Showing papers on "Reaction rate constant published in 1992"

Peroxynitrite, a cloaked oxidant formed by nitric oxide and superoxide.

Abstract: Peroxynitrite [oxoperoxonitrate(1-), ONOO-] may be formed in vivo from superoxide and nitric oxide. The anion is stable, but the acid (pKa = 6.8) decays to nitrate with a rate of 1.3 s-1 at 25 degrees C. The experimental activation parameters of this process are delta H++ = +18 +/- 1 kcal/mol, delta S++ = +3 +/- 2 cal/(mol.K), and delta G++ = +17 +/- 1 kcal/mol. Peroxynitrite (or its protonated form) oxidizes some compounds such as thiols and thioethers in a biomolecular reaction. The reactions with glutathione and cysteine have activation enthalpies of 10.9 and 9.7 kcal/mol, respectively, which are lower than that of the isomerization reaction. Peroxynitrite reacts with other compounds such as dimethyl sulfoxide and deoxyribose in a unimolecular reaction for which the activation of peroxynitrite is rate-limiting. In theory, activation could involve (1) heterolysis to OH- and NO2+ (delta rxn Gzero' = 13 kcal/mol at pH 7) or (2) homolysis to .OH and .NO2 (delta rxn Gzero = 21 kcal/mol), and these processes also could be involved in the isomerization to nitrate. However, thermodynamic and kinetic considerations indicate that neither process is feasible, although binding to metal ions may reduce the large activation energy associated with heterolysis. An intermediate closely related to the transition state for isomerization of ONOOH to HONO2 may be the strongly oxidizing intermediate responsible for hydroxyl radical-like oxidations mediated by ONOOH. Thus, peroxynitrite reacts with different compounds by at least two distinct mechanisms, and the hydroxyl radical is not involved in either.

Hydroxyl radical formation in aqueous reactions (pH 3-8) of iron(II) with hydrogen peroxide : the photo-fenton reaction

Abstract: Kinetics studies of the indirect photooxidation of trace hydroxyl radical ( . OH) probes in aqueous solutions were used to evaluate the nature and formation efficiency of the transient oxidants that are generated when hydrogen peroxide reacts with Fe(II) that is produced from photoreduction of Fe(III) (λ=436 nm)

Rate constants for reaction of hydroxyl radicals with several drinking water contaminants

Abstract: Rate constants for reactions of the hydroxyl radical with 25 potential organic drinking water contaminants, including solvents, haloalkanes, esters, aromatics, and pesticides (for example, aldicarb, atrazine, 1,2-dibromo-3-chloropropane, endrin, glyphosate, haloforms, lindane, picloram, etc.), have been measured in mater using relative rate methods. A variety of HO . -generating techniques mere used, including ozone decomposition, Fenton's reaction, and a convenient new method employing photo-Fenton's chemistry. In addition, rate constants for 19 other compounds mere estimated using structure-activity relationships

Kinetics of the hydrolysis and condensation of organofunctional alkoxysilanes: a review.

Abstract: A review of the literature is presented for the hydrolysis of alkoxysilane esters and for the condensation of silanols in solution or with surfaces. Studies using mono-, di-, and trifunctional silane esters and silanols with different alkyl substituents are used to discuss the steric and electronic effects of alkyl substitution on the reaction rates and kinetics. The influences of acids, bases, pH, solvent, and temperature on the reaction kinetics are examined. Using these rate data, Taft equations and Bronsted plots are constructed and then used to discuss the mechanisms for acid and base-catalyzed hydrolysis of silane esters and condensation of silanols. Practical implications for using organofunctional silane esters and silanols in industrial applications are presented.

A kinetic model for the oxidation of toluene near 1200 K

Abstract: An improved kinetic model for the high-temperature oxidation of toluene has been developed using previously established reaction mechanisms for benzene and toluene. The model is compared to benzene and toluene flow reactor experiments near 1100 and 1200 K, respectively. Fuel decay rates and many intermediate species profiles are reproduced successfully for both lean and rich equivalence ratios. A linear sensitivity analysis indicated that the reaction mechanism was most sensitive to the rate constant of C{sub 6}H{sub 5}CH{sub 3} + O{sub 2} {r_arrow} C{sub 6}H{sub 5}CH{sub 2} + HO{sub 2} (71). A value of k{sub 71} = 3.0 x 10{sup 14} exp(-20700/T) cm/mol/s was found to fit the experimental data best. The model revealed that the presence of resonantly stable radical such as benzyl and phenoxy can inhibit the reaction rate of the fuel by removing H atoms from the system. Specific shortcomings of the model are also discussed. 70 refs., 10 figs., 4 tabs.

Kinetics of nitroxide radical trapping. 2. Structural effects

Abstract: Laser flash photolysis and kinetic competition product studies have demonstrated that in isooctane at ambient temperatures the rate constant for coupling of carbon-centered radicals with persistent nitroxides, k T , depends upon the degree of steric hindrance to coupling and upon the extent of resonance stabilization of the carbon radical. Sterically induced reductions in the magnitude of k T are observed for changes in both the structure of the nitroxide and the structure of the carbon radical. Thus, for any particular carbon radical k T is largest for the Bredt's rule protected nitroxides, 9-azabicyclo[3.3.1]nonane-N-oxyl (ABNO) and nortropane-N-oxyl, while for the usual' di-tert-alkyl nitroxides k T decreases along the series, 1,1,3,3-tetramethylisoidoline-2-oxyl ≥2,2,5,5-tetramethylpiperidin-1-oxyl (Tempo)>di-tert-butyl nitroxide, i.e.,k T decreases on going from a five-membered ring to a six-membered ring to a noncyclic structure

Electron transfer in bis-porphyrin donor-acceptor compounds with polyphenylene spacers shows a weak distance dependence

Abstract: A series of phenylene-bridged bis-porphyrin adducts have been synthesized, containing one, two, or three phenyl bridges. Complete synthetic details are provided. For studies of photochemical electron transfer, mixed metals were incorporated, with zinc in one porphyrin macrocycle and Fe III (bis-imidazole) in the other macrocycle. When photoexcited, an electron is transferred from Zn to Fe III

Kinetics and thermodynamics of triple-helix formation: effects of ionic strength and mismatches.

Abstract: Thermodynamic and kinetic parameters for the triplex-forming reactions between a homopurine-homopyrimidine 22-base-pair duplex (sequence of the purine strand: 5'd[AAAGGAGGAGAAGAAGAAAAAA]3') and the four 22-dN third strands (22 dN: 5'd[TTTCCTCCTCTNCTTCTTTTTT]3', where N = A, C, T, or G) were determined from thermal denaturation and renaturation UV absorbance profiles. Cooling and heating curves were not superimposable and thus allowed us to determine the rate constants of association (k(on)) and dissociation (k(off)) as a function of temperature, assuming a two-state model analogous to that developed for duplex-forming reactions. Experiments were performed in 10 mM cacodylate buffer (pH 6.8) in the presence of NaCl concentrations ranging from 20 to 300 mM. Within experimental accuracy, the main results are the following: (i) The rate constants k(on) and k(off) result in linear Arrhenius plots, consistent with the prediction of two-state association and dissociation (ii) k(on) is independent of the nature of the base N located in the center of the third strand. (iii) k(on) strongly decreases when the NaCl concentration is decreased. (iv) The activation energy, E(on), is always negative and becomes more negative when the NaCl concentration is decreased. (v) k(off) is independent of NaCl concentration but depends on the base N, with its magnitude following the order C greater than G greater than A much greater than T. (vi) The activation energy, E(off), is independent of the base N. All these results are discussed in the light of a nucleation-zipping model similar to that developed for the duplex-coil transitions [Craig, M. E., Crothers, D. M., & Doty, P. (1971) J. Mol. Biol. 62, 383-401; Porschke, D., Eigen, M. (1971) J. Mol. Biol. 62, 361-381].

Oxidation of ferrous ions by ozone in acidic solutions

Abstract: The oxidation of ferrous ions by ozone in acidic solutions of pD 0-2 was studied using a stopped-flow spectro-photometer. The reaction can be characterized as an oxygen atom transfer from O 3 to Fe 2+ . An intermediate product assigned to be the ferryl ion, FeO 2+ , was found and its UV-vis spectrum measured.

Atmospheric photooxidation of isoprene part I: The hydroxyl radical and ground state atomic oxygen reactions

Abstract: The OH reaction with isoprene is studied. Methyl nitrite photolysis experiments were carried out in an outdoor smog chamber in an attempt to identify as completely as possible OH-isoprene product distribution. Emphasis was placed on identification and quantification of oxygenated products. A Tenax-based cryo-trap thermal desorber used to trap, concentrate, and dry chamber samples for identification on a GC/MS is described. Analysis of the products revealed that O(^3P) can form in reaction systems designed to study OH reactions that include high concentrations of NO, and consequently NO_2, hence, this reaction is also examined. The yields of methacrolein and methyl vinyl ketone are determined as 25 ± 3 and 35.5 ± 4%, respectively, with an additional 4 ± 2% as 3-methyl furan, totaling 65 ± 4%. These results, combined with those of previous studies allow 80% of isoprene's products to be explicitly identified, and the general structure of the remaining products to be ascertained. The O(^3P) reaction produces 84 ± 8% epoxides, and 8 ± 3% species which result in production of HO_2, and subsequently, OH. A heretofore unidentified product of the O(^3P) reaction, 2-methyl 2-butenal, is identified. The rate constant of the NO_2-isoprene reaction is measured.

Reversible triplet-triplet energy transfer within a covalently linked bichromophoric molecule

Abstract: The localized photoexcitation of a Ru(II) polypyridine complex covalently attached by a flexible link to pyrene results in a rapid intramolecular transfer of triplet energy with a forward rate constant of (1.3 {+-} 0.3) x 10{sup 8} s{sup -1} in deaerated methanol solution. On account of the energetic proximity of the states, reverse transfer occurs with a rate constant lower by a factor of 18{+-} 3. This equilibiration is rapid compared with the relaxation processes that are deactivating the system, which have a common lifetime of 11.2 {+-}4 {mu}s. These results may be the first direct observation of an equilibrium being reached in an intramolecular triplet energy transfer reaction. 17 refs., 4 fig.

Pathways of proton release in the bacteriorhodopsin photocycle

Abstract: The pH dependencies of the rate constants in the photocycles of recombinant D96N and D115N/D96N bacteriorhodopsins were determined from time-resolved difference spectra between 70 ns and 420 ms after photoexcitation. The results were consistent with the model suggested earlier for proteins containing D96N substitution: BR hv----K----L----M1----M2----BR. Only the M2----M1 back-reaction was pH-dependent: its rate increased with increasing [H+] between pH 5 and 8. We conclude from quantitative analysis of this pH dependency that its reverse, the M1----M2 reaction, is linked to the release of a proton from a group with a pKa = 5.8. This suggests a model for wild-type bacteriorhodopsin in which at pH greater than 5.8 the transported proton is released on the extracellular side from this as yet unknown group and on the 100-microseconds time scale, but at pH less than 5.8, the proton release occurs from another residue and later in the photocycle most likely directly from D85 during the O----BR reaction. We postulate, on the other hand, that proton uptake on the cytoplasmic side will be by D96 and during the N----O reaction regardless of pH. The proton kinetics as measured with indicator dyes confirmed the unique prediction of this model: at pH greater than 6, proton release preceded proton uptake, but at pH less than 6, the release was delayed until after the uptake. The results indicated further that the overall M1----M2 reaction includes a second kinetic step in addition to proton release; this is probably the earlier postulated extracellular-to-cytoplasmic reorientation switch in the proton pump.

Borohydride Oxidation at a Gold Electrode

Abstract: The multistage process of borohydride oxidation in an 8 e - reaction to borate at a Au electrode has been studied by means of fast-scan cyclic voltammetry (CV) and scanning electrochemical microscopy (SECM). The total irreversibility of this process observed previously is shown to be a result of the presence of very unstable intermediates. CV measurements showed that at least two stages of the process are quasi-reversible, and the presence of a coupled homogeneous chemical reaction was proved by SECM. The rate constant for this reaction as well as the electrochemical kinetic parameters for the first stage of oxidation are evaluated using digital simulation

Kinetics and thermochemistry of R + hydrogen bromide .dblarw. RH + bromine atom reactions: determinations of the heat of formation of ethyl, isopropyl, sec-butyl and tert-butyl radicals

Abstract: The reactions of alkyl radicals (R = CH{sub 3}, C{sub 2}H{sub 5}, i-C{sub 3}H{sub 7}, and t-C{sub 4}H{sub 9}) with HBr have been studied by excimer laser flash photolysis coupled with photoionization mass spectrometry. R + HBr rate constants are approximately a factor of 2 higher than previously reported. The source of this disparity is explained. The kinetics of reverse reactions, Br + RH (R = C{sub 2}H{sub 6}, C{sub 3}H{sub 8}, n-C{sub 4}H{sub 10}, i-C{sub 4}H{sub 10}), have also been investigated using laser flash photolysis/resonance fluorescence methods. These results, combined with previously obtained kinetic information, were used in second- and third-law thermochemical calculations to obtain accurate determinations of the heats of formation of the C{sub 2}-C{sub 4} alkyl radicals involved. Second- and third-law determinations agreed extremely closely (differences were under 1.3 kJ mol{sup {minus}1}). The heats of formation of the radicals thus obtained are in excellent agreement with values obtained from studies of dissociation/association equilibria, within 2.6 kJ mol{sup {minus}1}. Recommended alkyl-radical heats of formation (with uncertainties) at 298 K are provided that are based on an assessment of all the results of the current study and a review of other recent determination (kJ mol{sup {minus}1}): C{sub 2}H{sub 5},more » 121.0 {+-} 1.5; i-C{sub 3}H{sub 7}, 90.0 {+-} 1.7; sec-C{sub 4}H{sub 9}, 67.5 {+-}2.2; t-C{sub 4}H{sub 9}, 51.3 {+-} 1.8. Accurate determination of carbon-hydrogen bond enthalpies (298 K) are provided that are based on these heats of formation (kJ mol{sup {minus}1}): primary C-H in C{sub 2}H{sub 6} (422.8 {+-} 1.5); secondary C-H in C{sub 3}H{sub 8} (412.7 {+-} 1.7) and n-C{sub 4}H{sub 10} (411.1 {+-} 2.2); tertiary C-H in i-C{sub 4}H{sub 10} (403.5 {+-} 1.8). 44 refs., 4 figs., 8 tabs.« less

Atmospheric photooxidation of isoprene part II: The ozone‐isoprene reaction

Abstract: A series of experiments have been performed to study the ozone-isoprene reaction in a smog chamber by adding externally produced O_3 to the hydrocarbon in the dark. A chemical tracer, methyl cyclohexane, was added to probe the OH formation in the system. O(^3P) formation was also examined using the known distribution of products that are unique to the O(^3P)-isoprene reaction (part I). The results provide clear evidence that both OH and O(^3P) are produced by the O_3-isoprene reaction directly in large quantities; about 0.68 ± 0.15 and 0.45 ± 0.20 per O_3-isoprene reaction, respectively. These additional radicals severely complicate the analysis of the O_3 reaction, hence, computer kinetic modeling was necessary to ascertain the products of the O_3 reaction itself, corrected for OH and O(^3P) reactions. The product distribution, which differs dramatically from that published previously, is: 67 ± 9% methacrolein, 26 ± 6% methyl vinyl ketone, and 7 ± 3% propene, accounting for 100 ± 10% of the reacted isoprene. Applicability of these results to the gas-phase O_3 reaction with other unsaturated hydrocarbons is briefly discussed.

Kinetics of the growth of spinel, MgAl2O4, on alumina particulate in aluminum alloys containing magnesium

Abstract: The growth of spinel in melts of alumina-reinforced composites not only consumes the magnesium required for age hardening, but also acts to increase the viscosity and thus adversely affects the castability of the material. The kinetics of this growth have been studied in molten Al-1 pct Mg alloys in the temperature range of 948 to 1073 K. The results are shown to fit an equation of the form ln(1−α = k(t0 −t) describing deceleratory growth on fine particles, where α is a dimensionless reaction parameter. The rate constant,k, fits an Arrhenius equation giving an activation energy of 103 ± 7 kJ and a time constant of 50 s−1. The incubation time,t0, which is about 2000 seconds forT 1000 K.

Effect of calcium and phytic acid on the activation of trypsinogen and the stability of trypsin

Abstract: The kinetics of the activation of trypsinogen to ttrypsin has been investigated under different combinations of calcium (II) and phytic acid. The data describing the integrated process of active trypsin generation, inert protein generation, and active trypsin stability have been analysed in terms of 2 rate equations containing 4 rate constants. The effect of phytic acid on these rate constants at 30°C and either pH 6.0 or pH 8.1 is given

Kinetics of the gas-phase reactions of chloride anion, Cl- with CH3Br and CD3Br: experimental evidence for nonstatistical behavior?

Abstract: The reactions of Cl - with CH 3 Br and CD 3 Br have been studied as a function of ion-neutral average center-of-mass kinetic energy, (KE cm ), at several temperatures. The reactions are inefficient, proceeding at only a few percent of the collision rate. Both increasing temperature and increasing kinetic energy are found to decrease the rate constant as approximately T -0.8 or (KE cm ) -0.8 . The rate constants were found to be independent of pressure at 300 K