Rate Coefficient K1 Research Articles

Sorption, Degradation, and Mineralization of Carbaryl in Soils, for Single‐Pesticide and Multiple‐Pesticide Systems

AbstractSorption, degradation, and mineralization of carbaryl (1‐naphthyl N‐methylcarbamate) alone and in mixtures with atrazine(2‐chloro‐4‐ethylamino‐6‐isopropylamino‐1,3,5‐triazine) and diuron (N′‐(3,4‐dichlorophenyl)‐N,N‐dimethylurea) were characterized for two topsoils and their respective subsoils. In general, differences in sorption and transformation between single and multiple‐pesticide systems were less than differences due to soil type and between topsoil and subsoil. For a given soil, the topsoil containing more organic carbon (OC) than the subsoil exhibited a larger value for the Freundlich sorption coefficient (Kt). Similarly, a larger value for the soil solution degradation rate coefficient (k1) was obtained from topsoil than subsoil, likely due to greater microbial activity in the topsoil. However, for all soils, sorption of carbaryl on soils considerably increased the magnitude of the degradation half‐life (t1/2) for carbaryl in either single‐pesticide or multiple‐pesticide systems. In soils that contained <10 g kg−1 OC content, the effect of multiple‐pesticide systems was to enhance the mineralization of carbaryl. This was attributed to cometabolism. This study shows that carbaryl chemodynamic parameters (Koc and t1/2 values) can be used to adequately analyze the fate of carbaryl in the environment even when carbaryl exists in a mixture with atrazine and diuron.

Rate and mechanism of the reactions of OH and Cl with 2‐methyl‐3‐buten‐2‐ol

An environmental chamber/Fourier transform infrared system was used to determine the rate coefficient k1 for the gas‐phase reaction of OH with 2‐methyl‐3‐buten‐2‐ol (MBO, (CH3)2C(OH)CH=CH2), relative to the rate of its reaction with ethylene (k2) and propylene (k3). Experiments performed at 295±1 K, in 700 torr total pressure of air, gave k1 = (6.9±1.0) × 10−11 cm3 molecule−1 s−1. At 295±1 K, the reaction of OH with MBO yielded, on a per mole basis, (52±5)% acetone, (50±5)% glycolaldehyde, and (35±4)% formaldehyde. The production of acetone from the oxidation of MBO may be of significance globally. The kinetics and mechanism of the reaction of chlorine atoms with MBO (k15) have also been studied at 700 torr total pressure of air and 295±1 K. The rate coefficient was determined using a relative rate technique, with ethane (k16), ethylene (k17), and cyclohexane (k18) as reference compounds. The value of k15 was found to be (3.3±0.4) × 10−10 cm3 molecule−1 s−1 at 295 K. The major carbon‐containing products obtained in the Cl‐atom oxidation of MBO were acetone (47±5)%, chloroacetaldehyde (53±5)%, HCOCl (<11%), and formaldehyde (6 ± 2)%.

A Modified Water Quality Model for P. Redicting BOD and DO Variations in a Shallow Polluted Channel

This study developed a new water quality model by taking into consideration the effects of flow conditions and biofilm metabolism on the BOD and DO changes in a shallow polluted channel. Experimental tests were performed in a model channel under various flow conditions. The test results showed that the organic matter deoxygenation rate coefficient (k1) varies significantly with the water velocities instead of being a constant as previously believed. The influences of organic concentration on the deoxygenation rate of biofilms was found to decrease with an increase in the Reynolds number. The present model along with a simplified version and the conventional Streeter-Phelps model were utilized to compare of experimental data obtained from test runs under various flow conditions. The test results revealed that the residual BOD and DO concentrations predicted by the present study agree very well with the measured values while the predicted values using the two other models deviate significantly from the observed data.

The role of vibrationally excited oxygen and nitrogen in the ionosphere during the undisturbed and geomagnetic storm period of 6-12 April 1990

Abstract. We present a comparison of the observed behavior of the F-region ionosphere over Millstone Hill during the geomagnetically quiet and storm periods of 6–12 April 1990 with numerical model calculations from the IZMIRAN time-dependent mathematical model of the Earth's ionosphere and plasmasphere. The major enhancement to the IZMIRAN model developed in this study is the use of a new loss rate of O+(4S) ions as a result of new high-temperature flowing afterglow measurements of the rate coefficients K1 and K2 for the reactions of O+(4S) with N2 and O2. The deviations from the Boltzmann distribution for the first five vibrational levels of O2(v) were calculated, and the present study suggests that these deviations are not significant. It was found that the difference between the non-Boltzmann and Boltzmann distribution assumptions of O2(v) and the difference between ion and neutral temperature can lead to an increase of up to about 3 or a decrease of up to about 4 of the calculated NmF2 as a result of a respective increase or a decrease in K2. The IZMIRAN model reproduces major features of the data. We found that the inclusion of vibrationally excited N2(v > 0) and O2(v > 0) in the calculations improves the agreement between the calculated NmF2 and the data on 6, 9, and 10 April. However, both the daytime and nighttime densities are reproduced by the IZMIRAN model without the vibrationally excited nitrogen and oxygen on 8 and 11 April better than the IZMIRAN model with N2(v > 0) and O2(v > 0). This could be due to possible uncertainties in model neutral temperature and densities, EUV fluxes, rate coefficients, and the flow of ionization between the ionosphere and plasmasphere, and possible horizontal divergence of the flux of ionization above the station. Our calculations show that the increase in the O+ + N2 rate factor due to N2(v > 0) produces a 5-36 decrease in the calculated daytime peak density. The increase in the O++ O2 loss rate due to vibrational-ly excited O2 produces 8-46 reductions in NmF2. The effects of vibrationally excited O2 and N2 on Ne and Te are most pronounced during the daytime.Key words. Ion chemistry and composition · Ionosphere – atmosphere interactions · Ionospheric disturbances

The role of vibrationally excited oxygen and nitrogen in the ionosphere during the undisturbed and geomagnetic storm period of 6–12 April 1990

We present a comparison of the observed behavior of the F-region ionosphere over Millstone Hill during the geomagnetically quiet and storm periods of 6–12 April 1990 with numerical model calculations from the IZMIRAN time-dependent mathematical model of the Earth's ionosphere and plasmasphere. The major enhancement to the IZMIRAN model developed in this study is the use of a new loss rate of O+(4S) ions as a result of new high-temperature flowing afterglow measurements of the rate coefficients K1 and K2 for the reactions of O+(4S) with N2 and O2. The deviations from the Boltzmann distribution for the first five vibrational levels of O2(v) were calculated, and the present study suggests that these deviations are not significant. It was found that the difference between the non-Boltzmann and Boltzmann distribution assumptions of O2(v) and the difference between ion and neutral temperature can lead to an increase of up to about 3 or a decrease of up to about 4 of the calculated NmF2 as a result of a respective increase or a decrease in K2. The IZMIRAN model reproduces major features of the data. We found that the inclusion of vibrationally excited N2(v > 0) and O2(v > 0) in the calculations improves the agreement between the calculated NmF2 and the data on 6, 9, and 10 April. However, both the daytime and nighttime densities are reproduced by the IZMIRAN model without the vibrationally excited nitrogen and oxygen on 8 and 11 April better than the IZMIRAN model with N2(v > 0) and O2(v > 0). This could be due to possible uncertainties in model neutral temperature and densities, EUV fluxes, rate coefficients, and the flow of ionization between the ionosphere and plasmasphere, and possible horizontal divergence of the flux of ionization above the station. Our calculations show that the increase in the O+ + N2 rate factor due to N2(v > 0) produces a 5-36 decrease in the calculated daytime peak density. The increase in the O++ O2 loss rate due to vibrational-ly excited O2 produces 8-46 reductions in NmF2. The effects of vibrationally excited O2 and N2 on Ne and Te are most pronounced during the daytime.Key words. Ion chemistry and composition · Ionosphere – atmosphere interactions · Ionospheric disturbances

Kinetic study of methyl tert-butyl ether (MTBE) oxidation in flames

Three premixed MTBE/H2/O2/Ar flat flames burning at low pressure (30 torr) have been investigated by molecular beam mass spectrometry for equivalence ratios ranging from 0.18 to 1.84. Profiles of stable, atomic, and radical species concentrations have been measured as well as the temperature ones. The structure of these flames has been established in order to deduce the rate coefficients of H-atom abstraction reactions (R1-R3) from MTBE by highly reactive species like H and O atoms and OH radicals. C5H12O+H→Products (R1) C5H12O+O→Products (R2) C5H12O+OH→Products (R3) The rate coefficient of reaction R1 has been deduced in the rich flame k1=3.92×1013 exp(−2830/T) cm3 mole−1 s−1 between 700 and 1000 K. Rate coefficients of reactions R2 and R3 have been determined in the lean flame k2=1.78×1014 exp(−4000/T) cm3 mole−1 s−1 between 1000 and 1300 K and in the the stoichiometric flame k3=4.1×1013 exp(−1450/T) cm3 mole−1 s−1 between 650 and 900 K, respectively. Using the deduced rate coefficients k1, k2, and k3, and the experimental mole fraction profiles of the involved species, the relative contribution of reactions (R1-R3) to the MTBE consumption has been determined in the investigated flames. The analysis of the flame structures has allowed also to evaluate the pathways leading to the formation of isobutene and acetone that are intermediate species arising from the first steps of the MTBE decay. It has been demonstrated that all MTBE conversion proceeds through isobutene formation whereas reactions leading to the formation of acetone have a weak contribution.

Antigen-antibody binding kinetics for biosensor applications

The diffusion-limited binding kinetics of antigen (or antibody) in solution to antibody (or antigen) immobilized on a biosensor surface is analyzed within a fractal framework. The fit obtained by a dual-fractal analysis is compared with that obtained from a single-fractal analysis. In some cases, the dual-fractal analysis provides an improved fit when compared with a single-fractal analysis. This was indicated by the regression analysis provided by Sigmaplot (San Rafael, CA). These examples are presented. It is of interest to note that the state of disorder (or the fractal dimension) and the binding rate coefficient both increase (or decrease, a single example is presented for this case) as the reaction progresses on the biosensor surface. For example, for the binding of monoclonal antibody MAb 49 in solution to surface-immobilized antigen, a 90.4% increase in the fractal dimension (Df1 to Df2) from 1.327 to 2.527 leads to an increase in the binding rate coefficient (k1 to k2) by a factor of 9.4 from 11.74 to 110.3. The different examples analyzed and presented together provide a means by which the antigen-antibody reactions may be better controlled by noting the magnitude of the changes in the fractal dimension and in the binding rate coefficient as the reaction progresses on the biosensor surface.

Dual-fractal analysis for antigen--antibody binding kinetics for biosensor applications.

The diffusion-limited binding kinetics of antigen (or antibody) in solution to antibody (or antigen) immobilized on a biosensor and other surfaces is analyzed within a fractal framework. Often, the binding kinetics may be described by a single-fractal analysis. In some cases, the binding curve exhibits complexities. Then, for these cases, the dual-fractal analysis provides an improved fit when compared with a single-fractal analysis. This indicates a change in the reaction mechanism on the surface. It is of interest to note that the state of disorder (or the fractal dimension) and the binding rate coefficient both increase as the reaction progresses on the biosensor surface. For example, for the binding of 10 nM insulin growth factor-1 in solution to insulin growth factor binding protein-1 immobilized on a biosensor surface, a 64% increase in the fractal dimension from 1.73 (Df1) to 2.85 (Df2) leads to an increase in the binding rate coefficient by a factor of 31.8 from 3.92 (k1) to 125 (k2). Furthermore, as the IGF-1 concentration in solution increases from 10 to 80 nM in solution, k2 and Df2 exhibit a linear increase. k1 and Df1 exhibit a linear increase with the reciprocal of the IGF-1 concentration in solution. The different examples analyzed and presented together provide a means by which the antigen-antibody reactions may be better controlled by noting the magnitude of the changes in the fractal dimension and in the binding rate coefficient as the reaction progresses on the biosensor surface. Also, the magnitude of the changes in the binding rate coefficients (k1 and k2) and in the fractal dimensions (Df1 and Df2) as different parameters are changed for the different biosensor applications are of particular value, since they provide us with a measure or extent of changes in the binding rate coefficient on changing different experimental parameter values. It is of interest to note the effect of different parameters on the extent or heterogeneity that exists on the surface and how this influences the binding rate coefficients. This may be one method to help manipulate or control the binding rate coefficients on the reaction surface.

Mechanistic and kinetic study of formaldehyde production in the atmospheric oxidation of dimethyl sulfide

S. P. Urbanski, R. E. Stickel, Z. Zhao and P. H. Wine, J. Chem. Soc., Faraday Trans., 1997, 93, 2813 DOI: 10.1039/A701380I

Energy pooling in cesium vapor

The total rate coefficient k for the energy pooling process \(\) has been measured in dependence on the ratio of the number densities of the Cs 6P fine-structure sublevels. The measurements were performed applying the laser fluorescence method. From the set of the obtained data for k, the rate coefficients for the particular \(\) collisions were derived. The obtained values for the \(\) and \(\) collisions at T = 600 K are \(\), \(\) and \(\), respectively. The value for k1 and the estimate for k3 are consistent with the data previously published by other authors. The rate coefficient k2 is reported for the first time.

Elementary reactions of the methoxymethyl radical in the gas phase: CH 3OCH 3+F, CH 2OCH 3+CH 2OCH 3, CH 2OCH 3+O 2 and CH 2OCH 3+O

Elementary reactions of the methoxymethyl radical have been studied at low pressures (1–5 mbar) and temperatures between 240 and 700 K using the discharge flow reactor technique. Specific detection of labile and stable species was accomplished by mass spectrometers with low-energy electron-impact ionization coupled directly to the reactor by a molecular-beam sampling system. Methoxymethyl radicals were produced by CH3OCH3+F→CH2OCH3+HF (1) and the rate coefficient k1 was measured k1(T)=(1.1±0.1)×1014 (T/298 K)0.65 cm3 mol−1 s−1 (243≤T/K≤363 K) At temperatures of 600–700 K, decomposition of the radical prevailed (CH2OCH3→CH2O+CH3. lifetime The mechanism of the oxidation of the CH2OCH3 radical by atomic oxygen was found as CH2OCH3+O→HCOOCH3+H (4a) →CH2O+CH3O (4b) with (a): (95±18)%, (b):(5±1)%, and the rate coefficient was measured with respect to the reference reaction (00) C2H5+O (k00=1.33×1014 cm3 mol−1 s−1) k4(T)=(1.54±0.03)×1014 (T/298)0.15cm3 mol−1 s−1 (243≤T/K≤363).

Kinetics of the Reactions of Acetonitrile with Chlorine and Fluorine Atoms

The rate coefficients for the reactions of chlorine and fluorine atoms with acetonitrile have been measured using relative and direct methods. In the case of chlorine atoms the rate coefficient k1 was measured between 274 and 345 K using competitive chlorination and at 296 K using laser flash photolysis with atomic resonance fluorescence. The rate coefficient measured at ambient temperature (296 ± 2 K) is (1.15 ± 0.20) × 10-14 cm3 molecule-1 s-1, independent of pressure between 5 and 700 Torr (uncertainties are 2 standard deviations throughout). This result is a factor of 6 higher than the currently accepted value. The results from the three independent determinations reported here yield the Arrhenius expression k1 = (1.6 ± 0.2) × 10-11 exp[−(2140 ± 200)/T] cm3 molecule-1 s-1. Product studies show that the reaction of Cl atoms with CH3CN proceeds predominantly, if not exclusively, by hydrogen abstraction at 296 K. The rate coefficient for the reaction of fluorine atoms with acetonitrile was measured using...

Simultaneous laser absorption measurements of CN and OH in a shock tube study of HCN + OH → products

AbstractSimultaneous, quantitative, narrow‐line laser absorption measurements of CN time‐histories at 388.444 nm and OH time‐histories at 306.687 nm have been made in incident and reflected shock wave experiments using dilute mixtures of nitiric acid (HNO3) and HCN in argon. The thermal decomposition of HNO3 serves as a rapid source of OH upon shock‐heating, and the OH subsequently reacts predominantly with the HCN in the test gas mixture. The rate coefficient for the reaction equation image was determined in the temperature range 1120–1960 K via detailed kinetics modeling of the simultaneously acquired CN and OH measurements. These data are in good agreement with lower temperature measurements of the rate of the reverse reaction (−1a) when recent values of the heats of formation of CN and HCN are used. The expression valid for temperatures 500 to 2000 K, effectively represents the experimental measurements. The estimated uncertainty of the expression for k1a is ±30%, based on the experimental uncertainties of the individual rate coefficient studies. Analysis of the decay region of the experimental OH time‐histories yielded the total rate coefficient k1 (all product channels) for the reaction of HCN with OH for temperatures ranging from 1490 to 1950 K. These measurements are consistent with a previous theoretical analysis of the three primary addition‐isomerization‐dissociation processes for the HCN + OH reaction at combustion temperatures when the contribution to k1 from reaction (1a) is included. © 1995 John Wiley & Sons, Inc.

A Shock Tube Study of the Thermal Decomposition of Si2H6 Based on Si and SiH2 Measurements

AbstractIn the present investigation the thermal dissociation of disilane (Si2H6) mixed with different additives was measured behind reflected shock waves. Atomic Resonance Absorption Spectroscopy (ARAS) was applied for detecting Si atoms and Ring Dye Laser Absorption Spectroscopy (RDLAS) for detecting SiH2 radicals. Three different series of experiments were performed.In the first group of experiments mixtures of 0.1 to 0.25 ppm Si2H6 diluted in Ar were shock heated to temperatures 2000 K≤T≤3135 K and pressures between 0.8 and 1.4 bar. From the Si‐measurements silane and silylene were found to be the main primary products during disilane decomposition. The SiH2 absorption measurements were carried out at temperatures 1070 K ≤T≤ 1381 K and pressures between 0.35 and 1.3 bar. In these experiments the SiH2 formation was found to be mainly caused by the dissociation reaction Si2H6⇌ SiH4+SiH2 (R1). The rate coefficient k1 was determined by fitting SiH2 profiles calculated based on a simplified reaction mechanism to measured profiles with k1 to be a variable parameter. The value obtained was interpreted based on RRKM calculations. For bath gas concentrations of 3.1 × 10−6≤[M]≤ 4.6 × 10−6 mol cm−3 the rate coefficient k1 can be summarized by the Arrhenius expression k1 = 5.2×1010 exp(‐16850 K/T) s−1. For longer reaction times the SiH2 radicals were consumed due to the reaction SiH2+SiH2 ⇌Si2H2 (R4). The rate coefficient k4 was determined to be k4 = 6.5×1014 cm3 mol−1 s−1 (70%).The second group of experiments was carried out in mixtures with molecular hydrogen as an additive. SiH2measurements were performed in the temperature range 1082 K≤T≤1417 K at pressures 0.24 bar ≤p≤ 1.23 bar by using initial mixtures of 15 to 30 ppm Si2H6 and 5 to 50% H2 diluted in Ar. The aim of this part was to increase the influence of the recombination reaction SiH2 + H2→ SiH4 (R‐2) and to measure the dissociation barrier for the silane decomposition via the equilibrium constant. The data evaluation results in a mean value of E0,2 = 244.1 kJ/mol which is in good agreement to previous investigations.In the last part of this study the influence of silane used as an additive during Si2H6 decomposition was investigated. Initial mixtures of 30 ppm Si2H6 and 300 to 1000 ppm SiH4 diluted in Ar were used under conditions 1060 K ≤ T ≤ 1198 K at pressures between 0.36 and 1.3 bar. From SiH2 concentration measurements an estimation for the rate coefficient of the disproportionation reaction SiH2+SiH4 →H3SiSiH+H2 (R5), k5 = 1.3 × 1013 cm3 mol−1 s−1, was found.

Release rates of polynuclear aromatic hydrocarbons from natural sediments and their relationship to solubility and octanol-water partitioning

Sediment efflux rates of six polynuclear aromatic hydrocarbons (PAHs) were determined employing a novel boundary-layer flux chamber which concentrates trace organics using solid phase extraction techniques. Sediment cores were collected from the Elizabeth River, Virginia and were monitored over a 48 h period for PAH fluxes into uncontaminated seawater. The observed flux rates were related to solubility and octanol-water partition coefficients (KOW) of the PAHs. Flux rates for the six PAHs varied from 0.028 to 0.646 μg/L/d on a concentration basis and 43.0 to 1,000 μg/m2/d on an areal basis. The resulting flux rate coefficients (k2) were determined to be well correlated to solubility (R2=0.82) and inversely related to KOW (R2=0.75). The empirically determined sediment flux dynamics fit well with theoretical relationships that have been previously established for bioconcentration kinetics and octanol/water partitioning systems. Thus, it appears possible to predict flux rates of other hydrophobic organic chemicals from ambient sediment and water concentrations, KOW information, and the proposed empirical models.

Reactions of phenoxy radicals in the gas phase

The mechanisms and rates of the formation and the reactions of the phenoxy radical with atoms andradicals in the gas phase have been studied at room temperature (around 295 K) and low pressure (1–5 mbar) using the discharge flow technique. Samples were withdrawn continuously by a molecular-beam-sampling system coupled to a mass spectrometer with electron impact ionization for a specific and sensitive detection by applying low ionization energy and single-ion counting. Phenoxy radicals were produced by the reaction of phenol with chlorine atoms C6H5OH+Cl→C6H5O+HCl (1) and the rate coefficient k1 was determined to be k1=(1.43±0.25)·1014cm3/mol s (relative to the reference reaction C2H6+Cl, k0=3.4·1013 cm3/mol s). Phenoxy radicals react with atoms according to C6H5O+O→C6H4O2 (quinone)+H (2a) →C5H5+CO2 (2b) with reaction (2a) being the main pathway, and a rate coefficient of k2=(1.68±0.35)·1014 cm3/mol s (measured relative to the standard reaction (01) C2H5+O, k01=1.31·1014 cm3/mol s) and C6H5O+H→C6H6O/C6H5OH (3) with C6H6O being cyclohexadienone, and a rate coefficient k3=4·1013 cm3/mol s. (The reference reaction was (02) C2H5+H, k02=3.6·1013/mol s, and the ratio of the rate constants was determined as k3/k02=1.12±0.03.) Phenoxy radicals combine with the alkyl radicals C2H5 and CH3 in two different pathways with comparable efficiency:C6H5O+C2H5→C6H5OC2H5 (phenetol) (4a) →C2H5C6H4OH (ethylphenol) (4b) C6H5O+CH3→C6H5OCH3 (anisol) (5a) →CH3C6H4OH (cresol) (5b) ((4a):(4b)=63:37; (5a):(5b)=59:41.) The combination reaction of phenoxy radicals is fast:C6H5O+C6H5O→(C6H5O)2 k6=(2+2/−1)·1013 cm3/mol s (6)

Modelling of CF2HCl Consumption in H2/O2/Ar Flames

AbstractThe aim of this work is the investigation of the relative contribution of reactions (1) CF2HCl + H → CF2H + HCl, (2) CF2HCl + O → CF2Cl + OH and (3) CF2HCl + OH → CF2Cl + H2O to the initial CF2HCl attack in lean, stoichiometric and rich H2/O2/Ar flames. The mole fraction profiles have been computed by means of the PREMIX code. Using the rate coefficient k1 = 4.65 1014 exp(‐7730/T) cm3mole−1s−1, k2 = 6.62 1012 exp(‐4228/T) cm3mole−1s−1 and k3 = 1.28 1012 exp(‐1671/T) cm3mole−1s−1 the contribution of reactions (1), (2) and (3) has been calculated by integration throughout the flame front. The results show an obvious dependence on the equivalence ratio. In rich and stoichiometric flames CF2HCl is consumed nearly exclusively by H‐radicals, while in the lean flame H contributes to 43%, O to 32% and OH to 25% of the CF2HCl disappearance rate. The computed results of the stoichiometric flame compare favourably with experiments.

Shear dependence of the fibrin coagulation kinetics in vitro

Fibrin thrombus formation, in vivo and in vitro, preferentially occurs in regions of retarded, recirculating flow which promote local variations of the distribution of blood components, e. g. thrombin, and shear rates. To better understand the effects of shear forces on the thrombin induced fibrin coagulation process the time course of fibrin formation in a fibrinogen/thrombin solution was studied for different shear rates γ ̇ (0 s −1⩾ γ ̇ ≤500 s −1) and thrombin concentrations cthr (0.1 units/ml≤c thr≤1.0 units/ml). The clotting curves at zero shear and the shear induced alterations of these curves could essentially be described in terms of a reaction kinetics defined by two rate coefficients k 1, k 2 which can be attributed to fibrinogen cleavage by thrombin and fibrin polymerisation, respectively. For c thr≈0.5 units/ml and γ ̇ ≈15 s −1 an additional mechanism, presumably fibrin breakage, had to be assumed. The rate coefficient k2 was markedly more affected by cthr and shear forces then was k 1. The results fit well to the growth kinetics of fibrin thrombi formed in glas models of an arterial branching.

Third excimer continua in neon and argon

The emission of the third continuum of argon in the wavelength range between 175 and 250 nm and the vacuum ultraviolet emission of neon (λ < 100 nm) has been studied by timeresolved optical spectroscopy. The target gases at pressures between 2 and 100 kPa were excited with a pulsed beam of 100 MeV 32S9+ ions from the Munich Tandem van de Graaf accelerator. Wavelength spectra recorded in different time windows after the 2-ns beam pulses show that two different components contribute to the third continuum of argon. A radiative lifetime of 5.71 ± 0.08 ns for the Ar22+ molecule and a rate coefficient k3 = (1.46 ± 0.12) x 10-30 cm6/s for the reaction Ar2+ + 2Ar → Ar22+ + Ar were obtained from the pressure dependence of time spectra at a wavelength of 190 nm. From time- and pressure-dependent studies of the third continuum emission of neon at a wavelength of 99 nm, rate coefficients k2 = (3.6 ± 0.3) x 10-13 cm3/s for the bimolecular reaction Ne2+ + Ne→ 2Ne+ and k3 = (2.84 ± 0.09) X 10-31 cm6/s for the termolecular reaction Ne2+ + 2Ne → Ne22+ + Ne were determined.

Kinetic and equilibrium solvent isotope effects on the ionisation of a hydrogen-bonded proton and studies of the intramolecular hydrogen bond in phenylazoresorcinols

The kinetic solvent isotope effect on the removal of the hydrogen-bonded proton from 2-methyl-4-(4-nitrophenylazo)resorcinol monoanion by hydroxide ion has been measured at different hydroxide-ion concentrations in 60%(v/v) Me2SO–L2O (L = H or D) and in 80%(v/v) Me2SO–L2O. The isotope effect in 80%(v/v) Me2SO–L2O has been analysed to give isotope effects on the rate coefficient (k1) and equilibrium constant (k1) for formation of an open non-hydrogen-bonded species from the intramolecularly hydrogen-bonded form and the results (k1)H/(k1)D 1.26 and (k1)H/(k1)D 0.65 were obtained. One contribution to the equilibrium isotope effect is made by the difference in the values of the isotopic fractionation factors (φ) for the protons in the hydrogen-bonded and open forms. To provide estimates of these values, fractionation factors have been measured in 80%(v/v) Me2SO–L2O for the hydrogen-bonded proton in 2-isopropyl-4-(4-nitrophenylazo)resorcinol monoanion (φ 0.97) and for the hydroxy proton in phenol (φ 1.08). The isotope effect on the chemical shift of the hydrogen-bonded proton in 2-isopropyl-4-(4-nitrophenylazo)resorcinol monoanion has been determined and the result Δ[δ(1H)–δ(2H)] 0.61 shows that the hydrogen bond is of the double minimum type.