Partial Rate Constants Research Articles

The γ-Silicon Effect. IV. The Solvolysis Mechanism of 3-(Aryldimethylsilyl)propyl p-Toluenesulfonates

Abstract Solvolysis rates of 3-(aryldimethylsilyl)propyl p-toluenesulfonates were determined in various solvents. The reaction mechanism of this simple γ-silyl system was clarified based on the solvent effect and the substituent effect analyses. The solvent effect on this system clearly showed the nucleophilic assistance of solvent, but failed to correlate linearly with the extended Winstein-Grunwald equation, substantiating that the reaction should not proceed through either the formation of the cation intermediate or the SN2 mechanism. This suggests that the reaction takes place in competition between γ-silyl-assisted (kSi) and solvent-assisted (ks) pathways, and that the competition ratio varies with solvents and with aryl substituents. Product analysis revealed that the former pathway gave only cyclopropane and the latter gave only the substitution products. The overall kt value could be dissected into the partial rate constants kSi and ks for the two pathways by using product ratios. The effects of aryl substituents at the γ-silyl atom on kSi pathway were correlated with unexalted σ° parameter, giving the ρ values of -1.0 in 60E and -1.32 in 97Tw, and reflecting the delocalization of incipient carbocationic charge by participation of the Si-Cγ bond. The substituent effects on the ks pathway were negligibly small; this is in line with the remote reaction center in the concerted SN2 mechanism.

The influence of ionic strength on the dissolution process of silica

A general dissolution model for oxidic solids in water has been suggested. To confirm this model the dissolution kinetics of well-defined silica spheres is determined dependent on pH and ionic strength at 40°C. By analyzing dissolution data it is possible to study electrical double layer properties. In this approach, the charge state of the oxide is described by application of a diffuse layer model. From a theoretical point of view, some important, general conclusions for the overall rate constant of dissolution, consisting of three partial rate constants, may be drawn. The characteristic parameters such as the pristine point of zero charge, the acidity constants of the surface groups, the saturation concentration, and the partial rate constants, could be determined precisely. Experimental results for two different types of silica are compared. The model is able to elucidate well the dissolution behavior as a function of time, pH and ionic strength.

Deceleration of light-induced changes of selected pharmacons by means of light screening films

Photo-induced changes in three pharmacons (Promethazine HCl, Carbidopa and Niphedipine), under exposure to light rich in UV component, could be characterised with the rate constants of the occurring processes. Partial rate constants could be determined after exposure to selected wavelenght regions emitted by the light source. The most effective wavelength regions characteristics for the single pharmacons could be derived from the calculated partial rate constants. Six composite plastic films were applied for the desired light protection. A matrix LDPE film was particle filled — keeping the transparency of the extruded films — with colloidal titanium dioxide. By using three particle filled films of three different TiO 2 concentrations together with two commercial products, the photoactive ranges of wavelength could be cut off from the illuminating light. Satisfactory light stability of the pharmacons could be established with the help of the most appropriate single light screening films or with their best binary combination.

Theoretical treatment of inelastic thermal collisions

The production of excited mercury ions in thermal collisions is studied theoretically. The adiabatic potentials and the nonadiabatic radial couplings are calculated within the multielectron pseudopotential method. The coupled equations for the nuclear motion are solved fully quantum mechanically in a diabatic representation. The partial transition probabilities, the incoming and outgoing currents, the partial cross sections and the partial rate constants are determined in the range of interest and compared with the available experimental data. It is found that the total rate constant is mainly determined by the partial rate constant for excitation of the state.

Dissociative recombination of electrons and molecular ions

The existing techniques for the calculation of the dissociative recombination (DR) of electrons and molecular ions were compared. The advantages of the method of multichannel quantum defect (MQD), in which equations are formulated directly for the T-matrix of collisions and the unitarity of the scattering S-matrix is thus ensured, were demonstrated. The effect of molecular rotation and of the nonadiabatic electron-rotation coupling on the e− + H2+ →, H* + H reaction was investigated. A procedure was suggested based on the use of the adiabatic approximation (with respect to the nuclear rotation) in the near-threshold area while taking into account the contributions of the excited vibronic states of the Rydberg complex formed in an intermediate stage of the reaction. It is notable that the partial rate constants (and the corresponding cross-sections) arc very sensitive to the initial rotation excitation. However, the temperature-averaged rate constants under equilibrium conditions are only slightly affected by rotation.

Steady State Fischer-Tropsch Synthesis in Supercritical Propane

Fischer-Tropsch synthesis (FTS) on a precipitated iron catalyst was conducted in a fixed bed reactor at baseline conditions of 1.48 MPa, 250 C, 2 L(NTP)/(h [times] g of cat), H[sub 2]/CO = 0.67, and in supercritical propane (total pressure of 7 MPa) while keeping the synthesis partial pressure and gas flow rate constant. The catalyst activity and lumped hydrocarbon product distribution under the supercritical conditions were similar to those obtained during reaction a the baseline conditions; however, higher selectivities of 1-olefins were obtained during the supercritical phase FTS. This suggests that the rate of FTS is not diffusion limited under the reaction conditions used in this study, whereas the secondary olefin hydrogenation and isomerization reactions are diffusion limited. An independent test in which nitrogen was used instead of propane showed that operation at a total pressure of 7 MPa does not have any effect on olefin selectivity. The observed increase in olefin selectivity during the supercritical FTS is due to higher diffusivities and desorption rates of high molecular weight olefins relative to normal FTS where the catalyst pores are filled with liquid hydrocarbons (wax).

Interference in the kinetic determination of ascorbic acid by sulphide and sulphite

The reduction of 2,6-dichlorophenolindophenol (DCPI) by sulphides and sulphites has been studied kinetically by the stopped-flow technique. The reaction is first-order with respect to each of the reactants. From the distribution diagrams for the species DH + 2, DH and D − for DCPI and H 2Q, HQ − and Q 2− for sulphides or sulphites, a mechanism is proposed which suggests partial reactions of all possible combinations of the reacting species at any pH. An equation for calculation of the second-order reaction rate constants k at any pH is derived, which gives k as a function of [H +], the partial reaction rate constants and the dissociation constants of DCPI and H 2S or H 2SO 3. Values of the overall reaction rate constants over a wide pH-range have been determined, together with values of k for all possible partial reactions. For particular pH-values the second-order reaction rate constant was determined by four different methods. Mean values of k = 251 ± 1 and 240 ± 1 l.mole −1.sec −1 were obtained for pH 3.15 and 4.17, respectively, for the DCPI—Na 2S reaction and k = 137 ± 1, 127 ± 1 and 136 ± 1 l.mole −1.sec −1 for pH 2.02, 4.25 and 5.10, respectively, for the DCPI—Na 2SO 3 reaction. From the slopes of the linear Arrhenius plots activation energies of 6.6 ± 0.2 and 4.0 ± 0.1 kcal/mole for the DCPI—Na 2S and DCPI—Na 2SO 3 reactions, respectively were calculated. The effect of ionic strength on the reactions supports the proposed mechanism.

Kinetics and mechanism of the acid-catalysed hydrolysis of regioisomeric benzotriazole and 8-azaadenine nucleosides

First-order rate constants for the acid-catalysed hydrolysis of ribo- and 2′-deoxyribonucleosides derived from 1H-, 2H- and 3H-benzotriazoles and 7H-, 8H- and 9H-8-azaadenines have been determined over a wide pH range. Partial rate constants for reactions via mono- and di-protonated substrates have been calculated with the aid of spectrophotometrically determined pKa values. The mechanisms of hydrolysis reactions have been elucidated by the effects that polar substituents on the base moiety exert on the protonation and heterolysis steps. Reactivities of various regioisomers have been compared, and reasons for the exceptional hydrolytic instability of N3-alkylated purine nucleosides briefly discussed.

Kinetic and mechanistic study of the reduction of 2,6-dichlorophenol indophenol by dithionites

The reaction of 2,6-dichlorophenolindophenol (DCPI) and dithionites is studied kinetically by applying the stopped-flow technique. Reaction rate constants are given for the pH range 1.30–6.80. The reaction was found to follow first-order kinetics with respect to each of the reactants. For pH 3.97, 5.10 and 6.80, the second-order reaction rate constant was determined by applying four different technique. Mean values of k = 172±5, 200±2 and 276±4 l mol −1 s −1 are given for pH 3.97, 5.10 and 6.80, respectively. A mechanism is proposed for the reaction, which suggests partial reactions of all possible species of DCPI and dithionites at any pH. An equation for the calculation of k at any pH is derived, which gives k as a function of [H + ], the partial reaction rate constants and the dissociation constants of DCPI and H 2 S 2 O 4 . Values of reaction rate constants of all possible partial reactions are also presented.

Study of the mechanism of the photochemical oxidation of bilirubin by high-performance liquid chromatography

The photochemical degradation of bilirubin was studied in vitro using high-performance liquid chromatography and spectrophotometry. Attention was centred on the formation of biliverdin, which is produced as an intermediate in the photooxidation catalysed by riboflavin. Approximate values of the overall and partial relative rate constants were calculated using the physicochemical criteria for a pseudo-monomolecular reaction. A more precise evaluation was made by non-linear regression programmes on a Hewlett-Packard 9835 A computer. In addition to the formation of biliverdin, attention was also paid to accompanying processes affecting the reaction mechanism, which were explained as aggregation and dimerization of bilirubin and biliverdin. It was also found that during phototherapy of newborn babies suffering from hyperbilirubinaemia the level of biliverdin in the plasma increased. The results are discussed from the point of view of phototherapy.

Spectrocoulometric study of the hydrolysis of the tris (1,10-phenanthroline)iron(III) complex ion

The spectrocoulometric technique reported earlier is applied to verify the mechanism and to evaluate the contributions k B i of the individual bases to the total rate constant k of the hydrolysis of the tris (1,10-phenanthroline) iron(III) complex, Fe (phen) 3+ 3. Both normal and “open-circuit” spectrocoulometric experiments are used. Partial rate constants for four bases in the acetate-buffered solutions are k H 2O =(3.4±1.2) × 10 −4s −1 ( k H 2O includes the H 2O concentration), k OH=(1.20±0.06)×10 7 mol −1dm 3s −1, k phen=(1.4±0.2) mol −1dm 3s −1, k Ac=(3.8±0.3)×10 −2 mol −1dm 3s −1, at 25°C and ionic strength 0.5 mol dm −3. The Fe(phen) 3+ 3 hydrolysis, with (phen) 2 (H 2O) Fe-O-Fe (H 2O) (phen) 4+ 2 formation, is first order with respect to Fe (phen) 3+ 3 and the bases present in the solution. The rate-determining step in the hydrolysis is the entry of a base to the coordinating sphere of the complex, as in the hydrolysis of the analogous 2,2′-bipyridyl complex.

Reactivity of polyatomic complex esters in the reaction with trichloromethyl radicals

1. The kinetics of chlorination of pentaerythritol tetrapropionate and tetracaprate, neopentylglycol dipropionate, and 1,1,1-trimethylolpropane tripropionate in a solution of CCl4 at 373°K according to the scheme RH+CCl4→RCl + CHCl3 were studied. 2. Initiation of the reaction by benzoyl peroxide takes place according to a chain mechanism with the participation of CCl3 radicals in chain propagation; the rate constants of the reaction.of CCl3 with these esters were determined. 3. The partial rate constant of the reaction of CCl3 with the CH2OCOCH2CH3 fragment is 49 ±6 liter/ mole.sec at 373°K and is not dependent on the number of such fragments in the molecule.

Kinetic analysis of the protonation of a surface group of a macromolecule.

The dynamics of proton transfer between a surface-attached acidic moiety and the bulk of the solution was measured using the laser-induced proton pulse technique. [Gutman, M., Huppert, D. and Pines, E. (1981) J. Am. Chem. Soc. 103, 3709-3713]. The model system for this study consists of pH indicators (either neutral red or bromcresol green) adsorbed on Brij 58 micelles, as defined targets for protonation and a non-adsorbed proton emitter (2-naphthol-3,6-disulfonate) for generation of protons in bulk. The reaction was measured with 50-ns time resolution over a time period of about 200 microseconds. The results were analyzed by a numerical solution of the coupled nonlinear differential equation corresponding with the reaction system. [Gutman, M., et al. (1983) J. Am. Chem. Soc. 105, 2210-2216]. Quantitative analysis reveals two independent reactions which govern the observed dynamics: (a) a diffusion-controlled reaction between the proton and the surface targets; (b) translocation of the protonated target between the hydration layer of the interface and a more hydrophobic one. The contribution of the translocation reaction to the dynamics of surface protonation is more pronounced for compounds like carboxylates or phenolates which increase their hydrophobicity upon protonation. Amines and azoaromatic structures are more hydrophilic in their protonated states, the dynamics of their protonation is less affected by post-protonation distribution within the microenvironments of the interface. The interrelation between the partial rate constants and the macroscopic time constants and equilibrium parameters is analyzed.

Biochemical and Chemical Transformations of Terbufos, Terbufos Sulfoxide, and Terbufos Sulfone in Natural and Sterile, Mineral and Organic Soil

A sandy loam and a muck soil were sterilized by autoclaving, and samples were treated at 10 ppm with terbufos, terbufos sulfoxide, or terbufos sulfone. These were incubated in the dark at 28°C; samples were removed after 1, 2, 4, 8, 12, 16, and 20 weeks and were examined for microbial populations, pH, and insecticide concentrations. These variables were determined also in correspondingly treated natural soils. Residual insecticide determinations were simplified by acid treatment of the solid support used to prepare gas-liquid chromatography (GLC) columns, which permitted direct analysis of at least 0.05 ng of terbufos sulfoxide and separation from 100-fold greater quantities of terbufos sulfone by GLC. In the natural soils, all three applied insecticides disappeared much more rapidly than from the sterilized soils, and terbufos sulfoxide and terbufos sulfone were observed as intermediate transformation products. In the sterilized soils, terbufos oxidation to terbufos sulfoxide was the major observable process, but the rate of oxidation was slower than in the natural soils and further oxidation to the sulfone was not observed. Terbufos sulfone also was stable in the sterilized soils. These results show that biochemical processes are of major importance in the transformation of terbufos and its oxidation products in soil. Partial-rate constants for the processes described showed that the greater persistence of the “total residue” from terbufos treatment in the organic soil was due to more rapid conversion of the terbufos to terbufos sulfoxide combined with a slower rate of degradation of terbufos sulfoxide and terbufos sulfone.

Kinetics of tritium isotope exchange between H2S and CH3OH vapor in glass vessels

Two different mechanisms of tritium exchange between\(\mathop H\limits^* _2 S\) and CH3OH vapor in glass vessels have been found: (i) bimolecular exchange in the gas phase, (ii) pseudomonomolecular exchange between gaseous\(\mathop H\limits^* _2 S\) and CH3OH adsorbed on the surface. The total and partial orders, rate constants and activation energies are given.

Biochemical and Chemical Transformations of Phorate, Phorate Sulfoxide, and Phorate Sulfone in Natural and Sterile Mineral and Organic Soil

A sandy loam and a muck soil were sterilized by autoclaving, and samples were treated at 10 ppm with phorate, phorate sulfoxide, or phorate sulfone. These were incubated in the dark at 28°C, and samples were removed after 1, 2, 4, 8, 12, and 16 weeks and examined for microbial populations, pH, and insecticide concentrations. These variables also were determined in correspondingly treated natural soils. The three applied insecticides disappeared much more rapidly from the natural soils than from the sterilized soils. Phorate sulfoxide and phorate sulfone were observed as intermediate transformation products. In the sterile soils, phorate oxidation to phorate sulfoxide was the major process observed, and further oxidation of the sulfoxide did not occur. The expected increases in rates and changes in the products formed were observed as the sterile soils became nonsterile in time. Reduction of phorate sulfoxide to phorate was detected as a minor process in these contaminated soils. The results show that biochemical processes are of major importance in the transformation of phorate and its oxidation products in soil. The usefulness of appropriate partial rate constants in describing the processes involved is discussed.

Reactions of dichloride anion radicals with aliphatic peptides in aqueous solutions.

Rate constants for the reactions of dichloride anion radicals with aliphatic peptides were determined by a conventional flash photolysis of 1 M solutions of potassium chloride containing aliphatic peptides. The rate constants were found to increase with increasing number of C-H bond. Partial rate constants were assigned to each C-H bond of the aliphatic peptides, as estimated for the reactions of hydroxyl radicals with the same substrates. Trend in the partial rate constants and the effect of a protonated amino group on the reactivity indicate a reaction mechanism, hydrogen atom abstraction, similar to hydroxyl radicals.

Spektroskopische Untersuchung unelastischer Stöße zwischen Ionen oder metastabilen Atomen der Edelgase und Alkalimetallatomen bei thermischen Stoßenergien

AbstractThe present work deals with the spectroscopic investigation of Penning ionization and nonresonant charge exchange due to thermal‐energy collisions of metastable atoms or ions of the inert gases He, Ne with atoms of alkali metals K, Rb, Cs in a low temperature plasma of the corresponding binary mixtures. The rate constants of these reactions and partial rate constants of exciting several alkali metal ion levels were determined by the kinetics of the decay of ionized and excited particles in the afterglow of a pulsed discharge. Measured Penning and charge exchange constants are in order of 1010‐9 cm3 sec−1. Partial Penning constants in the HeCs system exhibit a small dependence of the energy defect of reaction, whereby an important part of collisions results in excited Cs+‐states. On the contrary, energy transfer by charge exchange in HeRb, Cs systems distinguishes by a significant energy defect dependence. Charge exchange from He+ to Rb0 leads above all to He(23S1)‐atoms and normal Rb+‐ions. It follows from the metal ion spectrums, that Penning ionization and charge exchange of the investigated systems under utilized conditions represent as processes of selective excitation of alkali metal ions in afterglow and stationary discharge.

Persistence of four pyrethroid insecticides in a mineral and an organic soil

Abstract Permethrin, cypermethrin, fenpropanate and fenvalerate (emulsifiable concentrates) were applied at 280 g AI/ha and incorporated into mineral and organic soil contained in small field plots. Radishes and carrots were grown to serve as indicators of insecticide uptake. Similar plots were treated with the same insecticides at 140 g AI/ha and the soil surface was left undisturbed following application. Soil cores were removed at appropriate intervals and the crops were harvested when mature. The insecticide concentration in all samples was determined by gas‐liquid chromato‐graphy. The amount of insecticide in the soil declined rapidly to <50% of the initial value in 1 month or less for most material‐soil‐treatment combinations and within 2 months for all cases. Concentrations remained in excess of 0.01 ppm in the organic soil for at least six months for all material‐treatment combinations but fell below this level over 2–5 months in the mineral soils. Organic soil incorporated fenvalerate was the most persistent combination overall with 25, 17 and 7% remaining at 6, 18 and 28 months respectively. The trans‐isomers of permethrin and cypermethrin disappeared more quickly than the cis‐isomers but the insecticidally active 1R isomers were not preferentially degraded relative to the inactive IS. No residues (<0.01 ppm) were found in the radish or carrot crops. First order disappearance rates were not constant for any of the combinations. A comparison of partial rate constants showed: 0–1 mo rates were generally greater in mineral than organic soil, 1–6 mo rates in organic soil were lower than 0–1 mo rates, 1–6 mo rates for surface applications to organic soil were generally less than for incorporated applications. In laboratory experiments, 0–1 mo rates for fenvalerate disappearance in a mineral soil were 2–3x greater for 0.5 ppm than for 10 ppm while 1–6 mo rates were independent of insecticide concentration but were 1.5x greater for 0.5% moisture than for 5%.

Persistence of four pyrethroid insecticides in a mineral and an organic soil.

Permethrin, cypermethrin, fenpropanate and fenvalerate (emulsifiable concentrates) were applied at 280 g AI/ha and incorporated into mineral and organic soil contained in small field plots. Radishes and carrots were grown to serve as indicators of insecticide uptake. Similar plots were treated with the same insecticides at 140 g AI/ha and the soil surface was left undisturbed following application. Soil cores were removed at appropriate intervals and the crops were harvested when mature. The insecticide concentration in all samples was determined by gas-liquid chromatography. The amount of insecticide in the soil declined rapidly to less than 50% of the initial value in 1 month or less for most material-soil-treatment combinations and within 2 months for all cases. Concentrations remained in excess of 0.01 ppm in the organic soil for at least six months for all material-treatment combinations but fell below this level over 2-5 months in the mineral soils. Organic soil incorporated fenvalerate was the most persistent combination overall with 25, 17 and 7% remaining at 6, 18 and 28 months respectively. The trans-isomers of permethrin and cypermethrin disappeared more quickly than the cis-isomers but the insecticidally active IR isomers were not preferentially degraded relative to the inactive 1S. No residues (less than 0.01 ppm) were found in the radish or carrot crops. First order disappearance rates were not constant for any of the combinations. A comparison of partial rate constants showed: 1) 0-1 mo rates were generally greater in mineral than organic soil, 2) 1-6 mo rates in organic soil were lower than 0-1 mo rates, 3) 1-6 mo rates for surface applications to organic soil were generally less than for incorporated applications. In laboratory experiments, 0-1 mo rates for fenvalerate disappearance in a mineral soil were 2-3x greater for 0.5 ppm than for 10 ppm while 1-6 mo rates were independent of insecticide concentration but were 1.5x greater for 0.5% moisture than for 5%.