Determination Of Absolute Rate Constants Research Articles

Determinação da constante de velocidade absoluta para a reação de abstração de hidrogênio fenólico pelo estado excitado triplete de xantona em acetonitrila e no líquido iônico hexafluorfosfato de 1-butil-3-metil-imidazólio [bmim.PF6

The triplet excited state of xanthone was generated and characterized by laser flash photolysis in acetonitrile (λmax=620 nm; t=1.8 ms) and in ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate [bmim.PF6] (λmax=620 nm; t=3.0 ms). It reacts with phenols yielding the corresponding xanthone ketyl radical. Stern-Volmer plots for the reaction of triplet xanthone with phenols led to the determination of absolute rate constants for phenolic hydrogen abstraction in the order of ~109 Lmol-1s-1 in acetonitrile and ~108 Lmol-1s-1 in [bmim.PF6]. The lower diffusioncontrolled rate constant for [bmim.PF6] is responsible for the difference in the phenolic hydrogen abstraction rate constants in this solvent.

Selenium-containing amino acids are targets for myeloperoxidase-derived hypothiocyanous acid: determination of absolute rate constants and implications for biological damage

Elevated MPO (myeloperoxidase) levels are associated with multiple human inflammatory pathologies. MPO catalyses the oxidation of Cl−, Br− and SCN− by H2O2 to generate the powerful oxidants hypochlorous acid (HOCl), hypobromous acid (HOBr) and hypothiocyanous acid (HOSCN) respectively. These species are antibacterial agents, but misplaced or excessive production is implicated in tissue damage at sites of inflammation. Unlike HOCl and HOBr, which react with multiple targets, HOSCN targets cysteine residues with considerable selectivity. In the light of this reactivity, we hypothesized that Sec (selenocysteine) residues should also be rapidly oxidized by HOSCN, as selenium atoms are better nucleophiles than sulfur. Such oxidation might inactivate critical Sec-containing cellular protective enzymes such as GPx (glutathione peroxidase) and TrxR (thioredoxin reductase). Stopped-flow kinetic studies indicate that seleno-compounds react rapidly with HOSCN with rate constants, k, in the range 2.8×103–5.8×106 M−1·s−1 (for selenomethionine and selenocystamine respectively). These values are ~6000-fold higher than the corresponding values for H2O2, and are also considerably larger than for the reaction of HOSCN with thiols (16-fold for cysteine and 80-fold for selenocystamine). Enzyme studies indicate that GPx and TrxR, but not glutathione reductase, are inactivated by HOSCN in a concentration-dependent manner; k for GPx has been determined as ~5×105 M−1·s−1. Decomposed HOSCN did not induce inactivation. These data indicate that selenocysteine residues are oxidized rapidly by HOSCN, with this resulting in the inhibition of the critical intracellular Sec-dependent protective enzymes GPx and TrxR.

Selenium-containing Enzymes and Amino Acids are Major Targets for Myeloperoxidase-derived Hypothiocyanous acid: Determination of Absolute Rate Constants and Assessment of Biological Relevance

Selenium-containing Enzymes and Amino Acids are Major Targets for Myeloperoxidase-derived Hypothiocyanous acid: Determination of Absolute Rate Constants and Assessment of Biological Relevance

How Drug Photodegradation Studies Led to the Promise of New Therapies and Some Fundamental Carbanion Reaction Dynamics along the Way

The photodegradation of nonsteroidal anti-inflammatory drugs (NSAIDs), a class of medications that includes aspirin and ibuprofen, has generated considerable interest since the 1990s, largely because of the phototoxic and photoallergic effects that frequently accompany their therapeutic use. Among NSAIDs, ketoprofen, which contains a benzophenone chromophore, has been extensively studied, reflecting both its notorious adverse effects and the fascination that photochemists have with benzophenone. The photochemistry of ketoprofen involves the intermediacy of an easily detectable carbanion with a remarkable lifetime of 200 ns in water; its life expectancy can in fact be extended to minutes under carefully controlled anhydrous conditions. Over the past decade, we have used some key properties of the ketoprofen carbanion to conduct mechanistic studies on carbanions under various conditions. In particular, its ease of photogeneration provides the temporal control required for kinetic studies, which, combined with its long lifetime and readily detectable visible absorption, have enabled extensive laser flash photolysis work. These studies have led to an intimate understanding of the reaction dynamics for carbanions in solution, including the determination of absolute rate constants for protonation, S(N)2, and elimination reactions. Together they provide excellent exemplars of reactivity patterns that today are part of all introductory curricula in organic chemistry and illustrate the fundamentals of nucleophilic substitution paradigms. More recently, we have begun to exploit the photochemistry of ketoprofenate and have developed the ketoprofenate photocage, a valuable tool for the photocontrolled cleavage of protecting groups and concomitant drug release. The photorelease has been illustrated with ibuprofen, among many other molecules. These photocages have been further improved with the use of the xanthone chromophore; the goal is the release of antiviral agents taking advantage of the improved UVA absorption of xanthone (xanthonate photocages). In this Account, we survey our work of the past few years on the photochemistry of ketoprofen and related chromophores. Beginning with studies on the phototoxicity of ketoprofen, we have made the journey to new prodrug candidates, unraveling mechanistic elements of aroyl-substituted benzyl carbanions along the way.

Substituent and Solvent Effects on the β-Heterolysis Reaction of β-Hydroxy Arylethyl Radicals

Time-resolved conversion of a series of beta-hydroxy arylethyl radicals with electron-donating and -withdrawing aromatic substituents to their corresponding styrene radical cation via heterolytic loss of the beta-hydroxy leaving group was examined with nanosecond laser flash photolysis. In all cases, the reaction was catalyzed by added perchloric acid. Radicals 2a-d reacted via a pre-equilibrium protonation mechanism in acidic 1,1,1,3,3,3-hexafluoroisopropanol (HFIP), and measuring rate constants for radical cation formation as a function of acid content allowed for the determination of absolute rate constants ranging from 3.6 x 10(6) to 3.8 x 10(7) s(-1) for the loss of water from the protonated beta-hydroxy arylethyl radicals 2a-d, as well as the acidity constants, pKa approximately 1.5 (in HFIP), for the protonated radicals. The 4-methoxy-substituted beta-hydroxy arylethyl radical 2e reacted by rate determining protonation in HFIP with a second-order rate constant of k(H+) = 7.8 x 10(8) M(-1) s(-1). However, in acetonitrile, 2,2,2-trifluoroethanol, and mixtures of these two solvents, 2e reacted by pre-equilibrium protonation, allowing for solvent effects on the rate constant for loss of water from the protonated radical 2e to be determined. With use of these data, substituent electronic effects on the kinetics of the beta-heterolysis reaction are discussed. Differences in the effect of solvent on the rate constant for loss of water from the protonated beta-hydroxy arylethyl radicals and other beta-substituted arylethyl radicals are also discussed.

Comparative study of the reactivities of substituted 3-(benzoyl)benzyl carbanions in water and in DMSO.

Benzyl-substituted carbanions produced by photodecarboxylation of ketoprofen derivatives have been examined in basic aqueous and DMSO solutions. Product studies, combined with kinetic measurements from laser flash photolysis, have allowed the determination of absolute rate constants for protonation and intra-S(N)2 reactions leading to five- and six-membered ring cyclizations; the former are significantly faster. Many of the well-known trends in carbanion reactivity are placed on an absolute rate basis; thus, intra-S(N)2 are favored in polar nonprotic solvents, and the effect is larger for the more hindered carbanion centers. Protonation by water is slightly dependent on the nature of the carbanion center and is approximately 400 times faster in nonhydroxylic solvents, compared with bulk water. As expected, the reactivity for halide leaving groups follows the usual order of decreasing bond strengths, i.e., I(-) > Br(-) > Cl(-).

First determination of absolute rate constants for the reaction of aroyl-substituted benzyl carbanions in water and DMSO.

The prompt generation of carbanions II and III within the duration of the nanosecond laser pulse provides a way of evaluating absolute rate constants for their two decay pathways, protonation and cyclization, the latter resulting from an intramolecular nucleophilic carbanion displacement of iodide tethered at the end of the lateral alkyl chain. Absolute rate constants are given for both carbanions (II and III) and show that the intra-S(N)2 reaction is favored in aprotic media, such as dimethyl sulfoxide (DMSO), while protonation is the dominant reaction in basic aqueous media.

Structure−Reactivity Relationships in Radical Reactions: A Novel Method for the Simultaneous Determination of Absolute Rate Constants and Structural Features

Analysis of the ESR line width in time-resolved (TR-ESR) experiments is shown to be a suitable tool for the measurement of addition constants of phosphinoyl and substituted benzoyl radicals. Compared with kinetic investigations, which make use of the resonance intensity or integral, observation of the line width as a function of monomer concentration has the advantage that the experimental parameter is not affected by spin-polarization processes and, therefore, a lengthy determination of these polarization parameters is avoided. The resulting addition constants are discussed with respect to the experimental hyperfine coupling constants and the geometry of the radicals. TR-ESR experiments simultaneously provide rate constants and ESR parameters and allow structure−reactivity relationships to be established.

Absolute Rate Constants for the β-Scission and Hydrogen Abstraction Reactions of the tert-Butoxyl Radical and for Several Radical Rearrangements: Evaluating Delayed Radical Formations by Time-Resolved Electron Spin Resonance

An analysis is presented for the determination of absolute rate constants for radical transformation reactions in complex reaction systems by time-resolved ESR during intermittent photochemical radical production. The technique is applied to obtain absolute rate constants in large temperature ranges for the β-scission of the tert-butoxyl radical in various solvents and its hydrogen abstraction from cyclohexane, cyclopentane, tert-butylbenzene, and anisole. Kinetic studies of the neophyl and the 2-methyloxiran-2-yl rearrangements and the decarboxylation of the tert-butoxycarbonyl radical also demonstrate the versatility of the method. Further, rate data are given for the addition of the methyl radical to benzene and fluorobenzene, as well as for the hydrogen abstraction of methyl from di-tert-butyl peroxide.

Comments on the determination of absolute rate constants in anionic polymerization

Claims have recently been made that absolute rate constants for chain propagation of the unassociated active centers can be made in systems where a high degree of association is present. Anionic polymerization of styrene in nonpolar solvents with lithium as counterion is a typical case. The conditions required to obtain these constants (and the associated aggregate dissociation constants) are described using data from styrene polymerization with lithium and potassium as counterions and data from o-methoxystyrene polymerization. The conclusion reached must be that the kp and Kds values obtained for styrene with counterion lithium cannot be obtained from existing literature data and are simply artifacts of the computer analysis. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1065–1068, 1998

Comments on the determination of absolute rate constants in anionic polymerization

Claims have recently been made that absolute rate constants for chain propagation of the unassociated active centers can be made in systems where a high degree of association is present. Anionic polymerization of styrene in nonpolar solvents with lithium as counterion is a typical case. The conditions required to obtain these constants (and the associated aggregate dissociation constants) are described using data from styrene polymerization with lithium and potassium as counterions and data from o-methoxystyrene polymerization. The conclusion reached must be that the kp and Kds values obtained for styrene with counterion lithium cannot be obtained from existing literature data and are simply artifacts of the computer analysis. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1065–1068, 1998

ChemInform Abstract: Determination of Absolute Rate Constants for the Reversible Hydrogen‐ Atom Transfer Between Thiyl Radicals and Alcohols or Ethers.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Determination of absolute rate constants for the reversible hydrogen-atom transfer between thiyl radicals and alcohols or ethers

Absolute rate constants have been determined for the reversible hydrogen-transfer process R˙+ RSH ⇄ RH + RS˙ by pulse radiolysis, mainly through direct observation of the RS˙ radical formation kinetics in water–RH (1 : 1, v/v) mixtures. The thiols investigated were penicillamine and glutathione; the RH hydrogen donors were methanol, ethanol, propan-1-ol, propan-2-ol, ethylene glycol, tetrahydrofuran and 1,4-dioxane with the abstracted hydrogen being located α to the hydroxy or alkoxy function. Rate constants for the forward reaction of the above equilibrium (in radiation biology referred to as ‘repair’ reaction) were typically of the order of 107– 108 dm3 mol–1 s–1 while hydrogen abstraction from RH by thiyl radicals (reverse process) occurred with rate constants of the order of 103– 104 dm3 mol–1 s–1. This yields equilibrium constants of the order of 104. Based on these data, standard reduction potentials could be evaluated for the R′R″C˙OH/H+//R′R″CHOH, R′R″CO/H+//R′R″C˙(OH) and R′R″CO//R′R′C˙O– couples from methanol, ethanol and propan-2-ol. Effective hydrogen-atom abstraction by RS˙ required activation by neighbouring groups of the C—H bond to be cleaved in RH. No such process was observed for the RS˙ reaction with —CH3 groups, e.g. in 2-methylpropan-2-ol. Several halogenated hydrocarbons, including some anaesthetics (e.g. halothane) and Fe(CN)63– have been tested with respect to their ability to disturb the (CH3)2C˙OH + RSH ⇄(CH3)2CHOH + RS˙ equilibrium through an irreversible electron-transfer reaction with the reducing α-hydroxyl radical, thereby drawing the equilibrium to the lefthand side. The respective efficiencies are found to be related to the electronegativities of the electron acceptors. The results are briefly discussed in terms of their biological relevance.

Laser flash photolysis determination of absolute rate constants for reactions of bromine atoms in solution

The photodecomposition of vicinal dibromides at 266 nm produces bromine atoms with a quantum yield of ∼2.0. This results from an efficient primary photocleavage of a C-Br bond, followed by rapid elimination of a second bromine atom from radicalsofthe type RCH-CH 2 Br. This cleavage occurs with a lifetime of <20 ns at room temperature. Bromine atoms react with bromine ions with a rate constant of 1.6×10 10 M -1 s -1 to yield Br 2 .- , an easily detectable and long-lived radical ion. This reaction can be used as a probe in order to determine absolute rate constants for other reactions of bromine atoms

Determination of absolute rate constants for radical polymerization of dialkyl itaconates with various ester groups by electron spin resonance spectroscopy

It was found that the polymerization rate of dialkyl itaconates (DRI) with various ester groups initiated by dimethyl 2,2'-azobis(isobutyrate) in benzene at 60°C depended mainly on the structure of the ester substituents. Propagation rate constants (k p ) for all DRIs were determined to be much smaller (0.2-3.9 M -1 s -1 ) than those for ordinary vinyl monomers and were found to decrease as the ester substituents of the DRIs became bulkier. Termination rate constants for all DRIs were also determined to be smaller than those for ordinary vinyl monomers and were srevealed to be lowered with increasing bulkiness of the ester substituents

Application of time-resolved infrared spectroscopy to the determination of absolute rate constants for chlorine atom + ethane and chlorine atom + chloroethane

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTApplication of time-resolved infrared spectroscopy to the determination of absolute rate constants for chlorine atom + ethane and chlorine atom + chloroethaneE. W. Kaiser, L. Rimai, E. Schwab, and E. C. LimCite this: J. Phys. Chem. 1992, 96, 1, 303–306Publication Date (Print):January 1, 1992Publication History Published online1 May 2002Published inissue 1 January 1992https://doi.org/10.1021/j100180a057RIGHTS & PERMISSIONSArticle Views68Altmetric-Citations21LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (499 KB) Get e-Alerts

Determination of absolute rate constants for radical polymerization of diisopropyl fumarate based on a quantitative scavenge of propagating radical

During polymerization of diisopropyl fumarate (DiPF), the propagating poly(DiPF) radical was scavenged quantitatively by a stable free radical, because of its long-lived nature and high steady-state concentration. By addition of a known amount of 1,3,5-triphenylverdazyl to the polymerization mixture, the concentration of poly(DiPF) radical was determined to be 10 −5−10 −4 mol dm −3, which allowed the evaluation of the propagation rate constant ( k p) from the overall rate of polymerization: k p = 0.31 ± 0.07 dm 3 mol −1 s −1 at 30°C. The lifetime of the polymer radical was too long for a rotating sector experiment. The rate constant for bimolecular termination of the radical ( k t) was calculated from the decay of poly(DiPF) radical followed by the scavenger method: k t = 0.84 dm 3 mol −1 s −1. These rate constants, which are exceptionally small, are accounted for by the steric factor of the ester alkyl group accumulated along the polymer chain and extremely slow diffusion of the polymer radical. Because the termination rate determined is too slow to interpret the molecular weight of the polymer formed, primary radical termination is to be considered as an additional termination process in the actual polymerization.

Determination of absolute rate constants for elementary reactions in radical polymerization of diethyl fumarate

Les constantes de vitesse de propagation et de terminaison a 30 o C sont evaluees respectivement a 0,015 et 164 L/mole•sec par la methode du secteur tournant. La duree de vie du radical et la concentration en regime stationnaire du radical de propagation sont evaluees respectivement a 166 s et 3,6•10 −5 mole/l dans les conditions etudiees

Initiation in hydrocarbon autoxidation at elevated temperatures

AbstractKinetic and mechanistic investigations of liquid phase autoxidation of hexadecane at 120 to 190°C have shown that, in the early stages of oxidation, the initiation process involves homolytic decomposition of hydroperoxides. The values of a composite first order rate constant for this decomposition, 𝓀1, have been determined in stirred flow reactor experiments from the rate of formation of termination products and in batch reactor studies using inhibitor methods. Arrhenius parameters derived from these studies (log (A/s−1) = 8.5 and Ea = 26 kcal mol−1) allow calculation of 𝓀1 as a function of temperature. This permits determination of the rate of initiation as a function of hydroperoxide concentration and facilitates determination of absolute rate constants for other oxidation reactions occurring in this system.

Reversible H-atom abstraction from alcohols by thiyl radicals: determination of absolute rate constants by pulse radiolysis.

Penicillamine thiyl radicals, PenS., are shown to abstract hydrogen atoms from 2-propanol and to establish an equilibrium PenS. + (CH3)2 CHOH in equilibrium PenSH + (CH3)2 COH. The rate constants for the forward and back reaction have been determined to (1.4 +/- 0.3) x 10(4) and (1.2 +/- 0.3) x 10(8) M-1 s-1, respectively, by means of pulse radiolysis. The data have been obtained from various independent methods which include direct measurements and competitive schemes involving irreversible interception of the alcohol radical by electron acceptors (e.g. CCl4, PNAP) and/or the thiyl radical by antioxidants (e.g. alpha-tocopherol). The results demonstrate that the reaction of carbon-centered radicals with thiols, in radiation biology commonly known as "repair" reaction, may be reversed and thus imply the possibility of thiyl radical induced biological damage.