Showing papers on "Reaction rate constant published in 1987"

Photooxidation of organic impurities in water using thin films of titanium dioxide

Abstract: Results of the destruction of organic solutes in a simple, thin film TiO2 reactor are described. The reactor was illuminated with a 20-W blacklight UV fluorescent tube and the aqueous stream containing the organic solute flowed past the stationary photocatalyst. In the continuous recirculation mode, the destructive rate of each solute obeyed approximately first-order kinetics. The reaction rate constant decreased with increasing solute concentration. The times for 50% destruction of 500 cmT of 10 M solutions of each of the solutes salicylic acid, phenol, 2-chlorophenol, 4-chlorophenol, benzoic acid, 2-naphthol, naphthalene, and fluorescein were 7.1, 7.2, 8.2, 8.7, 6.9, 8.5, 4.3, and 6.4 min, respectively. It was found that the observed apparent first-order dependence and the change in rate constant with concentration could by explained in terms of the integrated form of the Langmuir adsorption isotherm. A marked dependence of the destruction rate on flow rate was observed and an expression developed which allows the calculation of the destruction curve with good precision at any solute concentration and flow rate. A corresponding curve was observed for the formation of carbon dioxide from salicylic acid solution. It was shown that hydroxylation of the aromatic ring to give salicylic acid is a minormore » reaction path in the destruction of benzoic acid. The maximum quantum yield for the destruction of salicyclic acid at 25C was found to be 0.022. The activation energy for the photooxidation of salicyclic acid was determined to be 11.0 +/- 0.8 kJ mol .« less

A structure-activity relationship for the estimation of rate constants for the gas-phase reactions of OH radicals with organic compounds

Abstract: A previous technique for the calculation of rate constants for the gas-phase reactions of the OH radical with organic compounds has been updated and extended to include sulfur- and nitrogen-containing compounds. The overall OH radical reaction rate constants are separated into individual processes involving (a) H-atom abstraction from CH and OH bonds in saturated organics, (b) OH radical addition to >CC NH, >N, SH, and S groups. During its development, this estimation technique has been tested against the available database, and only for 18 out of a total of ca. 300 organic compounds do the calculated and experimental room temperature rate constants disagree by more than a factor of 2. This suggests that this technique has utility in estimating OH radical reaction rate constants at room temperature and atmospheric pressure of air, and hence atmospheric lifetimes due to OH radical reaction, for organic compounds for which experimental data are not available. In addition, OH radical reaction rate constants can be estimated over the temperature range ca. 250–1000 K for those organic compounds which react via H-atom abstraction from CH and OH bonds, and over the temperature range ca. 250–500 K for compounds containing >CC< bond systems.

The kinetics of solubilisate exchange between water droplets of a water-in-oil microemulsion

Abstract: Exchange rates of aqueous solubilisates between water droplets in a water-in-oil microemulsion stabilised by sodium bis(2-ethyl-hexyl) sulphosuccinate (AOT) have been measured as a function of droplet size, temperature and the chain length of the oil The effects of additives (eg alcohols) on the exchange kinetics have also been investigated Exchange rates were measured using very fast chemical reactions as indicators for exchange Three types of reaction were investigated: proton transfer, metal–ligand complexation and electron transfer Similar exchange rates were found for all three reactions The indicator reactions involve the exchange of reactant ions of differing size and charge type; exchange rates were, however, independent of the ion transferred, but dependent on droplet size and temperature For AOT as dispersant, exchange occurs with a second-order rate constant of 106–108 dm3 mol–1 s–1, two to four orders of magnitude slower than the droplet encounter rate as predicted from simple diffusion theory The apparent activation enthalpy is high (and increases with droplet size) but is largely compensated by a positive activation entropy Exchange, on balance, is a relatively facile process which typically takes place on a millisecond timescale (depending on the droplet concentration)The exchange mechanism involves transient water droplet coalescence and separation This is the dynamic process whereby the equilibrium properties of the microemulsion, eg droplet size and polydispersity, are maintained There is a correlation between the exchange rate constants and the stability of the single-phase microemulsion This relationship between the kinetic and equilibrium properties is discussed in terms of the ‘natural curvature’ of the surfactant interface and inter-droplet interactions

The laser-flash-initiated polymerization as a tool of evaluating (individual) kinetic constants of free-radical polymerization, 2†. The direct determination of the rate of constant of chain propagation‡

Abstract: When a polymerizable system is subjected to periodic light flashes, which induce the formation of primary radicals, a pseudostationary state is established which is characterized by a periodic profile of the (polymer) radical concentration. Within such a period of length t0 the radical concentration will decay according to a second-order rate law. At the end of this period the radicals, which have escaped termination up to this moment, have propagated up to a chain length L0 = t0·kp·cM, kp representing the propagation rate constant and cM the monomer amount concentration. When the next flash arrives these radicals are opposed to a strongly increased overall concentration of radicals which leads to an enhanced probability for their termination. As a consequence the formation of dead polymer molecules with a chain length close to L0 is favoured. The chain-length distribution of polystyrene prepared under such pseudostationary conditions, which was evaluated by gel permeation chromatography, in fact exhibits such a peak. The analysis of the theoretical distribution curves, derived in this communication, reveals that it is easily possible to correlate this peak to L0, independently of the mode of termination (disproportionation or combination). Thus, a method of evaluating kp is derived without any reference to the termination rate constant kt and largely independent of all features which usually cause problems in the evaluation of kp and kt (such as primary radical termination etc.). The experimental results agree fairly well with the data reported in literature, especially with those obtained from the number of particles and the rate of polymerization in emulsion systems.

Ferrous-salt-promoted damage to deoxyribose and benzoate. The increased effectiveness of hydroxyl-radical scavengers in the presence of EDTA.

Abstract: Hydroxyl radicals (OH.) in free solution react with scavengers at rates predictable from their known second-order rate constants. However, when OH. radicals are produced in biological systems by metal-ion-dependent Fenton-type reactions scavengers do not always appear to conform to these established rate constants. The detector molecules deoxyribose and benzoate were used to study damage by OH. involving a hydrogen-abstraction reaction and an aromatic hydroxylation. In the presence of EDTA the rate constant for the reaction of scavengers with OH. was generally higher than in the absence of EDTA. This radiomimetic effect of EDTA can be explained by the removal of iron from the detector molecule, where it brings about a site-specific reaction, by EDTA allowing more OH. radicals to escape into free solution to react with added scavengers. The deoxyribose assay, although chemically complex, in the presence of EDTA appears to give a simple and cheap method of obtaining rate constants for OH. reactions that compare well with those obtained by using pulse radiolysis.

Calculations of parabolic reaction rate constants

Abstract: The oxidation kinetics of only a very limited number of pure metals or binary alloys can be described by the simplest parabolic law, Δm2=kpt, Thus for a transient period of faster kinetics, the steady state parabolic law is given by (Δm−Δmi)2 = kp(t−ti) when the initial weight gain Δmi does not contribute to steady state rate control. In such a case, a plot of the kinetics data as Δm vs t1/2 is inherently superior to the Δm2 vs t plot for an accurate determination of the steady state parabolic rate constant, as well as for the analysis of the transient, faster kinetics.

Hydroxide decomposition of dimethylsulfoniopropionate to form dimethylsulfide

Abstract: The kinetics of DMS production resulting from reaction of OH(-) with DMSP were investigated as a function of hydroxide concentration and temperature. The reaction was first-order with respect to DMSP and OH(-). The second order rate constant at 20+/-1 C is 0.0044/M/sec. The activation energy for this reaction is 14.4 kcal/mode. The investigation indicates that the rate of reaction of DMSP with OH(-) is very slow at the pH of seawater, suggesting that DMSP, which may be a major precursor of DMS in seawater, decomposes in the ocean by other mechanisms. A bacterium which produces DMS from DMSP quantitatively at rates many orders of magnitude higher than indicated by OH(-1) decomposition has been cultured, suggesting that enzymatic processes accelerate the production of DMS from DMSP in seawater.

Development of Positive Photoresists

Abstract: A mechanism for the development of positive optical photoresists is proposed, leading to the derivation of a development rate equation. This rate equation compares favorably with experimentally determined development rates. Typical values of the rate constants involved are given. Empirical models are given for the surface induction and substrate adhesion effects. An overall positive resist processing model requires a mathematical representation of the development process. Previous attempts have taken the form of empirical fits to development rate data as a function of exposure (1, 2). The model formulated below begins on a more fundamental level, with a postulated reaction mechanism which then leads to a development rate equation. The rate constants involved can be determined by comparison with experimental data. Deviations from the expected development rates have been reported under certain conditions at the surface of the resist and near the resist-substrate interface. These effects, called the surface induction and substrate adhesion effects, respectively, can be related empirically tothe expected development rate, i.e., to the bulk development rate as predicted by a kinetic model. Bulk Development Model In order to derive an analytical development rate expression, a kinetic model of the development process will be used. This approach involves proposing a reasonable mechaffism for the development reaction and then applying standard kinetics to this mechanism in order to derive a rate equation. We shall assume that the development of a diazo-type positive photoresist involves three processes: diffusion of developer from the bulk solution to the surface of the resist, reaction of the developer with the resist, and diffusion of the product back into the solution. For this analysis, we shall assume that the last step, diffusion of'the dissolved resist into solution, occurs very quickly so that this step may be ignored. Let us now look at the first two steps i n the proposed mechanism. The diffusion of developer to the resist surface can be described with the simple, diffusion rate equation rD = k,~(D - Ds) [1] where rD = rate of diffusion of the developer to the resist surface,. D = bulkdeveloper concentration, Ds = developer concentration at the resist surface, and kD = rate constant. We shall now propose a mechanism for the reaction of developer'with theresist. The resist is composed of large macromolecules of resin R along with a photoactive compound M, which converts to product P upon exposure to UV light. The resin is quite soluble in the developer solution, but the presence of the PAC (photoactive compound) acts as an. inhibitor to dissolution, making the development rate very slow. The product P, however, is very solubie in developer, enhancing the dissolution rate of the resin. Let us assume that n molecules of product p react with the developer to dissolve a resin mo'lecule. The rate of the reaction is rR = - kRDsP '~ [2] where rR = the rate of reaction of the developer with the resist and kR = rate constant. From the stoichiometry of the exposure reaction P = M,, - M [3]

Kinetic studies of the reduction of aromatic AZO compounds in anaerobic sediment/water systems

Abstract: The reductive transformation of azobenzene and selected derivatives was investigated in anaerobic sediment/water systems. The azo compounds exhibited pseudo-first-order disappearance kinetics through at least three half-lives. The reduction kinetics of these compounds was studied as a function of their reduction potential and sediment/water distribution coefficient. There was no apparent correlation between the observed disappearance rate constant and reduction potential. In general, as the distribution coefficient increased, the rate of reduction decreased. Values for the pseudo-first-order rate constant for disappearance ranged from 5.11 × 10−3 min−1 for methyl red to 6.03 × 10−6 min−1 for 4,4′-dichloroazobenzene. Removal of the solid phase from the sediment/water samples gave a filtrate with little or no reactivity. Chemical sterilization of the sediment/water sample with formaldehyde and treatment with m-cresol, a dehydrogenase inhibitor, or sodium azide, a metabolic inhibitor, had little effect on the observed reduction rate constants for azobenzene, indicating an abiotic reduction process. Heat sterilization indicated that the reducing agent was heat labile. In studies with 4,4′-dimethoxyazobenzene, the observed rate constant for reduction increased with increasing sediment concentration. Based on the results of these studies, a model for the reduction process was developed that incorporates a nonreactive sorptive sink and a reactive site, both of which are associated with the sediment.

Mechanism of superoxide ion disproportionation in aprotic solvents

Abstract: The electrochemical reduction of dioxygen in dimethyl sulfoxide is investigated as a function of the addition of acids by means of double potential step chronoamperometry. Analysis of the kinetics as a function of dioxygen and acid concentrations and of the measurement time in a series of acids involving five phenols and nitromethane allowed the determination of the reaction mechanism and of the characteristic rate constants. The HO/sub 2/ radical resulting from the neutralization of the initial superoxide anion radical, O/sub 2/, undergoes an electron-transfer reduction by O/sub 2//sup -/ itself rather than abstracting a hydrogen atom from the solvent or exchanging an H atom with another HO/sub 2/ radical, a commonly accepted mechanism. The mechanism appears to be the same in dimethylformamide, pointing to the conclusion that disproportionation of superoxide ions follows the same reaction pathway in aprotic organic solvents and in water.

Bimolecular QRRK analysis of methyl radical reactions

Abstract: Reactions which proceed through energized adducts, including radical recombinations, insertions, and addition to unsaturates, frequently exhibit unusual kinetic behavior. The branching ratios among various product channels are often complex functions of both temperature and pressure. Four such reactions involving methyl radicals are analyzed by combining chemical activation distribution functions with QRRK methods to predict rate constants for each channel. These include three oxidation paths, CH3 + O, CH3 + O2, CH3 + OH, and the addition reaction CH3 + C2H2. These predictions are compared to experiments wherever possible; generally, the agreement is quite satisfactory. Analysis of the energetics of the various reaction channels, using parameters which are readily available, provides a convenient framework for prediction. Suggested rate constants for the various channels for the four reactions are given at three pressures, 20, 760, and 7600 Torr, for the temperature range 300–2500 K. The approach used here can easily be applied to other reactions.

Extreme solvent effects on reaction rate constants at supercritical fluid conditions

Abstract: The reaction rate constant was adjusted continuously over two orders of magnitude for the unimolecular decomposition of α-chlorobenzyl methyl ether using the supercritical fluid solvent 1, 1 diffluoroethane. Activation volumes were observed as low as −6,000 cm3/mol, which is about an order of magnitude more negative than those reported previously in the literature for a homogeneous reaction. Spectral shift (solvatochromic) data were measured for phenol blue in the same fluid in order to interpret the rate data. A method is presented to predict solvent effects on rate constants at supercritical fluid conditions.

Steady-state near-infrared detection of singlet molecular oxygen: a stern-volmer quenching experiment with sodium azide

Abstract: — A sensitive near-infrared detection system incorporating improvements to existing methodologies has been used to characterize the sodium azide quenching of the steady-state luminescence of singlet molecular oxygen at 1270 nm. Stern-Volmer plots which were linear up to 80% quenching of the 1O2 generated by rose bengal and eosin Y yielded a rate constant of 5.8 ± 0.1 times 108M−1 s−1 for the quenching of 1O2 in water, while the rate constants obtained in deuterium oxide with the same sensitizers were 6.28 times 108M−1 s−1 and 6.91 times 108M−1 s−1 respectively. A flow system minimized the effects of photobleaching of the rose bengal. With a mercury arc light source, the instrument can be used in photosensitization experiments to detect low levels of 1O2 production in aqueous media.

Absolute rate constants of alkoxyl radical reactions in aqueous solution

Abstract: The pulse radiolysis technique was used to generate the alkoxyl radical derived from tert-butyl hydroperoxide (/sup t/BuOOH) in aqueous solution. The reactions of this radical with 2,2'-azinobis(3-ethyl-6-benzothiazolinesulfonate) (ABTS) and promethazine were monitored by kinetic spectroscopy. The unimolecular decay rate constant of the tert-butoxyl radical (/sup t/BuO) was determined to be 1.4 x 10/sup 6/ s/sup -1/. On the basis of this value, the rate constants for /sup t/BuO attack on quercetin, crocin, crocetin, ascorbate, isoascorbate, trolox c, glutathione, thymidine, adenosine, guanosine, and unsaturated fatty acids were determined. In addition, the reaction of /sup t/BuO with the polyunsaturated fatty acids (PUFA) was observed by directly monitoring the formation of the fatty acid pentadienyl radicals. Interestingly, the attack of /sup t/BuO on PUFA was found to be faster by about one order of magnitude as compared to the same reaction in a nonpolar solvent.

Charge separation in carotenoporphyrin-quinone triads: synthetic, conformational, and fluorescence lifetime studies

Abstract: Carotenoid-porphyrin-quinone triad molecules undergo a photodriven two-step electron-transfer reaction which results in the generation of a high-energy charge-separated state with a lifetime on the microsecond time scale at ambient temperatures in fluid solution. These systems mimic the initial charge separation steps of photosynthesis. A series of these tripartite molecules which differ systematically in the nature of the linkages joining the porphyrin to the quinone and carotenoid moieties has been synthesized in order to investigate the effect of structure on the yield and lifetime of the charge-separated state. The time-averaged solution conformations of these molecules have been determined from porphyrin ring current induced shifts in the /sup 1/H NMR resonances of the carotenoid and quinone moieties. Studies of the triads and related molecules in dichloromethane solution using time-correlated single photon counting fluorescence lifetime techniques have yielded the rate constant for the first of the photoinitiated electron-transfer steps as a function of the linkage joining the porphyrin and the quinone. The rate constants range from 1.5 x 10/sup 8/ to 9.7 x 10/sup 9/ s/sup -1/. For most members of the series, the results are consistent with an exponential dependence of the electron-transfer rate on the experimentally determined donor-acceptor separation, with themore » exponential factor ..cap alpha.. = 0.6 A/sup -1/.« less

Specific rate constants k(E,J) and product state distributions in simple bond fission reactions. II. Application to HOOH→OH+OH

Abstract: Detailed and simplified statistical adiabatic channel calculations of specific rate constants k(E,J) and product quantum state distributions for the simple bond fission reaction HOOH→2 OH are compared with recent measurements of state‐resolved dissociation rates, product state distributions, and thermally averaged rate coefficients. A simple modification of phase space theory based on the statistical adiabatic channel model successfully predicts product state distributions and rate constants as well. Because of the amount of experimental data and theoretical analysis available, the dissociation of hydrogen peroxide is becoming a model case for simple unimolecular bond fission processes.

The gas phase reactions of hydroxyl radicals with a series of aliphatic ethers over the temperature range 240–440 K

Abstract: Absolute rate constants were determined for the gas phase reactions of OH radicals with a series of linear aliphatic ethers using the flash photolysis resonance fluorescence technique. Experiments were performed over the temperature range 240–440 K at total pressures (using Ar diluent gas) between 25–50 Torr. The kinetic data for dimethylether (k1), diethylether (k2), and dipropylether (k3) were used to derive the Arrhenius expressions and At 296 K, the measured rate constants (in units of 10−13 cm3 molecule−1 s−1) were: k1 = (24.9 ± 2.2), k2 = (136 ± 9), and k3 = (180 ± 22). Room temperature rate constants for the OH reactions with several other aliphatic ethers were also measured. These were (in the above units): di-n-butylether, (278 ± 36); di-n-pentylether, (347 ± 20); ethyleneoxide, (0.95 ± 0.05); propyleneoxide, (4.95 ± 0.52); and tetrahydrofuran, (178 ± 16). The results are discussed in terms of the mechanisms for these reactions and are compared to previous literature data.