The Free-Radical Cleavage of the Disulfide Bond (Studied by Pulse Radiolysis)

Reaction of H atoms with glutathione leads rapidly to H + RSSR → RS · + RSH. The first observed product is RS, the spectrum of which is obtained. The spectrum of the RS?SR radical was obtained by direct attack of e^?_aq on glutathione. The rate constants of these processes were also measured. k_e?aq + RSSR = (2.7 ± 0.3) × 10⁹ M^?1 sec^?1 k_{H + RSSR} = (1.0 ± 0.2) × 10¹⁰ M^?1 sec^?1 When the OD of RS?SR is plotted vs pH a titration curve is obtained. This is due to the protonation of RS?SR with a rate constant of 2.6 × 10¹⁰ M^?1 sec^?1 which is probably followed by a cleavage to RS and RSH. In both cases the RSSHR radical cannot be detected. The spectrum attributed to the RSSHR radical is more likely to be that of RS.     

Absolute reactivity in the cationic polymerization of methyl and other alkyl vinyl ethers

The cationic polymerizations of methyl-, 2-chloroethyl-, ethyl-, cyclohexyl- and t-butyl- vinyl ethers initiated by cycloheptatrienyl hexachloroantimonate in methylene chloride solutions have been studied in detail. Reaction rates were measured by an adiabatic calorimetric technique and rate constants for propagation of each of the monomers, k_p (obs), were determined by appropriate kinetic analysis of the experimental curves. The results obtained are discussed in terms of current theories regarding ion pair/free ion equilibria in non-aqueous solvents. Although ethyl-, cyclohexyl- and t-butyl- vinyl ethers behave very similarly to isobutyl vinyl ether, and their reactivities are comparable [k_p (obs) ～ 3 × 10³M^?1sec^?1 at 0°C] both methyl- and 2-chloroethyl- vinyl ethers show markedly different characteristics to the others, and in particular exhibit a reactivity approximately one order of magnitude less [k_p (obs) ～ 2 × 10²M^?1sec^?1 at 0°C]. These variations in reactivity are discussed in terms of preferred monomer conformations, and the resulting differences in activation energy which are likely to arise when such conformers are approached by an electrophile.     

Proton Transfer in Liquid Ammonia Solutions Containing Water and Hydroxide Ion

The rate of proton transfer in ammonia solutions containing water and hydroxide ion was determined as a function of temperature and concentration by means of PMR spectroscopy. A first order dependence on hydroxide ion and second order dependence on water concentration was found with a rate constant of k₃ = 6.1 ± 1.2 × 10³ M^?2 sec^?1 at 25°C and ΔE^? = 4.5 ± 0.5 kcal/mole. The value of k₃ is in good agreement with that reported by other workers.     

Reactivity and mechanism in the cationic polymerization of p-methoxystyrene initiated by cycloheptatrienyl hexachloroantimonate

The cationic polymerization of p-methoxystyrene initiated by cycloheptatrienyl hexachloroantimonate in dichloromethane solution has been studied in some detail. Reactions proved to be highly exothermic, and rates of monomer consumption were measured using an adiabatic calorimetric technique. Termination was deduced to be insignificant during kinetic lifetimes, and $\text{M}\text{?}{}_{\mathrm{n}}$ values in the range 10000–50000 showed chain breaking to occur by transfer mechanisms. Appropriate analysis of conversion/time curves allowed computation of enthalpies of polymerization and rate coefficients for propagation, k_p (obs), under various conditions. Data for k_p (obs) were found to vary with the initial concentrations of initiator and monomer employed, and these dependences are discussed in terms of current theories regarding ion pair/free ion equilibria in nonaqueous solvents. In particular values of 3.6 × 10³ M^?1s^?1 and 4.8 × 10³ M^?1s^?1 at 0° and +10°C respectively for the rate constant for propagation by free poly (p-methoxystyryl) cation have been deduced, and a tentative value of 450 M^?1s^?1 at 0°C has been estimated for the rate constant for propagation by the corresponding hexachloroantimonate ion pairs. These data and the related activation parameters are compared with independent results in the literature. Polymerizations carried out in the presence of excess common ion salt, dimethyl benzyl phenyl ammonium hexachloroantimonate, showed rate depressions far in excess of those predicted by a simple mass law effect, arising possibly as a result of a more dramatic ionic association than simple ion pairing.     

Decay of the HO2 and O−2 Radicals Catalyzed by Superoxide Dismutase. A Pulse Radiolytic Investigation

The pulse radiolysis technique has been employed in the investigation of the dismutation of superoxide radicals, O^?₂ and HO₂, in the presence of superoxide dismutase in aqueous solutions. The decay of superoxide radicals in the presence of the enzyme was found to be first order in both enzyme and superoxide concentrations. An apparent second order reaction rate constant was found to be about 2 × 10⁹ M^?1 sec^?1, decreasing slightly as the pH is increased from 5 to 9.5. A mechanism which accounts for all our observations is proposed. It includes two steps: (1) formation of a product (EO^?₂ or E^?) from one enzyme (E) molecule and one O^?₂ radical ion; (2) regeneration of E by a reaction of this product with an additional O^?₂ ion radical. The reaction rate constants k = (1.4 ± 0.2) × 10⁹ and k = (1.9 ± 0.6) × 10⁹ M^?1 sec^?1 were measured at pH = 7 in an oxygenated 0.16 M sodium formate solution.     

Flash photolysis investigation on primary processes of the sensitized polymerization of vinyl monomers: 2. Experiments with benzoin and benzoin derivatives

Benzene solutions of benzoin (B), benzoin acetate (BA), benzoinmethyl ether (BME) and benzoinisopropyl ether (BIPE) were irradiated with 0.1 or 25 nsec flashes of 347.1 nm light at room temperature. With B and BA optical absorptions due to radical precursors were detected. The first order decay constants of these absorptions are 1.1 × 10⁸ sec^?1 (B) and 5.3 × 10⁷ sec^?1 (BA). The radical spectra decayed according to second order laws. In the case of B only one mode of decay was observed which accounts for the major part of the reaction (benzaldehyde formation by disproportionation of unlike radicals). During the experiments with BA, BME and BIPE two modes of radical-radical reactions were detected. Styrene was found to react with the rapidly decaying species with k_M = (1.3 ± 0.2) × 10⁵ M^?1 sec^?1, but did not react with the slowly decaying species. By considering the end product analyses of other authors it was concluded that the rapidly decaying mode of the absorption decay is due to the combination of benzoyl radicals forming benzil. The slowly decaying mode was assigned to the combination of alkoxybenzyl radicals (in the case of BME and BIPE) or of acetonbenzyl radicals (in the case of BA) forming pinacol derivatives. The rate constants of the reaction of naphthalene with triplet excited benzoin (7.7 ± 0.8) × 10⁹ M^?1 sec^?1 and benzoin acetate (5.0 ± 0.5) × 10⁹ M^?1 sec^?1 were determined.     

Anionic polymerization of ethylene oxide with cryptates as counterions: 1

A kinetic study of the anionic polymerization of ethylene oxide has been made in tetrahydrofuran at 20°C, with the cryptate K⁺ + [222] as counterion. The ion pair dissociation constant K_D has been deduced from conductivity measurements made on seeds solutions. Some ionic associations higher than cryptated ion pairs could be detected but they are negligible for living end concentrations lower than 6 × 10^?4 mol/l. k_± and k_? were determined from both sets of kinetic data obtained with and without added salt. Free alkoxide ions are one hundred times more reactive than cryptated ion pairs.     

Anionic polymerization of ethylene oxide with cryptates as counterions: 2

A kinetic study of the anionic polymerization of ethylene oxide has been made in tetrahydrofuran at 20°C, with the cryptate Cs⁺ + [TC] as counterion, [TC] being a spheroidal macrotricyclic ligand. Conductance measurements have been made on THF solutions of ?₄BCs + [TC]. Ionic associations higher than cryptated ion pairs are negligible for living end concentrations lower than 3 × 10^?4 moll^?1. k_± and the alkoxide ion pair dissociation constant K_D were determined from both sets of kinetic data obtained with and without added salt knowing the value of k_? from kinetic data performed with K⁺ + [222] as counterion. Free alkoxide ions are about twenty times more reactive than cryptated caesium ion pairs.     

Photocatalysis in a slurry reactor

A series of experiments have been conducted on the photocatalytic reduction of methylene blue with zinc oxide catalyst in a slurry reactor. It is shown that by using a procedure involving separate experiments to determine the shielding effects of the suspended solid (inert solid of similar particle size with a standard actinometric solution) it is possible to identify experimentally the region in the reaction system where imperfect solids mixing influences the rate of reaction. The reactor employed here is also shown to provide a relatively simple means for kinetic measurements in photocatalytic reactions. The rate constants obtained in this study for methylene blue reduction were weakly dependent on zinc oxide concentration in the suspension. A value of 3·67 M^?1 sec^?1 at 0·1 wt.% ZnO at 31·9°C was obtained, where M is the molarity of the solution in methylene blue. This may be compared with 1·25 × 10⁴M^?1 sec^?1 reported by Yoneyama [14] for methylene blue reduction on a rutile crystal of titanium dioxide at room temperature, and 3·33 M^?1 sec^?1 for p-nitrosodimethylaniline reduction in a 0·1 wt. % ZnO suspension, also at room temperature[13].     

Laser flash photolysis investigations on primary processes of the sensitized polymerization of vinyl monomers: 1. Experiments with benzophenone

The laser flash photolysis technique was used in order to evaluate kinetic parameters concerning the efficiency of benzophenone (BP) as a photosensitizer for polymerizations. Rate constants (in M^?1 sec^?1) of the reaction of triplet BP with monomers were measured (e.g. styrene, 3.3 × 10⁹; methyl methacrylate, 6.9 × 10⁷; acrylonitrile, 3.4 × 10⁷; vinyl acetate, 5.4 × 10⁶). The rate constant of the reaction of triplet BP with tetrahydrofuran is 3 × 10⁶M^?1sec^?1. From these results it can be derived, for example, that the BP photosensitized polymerization of styrene is not feasible. Ketyl radicals of BP were found to react relatively slowly with vinyl acetate (5.5 × 10³M^?1sec^?1), acrylonitrile (3.8 × 10³M^?1sec^?1) and methyl methacrylate (9.0 × 10³M^?1sec^?1). Based on these data it was estimated that benzpinacol should not be formed as a major reaction product at relatively low incident light intensities and at monomer concentrations greater than 1 mol/l.     

Kinetic Study of the Photoexcited Triplet State of Free Base Porphyrins by EPR

An electron paramagnetic resonance (EPR) study of the photoexcited triplet state of four free base porphyrins is presented. The zero field splitting parameters (ZFS) |D| and |E| were calculated from the EPR spectra of the porphyrins dissolved in n-octane matrices at 80°K. |D| = 0.0359 cm^?1, |E| = 0.0079 cm^?1 for tetra phenyl porphyrin (H₂ TPP), |D| = 0.0432 cm^?1, |E| = 0.0037 cm^?1 for tetra (per-fluoro) phenyl porphyrin H₂T (per-F) PP, |D| = 0.0366 cm^?1, |E| = 0.0078 cm^?1 for tetra (para-chloro) phenyl porphyrin H₂T(P-Cl)PP, |D| = 0.0369 cm^?1, |E| = 0.0076 cm^?1 for tetra (para-methyl) phenyl porphyrin H₂T(P-Me)PP. The transient behavior of the EPR signal intensities in the last two porphyrins is discussed. The depopulation rate constants of the triplet sublevels k_p, the ratio between the population rate constants A_p (at zero field, p = x,y,z), and the spin lattice relaxation rate W within the triplet manifold, were calculated. k_x = (12 ± 2) × 10² sec^?1, k_y = (0.5 ± 0.1) × 10² sec^?1, k_z = (1.2 ± 0.4) × 10² sec^?1, A_x:A_y:A_z ? 0.63:0.01:0.33, W = (0.4 ± 0.1) × 10⁴ sec^?1 for H₂T(P-Cl)PP, k_x = (7 ± 2) × 10² sec^?1, k_y = (4 ± 1) × 10² sec^?1, k_z = (1.5 ± 0.5) × 10² sec^?1, A_x:A_y:A_z ? 0.56:0.31:0.13, W = (1.7 ± 0.4) × 10³ sec^?1 for H₂T(P-Me)PP.     

Kinetics of absorption of oxygen in aqueous solutions: (1) acidic chromous chloride, (2) acidic titanous chloride and (3) ammoniacal cuprous chloride

The kinetics of absorption of oxygen in aqueous solutions of acidic chromous chloride and ammoniacal cuprous chloride were studied in a stirred cell. Both these reactions were found to be very fast and the theory of gas absorption accompanied by fast pseudo-m^th order reaction was used to analyse the results. The kinetics of absorption of oxygen in aqueous acidic solutions of titanous chloride were investigated in a bubble column where the reaction was found to be free from diffusional resistance.The reaction between oxygen and chromous chloride was found to be first order with respect to oxygen and second order with respect to chromous ion. The value of the third order rate constant for 2·0 M HCl solution was found to be 2·7 × 10¹² (cm³/g mole)² sec^?1 at 30°C.The oxidation of titanous chloride was observed to be zero order with respect to titanous ion and first order with respect to oxygen. The value of the first order rate constant was found to be 8·6 × 10^?3 sec^?1. It has been suggested that in this case hydrolysis precedes the oxidation and it is likely that the reaction of oxygen with the hydrolysed product is the rate controlling step.The absorption of oxygen in aqueous ammoniacal cuprous chloride was found to be third order; first order with respect to each species, namely Cu⁺, oxygen and free ammonia. The value of the third order rate constant was found to be 7·5 × 10¹⁰ (cm³/g mole)² sec^?1. The higher value of the rate constant compared to that for oxidation of acidic solutions of curpous chloride indicates that the presence of a complexing agent such as ammonia increases the rate of absorption of oxygen substantially.     

The Reduction of Ferricytochrome c Studied by Pulse Radiolysis

The formation of ferrocytochrome c upon pulse radiolysis of aqueous solutions of ferricytochrome c has been studied by following spectral shifts at 550 nm and 700 nm. The reaction between ferricytochrome c and hydrated electrons was studied by following the decay of hydrated electrons as well as the ferrocytochrome c formation. The two processes proceed with the same rate which indicate diffusion-controlled processes with a second order rate constant of 1.1 × 10¹¹ M^?1 sec^?1. The reaction between ferricytochrome c and H atoms was studied in a system containing H atoms only and found to be a second order reaction with a rate constant of 1.5 × 10¹⁰ M^?1 sec^?1. The formation of ferrocytochrome c by reaction between ferricytochrome c and OH radicals proceed by a first step with a second order rate constant of 1.4 × 10¹⁰ M^?1 sec^?1 during which OH radicals react with the protein part of ferricytochrome c forming an organic radical. Some of these radicals then undergo intramolecular reduction with a rate constant of 3.1 × 10³ sec^?1.     

Comments on the reactivity of methoxy pentadecyl phenyl isocyanate isomers

The reactions of 2-methoxy-4-pentadecyl phenyl isocyanate and 4-methoxy-2-pentadecyl phenyl isocyanate with excess 2-ethyl hexanol originally reported by Ghatge and co-workers to follow zero order kinetics have been re-examined on the basis of their data and shown to follow more realistically the product catalyzed pseudo first order kinetics. The new rate constant, k_s (sec^?1) for the spontaneous reaction and k_p (li. mole^?1 sec^?1) for the product catalyzed reaction are found to be: k_s = 0.57 × 10^?6 and k_p = 34 × 10^?6 for 2-methoxy-4-pentadecyl phenyl isocyanate and k_s = 1.2 × 10^?6 and k_p = 82 × 10^?6 for 4-methoxy-2-pentadecyl phenyl isocyanate.     

Faradaic rectification studies of the ferrous-ferric redox couple in the acid media

The value of α, the transfer coefficient, determined using varying redox concentration ratios of the ferrous-ferric couple by the extrapolation method in the three acid media is 0·52. the values of α obtained by zero point method are 0·69 in 1·0 N H₂SO₄, 0·58 in 1·0 N HClO₄ and 0·54 in 1·0 N HCl. The values of the rate constant determined by the extrapolation method are of the order of 5 × 10^?2 cm/sec, 10 × 10^?2 cm/sec, 10·5 × 10^?2 cm/sec in the three respective acids. However, the k⁰_a-values obtained by the zero point method differ appreciably and they are found to increase from 1·5 to 3 times in magnitude. It is found that the rate of reaction in HClO₄ is much faster as compared to that in the other two acids.     

Radiation-induced graft copolymerization of styrene to cellulose

The radiation-induced graft copolymerization of styrene to cellulose has been studied in vacuo at 30°C and at dose rates from (0.37 to 8.73) × 10^?2 W/kg. Dioxan was used as solvent for monomer and polystyrene homopolymer, and water (2% total volume) was incorporated as swelling agent for cellulose. The concentration of styrene in the bulk medium was varied from 0.432 to 3.46 moles/l., and the rates of both grafting and homopolymerization were shown to be proportional to [monomer] · [intensity]^1/2. The value of 3.3 × 10^?4 l. mole^?1 sec^?1 derived for k_p²/k_t in homopolymerization is similar to that for normal free-radical polymerization of styrene. However, reduced termination during grafting yielded a much higher value (58 l. moles^?1 sec^?1). Degradation of cellulose in the absence of monomer was followed viscometrically, and values of 13.5 and 24.6 were derived for G (scission) in vacuo and in air, respectively.     

The Kinetics of Complexed and Free Ho2 Radical in the Reaction of Ce+4 and H2O2

The intermediates formed in the reaction of Ce⁺⁴ with H₂O₂ in 0.5 M perchloric acid were studied, spectrophotometrically and through the quenching method, using a stopped-flow system. The spectrum in the UV range and the kinetics of the generated radicals and their dependence on Ce⁺³ concentration were investigated. On adding Ce⁺³ to the mixture, a change in the spectrum as well as in the recombination rate constant was observed. This behaviour was attributed to the formation of a complex between the cerous ion and the HO₂ radical. The values of the recombination rate constant of the complexed radical is 4.0 ± 0.4·10⁶ F^?1 sec^?1 while that of the free HO₂ radical is 0.9 ± 0.1·10⁶ F^?1 sec^?1. The stabilization constant of the complex was found to be 60 ± 18 F^?1.     

Dilithium initiators based on 1,3-bis(1- phenylethenyl)benzene. Tetrahydrofuran and lithium sec-butoxide effects

The adduct (3) from 2 moles of sec-butyllithium with 1,3-bis(1-phenyl-ethenyl)benzene has been prepared and evaluated as a hydrocarbon-soluble, dilithium initiator for butadiene and styrene polymerization in cyclohexane or benzene solution. Under high vacuum conditions with purified reagents, bimodal molecular weight distributions are observed for polybutadienes with M _n < 150 × 10³ g/mol and for polystyrenes with M _n < 50 × 10³ g/mol; monomodal distributions are observed only for higher molecular weights. Addition of tetrahydrofuran ([THF]/[Li] = 14 to 32) or preformed lithium sec-butoxide ([LiOBu]/[3] = 1·1) produces narrow, monomodal molecular weight distributions even at M _n = 26·2 × 10³ g/mol polybutadiene and at M _n = 10 × 10³ g/mol for polystyrene. Added lithium sec-butoxide is the preferred additive since high 1,4-polybutadienes are obtained. These results provide an explanation for the contradictory results reported previously; under less rigorous experimental conditions, oxygen and hydroxylic impurities can form lithium alkoxides in situ.     

Fast Consecutive Intramolecular Processes Involving Protein and Fe(III) in Ferri-Cytochrome-c in Aqueous Solution

The reaction of H atoms (produced by nanosecond pulse radiolysis) with ferri-cytochrome-c in aqueous solution results in the second order formation of an absorption spectrum due to the addition of H atoms to the enzyme protein. This spectrum is not specific to the iron moiety. The rate constant of the addition is 1×10¹⁰M^?1 sec^?1. This is followed by two first order intramolecular processes with specific rates of ～ 1×10⁵ sec^?1 and ～ 2×10⁴ sec^?1. In these processes, specific spectra related to the α and β bands of ferro-cytochrome-c appear. The results are interpreted to show reduction equivalent transfer through the protein to the iron moiety.     

Alkaline hydrolysis of alkylated acidic groups on carbon black

Potentiometric pH-stat titration was performed on alkylated carbon black dispersed in aqueous solution at pH 9.0, 10.0 and 11.0 and alkaline hydrolysis rate constant of the alkylated functional groups on the carbon black was obtained therefrom. Carbon black treated with C₁-C₈n-alcohols in vapor or liquid phase was used in this experiment. Hydrolysis proceeded more rapidly for carbon black treated with lower alcohols. For each alkylated carbon black, the hydrolysis rate constant decreased with time and, after the hydrolysis rate diminished, a hydrolysis resistant fraction of alkylated groups remained. For example, the hydrolysis rate constants at 25°C were 24.33, 17.50 and 4.501 · mol^?1 · sec^?1 at 15 min and 6.00, 6.00 and 3.671 · mol^?1 · sec^?1 at 45 min after the start of hydrolysis at pH 9.0 for carbon black treated with C₂, C₃ and C₄ alcohols respectively.