the accumulation of oxidation intermediates. The oxidation rate under conditions excluding these rate retardations was independent of the potential in the potential range of about 0.4–0.6 V vs. a reversible hydrogen electrode and corresponded to first-order kinetics with respect to the formaldehyde concentrations. These kinetic data are explained in terms of a slow adsorption of formaldehyde molecules.

The Retardation of Methanol Oxidation at a Platinum Electrode in an Acid Solution

Abstract The rate retardation of the oxidation of methanol at the potential range of about 0.65–0.8 V vs. a reversible hydrogen electrode on a platinum electrode in 0.5 mol dm−3 H2SO4 was studied. The rate retardation of the overall oxidation was caused by that of the oxidation, Reaction D, not via COad. From the relationship among the rate of Reaction D, the COad coverage, and the potentials, three types of rate retardation were found out: Type 1– Reaction D is not accelerated by the potential, and the rate of the reaction is determined by the COad coverage and the methanol concentration. Type 2– the rate of Reaction D decreases at stationary COad coverages as the oxidation is prolonged. Type 3– the rate decreases at COad coverages close to the limiting value. It is proposed that Types 1 and 2 of the rate retardations take place when the adsorption of methanol molecules is rate-determining, and when the formaldehyde and formic acid formed from methanol are accumulated in the vicinity of the electrode, respectively. Type 3 of the rate retardation has been explained in a preceding paper in terms of the aggregate damaging effect of COad.

Kinetics of organomercurial reactions with model thiols: Sensitivity to exchange of the mercurial labile ligand

Stop-flow kinetic studies are reported for the reaction of glutathione (GSH) with 2-chloromercuri-4-nitrophenol. The second-order rate constant was found to vary by less than twofold in going from pH 5.9 to 7.3, suggesting that the reaction involves attack of the un-ionized glutathione thiol on the mercurial around pH 7. The rate constant was 1.5 × 10 7 m −1 s −1 at pH 7.0 and 8 °C in dilute buffer of ionic strength 0.2. The sensitivity of the kinetics to the nature of the leaving group in the mercurial i.e., to the labile ligand, was investigated in complexes of the mercurial with OH −, N 3 −, Br −, Cl −, CN −, SCN −, pyrophosphate, ADP, ATP, EDTA, and adenosine. The addition of most of these substances was observed to lead to rate enhancements or rate retardations, the kinetic effects showing typical binding isotherm behavior when plotted as a function of the added ligand concentration. The kinetic midpoints of such plots showed very good correlation with the independently determined affinities they had toward the mercurial. The results demonstrate that many commonly added compounds of biochemical interest can significantly influence mercurial reactivity toward thiols through rapid exchange at the labile ligand position of the mercurial.

A kinetic and isotope study of the mechanism of decomposition of the tetraperoxochromate(V) ion in basic solution

The decomposition of K 3CrO 8 in aqueous alkaline solution has been reinvestigated. Over the pH range from 8.0 to 11.2, the rate is first-order each in tetraperoxochromate ion and hydrogen ion concentrations. No evidence was found for any rate effect due to carbonate or phosphate buffer. Inhibition by H 2O 2, NH 3 and by ethylenediamine occurs; the nature of these rate retardations was studied. Isotope labelling of the peroxide oxygen atoms showed that all of the oxygen atoms in product O 2 originated as peroxide oxygens in the anions, and that small but real amount of scrambling occurs. The marked complexities which obtain in this systems are discussed in terms of the kinetic and isotope data.

The Effects of Deuterium Substitution on the Rates of Organic Reactions. VIII. The Solvolysis of t-Butyl-d1, -d2, -d3, -d6 and -d9 Chlorides

The solvolysis rate retardations caused by substitution of 1, 2, and 3 deuterium atoms in one methyl group of tbutyl chloride are not quite cumulative; the isotope effect per deuterium increases slightly as the number of deuterium atoms increases. The previously proposed conformational dependence of the BETA - deuterium isotope effect quantitatively predicts just such a trend. The retardations caused by complete deuteration of 1, 2, and 3 methyl groups are more nearly cumulative.