Comparison of the Kinetics of Electron Transfer in the Diffusion Limit for the Singlet and Triplet Quenching of Eosin Y by Quinones

Electron transfer (ET) rate constants were determined by means of lifetime measurements for the fluorescence quenching and by laser flash photolysis for the triplet quenching of the dye eosin Y by benzoquinones in acetonitrile. The results represent a new aspect of the dependence of the rate constants with the driving force in the diffusion limit region. That is, the rate constants for singlet quenching in the highly negative region of ΔG_et do not decrease as predicted by Marcus theory, but rather show a small positive dependence on the driving force. However, it is found that, in the same free energy range, the triplet rate constants are lower than those for the singlet process. They also increase with the exergonicity of the reaction, but the dependence with ΔG_et is less marked than that found for the singlet reaction. Even at a Gibbs energy change of ?1.0 eV the triplet quenching rate constants do not reach the theoretical diffusion limit. The results are analyzed using the current theories for diffusion‐mediated ET reactions.     

Cross-termination rate constants in copolymerizations of some methacrylates and styrene

Cross-termination rate constants in the copolymerization of some methacrylates with styrene were determined. The rate constants obtained decrease with increasing r₁ and increase with decreasing r₂. The relationship between the cross-termination and propagation rate constants was discussed on theoretical and experimental bases.     

Kinetics of Living Polymerization with Propagation Rate Constants Depending on Degree of Polymerization

A study is presented on the kinetics of living polymerization in which the propagation rate constants decrease to zero at a certain degree of polymerization of the propagating chain. The general solution for the distribution function and the rate of polymerization is given and two special cases are discussed. When all the propagation rate constants are the same up to a critical degree of polymerization and null beyond it, the polymerization proceeds approximately as a normal living polymerization until the number-average degree of polymerization reach 85 to 90% of the critical value. When the propagation rate constants decrease linearly with the degree of polymerization, the distribution of living polymer is narrower than the usual Poisson distribution and the reaction order of the rate of polymerization with respect to monomer concentration is between first and second and is affected by the initial monomer and catalyst concentrations.     

Rate of complex formation for poly-n-vinyl carbazole and n-ethyl carbazole with various electron acceptors

The rates of complex formation for poly-N-vinyl carbazole and its saturated monomer analogue, N-ethyl carbazole, with various electron acceptors (chloranil, trinitrobenzene and picric acid) have been investigated. In mole ratios 1:1, the complex forming reactions in chloroform proceed rapidly, as measured by a “stopped flow” method; the rate constants are of the order of 10³ 1. mole^?1 sec^?1. The rate constants are smaller than those of the complexes with tetracyanoethylene as acceptor and decrease with decreasing electron affinity of the acceptor. The rate constants also depend on the molecular weight of the donor.     

Effect of Some Solvents on the Radical Polymerization of Methyl Methacrylate Studied by Differential Scanning Calorimetry

The course of the isothermal polymerization of methyl methacrylate in different concentrated solutions in toluene, n-butyl acetate, cyclohexanone, and dimethylformamide was measured at four temperatures by differential scanning calorimetry. The conversions at the sharp increase of the reaction rate were found to increase with dilution of the reaction mixture. The polymerization enthalpies and the composite rate constants were calculated. The polymerization enthalpies seem to be solvent independent. The composite rate constants for polymerization in butyl acetate are lower and those for polymerization in toluene are equal to the constants for bulk polymerization. They are independent of the concentration of the reaction system. The constants for polymerization in cyclohexanone and dimethylformamide are, however, concentration dependent. An interrelation between the composite rate constants and solubility parameters of the solvents and methyl methacrylate was found. The relative molecular weight averages decrease with decreasing concentration of the reaction mixture. The MW distributions were very broad.     

The complexation of protonated peptides with saccharides in the gas phase decreases the rates of hydrogen/deuterium exchange reactions

Gas-phase noncovalently bound complexes are probed by hydrogen/deuterium exchange. The complexes, composed of a protonated amino acid and a monosaccharide, are investigated to observe the effects of complexation on the rates of exchange. Rate constants are determined and compared for complexed and uncomplexed amino acids. The overall rate constant, which corresponds to exchange of a specific number of hydrogens, is deconvoluted to yield site-specific rate constants. Complexation of amino acids with saccharides significantly decreases the rate constants of the exchange. Results of molecular orbital calculations are provided to explain the decrease in the rates.     

Hydrolysis of N-Cyanoazoles

The hydrolysis kinetics of N-cyanoazoles in alkaline solutions is described by a first-order kinetic equation with respect to the substrate and concentration of hydroxide ions. The hydrolysis rate constants increase with increasing number of nitrogen atoms in the heterocyclic moiety and decrease with introduction of the annelated benzene ring.     

Spectrophotometric Determination of the Stability Constant of the Inclusion Complexes of Some Catechol Derivatives with β‐Cyclodextrin Based on Their Reaction with Iodate

The effects of β‐cyclodextrin (β‐CD) inclusion on the kinetics of the oxidation of several cathechol derivatives, including 4‐tert‐butylcatechol, 3‐methylcatechol and 3‐methoxycatechol, with iodate was studied spectrophotometrically. The rate of the oxidation reactions decreased by increasing β‐CD concentration as a result of inclusion. The stability constants for the inclusion complexes of the investigated compounds were determined based on the changes in the rate constants as a function of β‐CD concentration at pH 3.0. The rate constants for the free and complexed forms and also the stability constants for the inclusion complexes were calculated. The role of the hydrophobic effect was evaluated by studying the influence of the presence of different amounts of ethanol on the β‐CD: guest interaction. In a given H₂O‐EtOH mixture the stability of β‐CD complexes shows the order of 4‐tert‐butylcatechol ≈ 3‐methylcatechol ≈ 3‐methoxycatechol. Increasing ethanol content caused a decrease in the stability constant of the inclusion complexes and an increase in observed rate constants.     

Position dependent heavy atom effect in physical triplet quenching by electron donors

The radical yields and rate constants in the quenching reaction of thionine triplet with the complete series of monohalogen substituted anilines as electron donors were determined by flash spectroscopy. Whereas the quenching rate constants show little and unsystematic variation, the radical yields decrease with increasing spin—orbit coupling constant of the halogen substituent. This effect is very sensitive to the position of the halogen in the donor. The results are explained in terms of a heavy atom effect on the intersystem crossing rate constant in a triplet exciplex.     

Theoretical dynamic studies on the reactions of CH3C(O)CH3-nCl(n) (n = 0-3) with the chlorine atom

The theoretical investigations were performed on the reaction mechanisms for the title reactions CH(3)C(O)CH(3) + Cl --> products (R1), CH(3)C(O)CH(2)Cl + Cl --> products (R2), CH(3)C(O)CHCl(2) + Cl --> products (R3), and CH(3)C(O)CCl(3) + Cl --> products (R4) by ab initio direct dynamics approach. Two different reaction channels have been found: abstract of the H atom from methyl (--CH(3)) group or chloromethyl (--CH(3-n)Cl(n)) group of chloroacetone and addition of a Cl atom to the carbon atom of the carbonyl group of chloroacetone followed by methyl or chloromethyl eliminations. Because of the higher potential energy barrier, the contribution of addition-elimination reaction pathway to the total rate constants is very small and thus this pathway is insignificant in atmospheric conditions. The rate constants for the H-abstraction reaction channels are evaluated by using canonical variational transition state theory incorporating with the small-curvature tunneling correction. Theoretical overall rate constants are in good agreement with the available experimental values and decrease in the order of k(1) > k(2) > k(3) > k(4). The results indicate that for halogenated acetones the substitution of halogen atom (F or Cl) leads to the decrease in the C--H bond reactivity and more decrease of reactivity is caused by F-substitution.     

Effect of borate buffer on the photolysis of riboflavin in aqueous solution

The photolysis of riboflavin (RF) in the presence of borate buffer (0.1-0.5M) at pH 8.0-10.5 has been studied using a specific multicomponent spectrophotometric method for the determination of RF and photoproducts, formylmethylflavin (FMF), lumichrome (LC) and lumiflavin (LF). The overall first-order rate constants for the photolysis of RF (1.55-4.36 x 10(-2)min(-1)) and the rate constants for the formation of FMF (1.16-3.52 x 10(-2)min(-1)) and LC (0.24-0.84 x 10(-2)min(-1)) have been determined. The values of all these rate constants decrease with an increase in buffer concentration suggesting the inhibition of photolysis reaction by borate species. The kinetic data support the formation of a RF-borate complex involving the ribityl side chain to cause the inhibition of photolysis. The second-order rate constants for the borate inhibited reaction range from 1.17-3.94 x 10(-2)M(-1)min(-1). The log k-pH profiles for the reaction at various buffer concentrations indicate a gradual increase in rate, with pH, up to 10 followed by a decrease in rate at pH 10.5 probably due to ionization of RF and quenching of fluorescence by borate species. A graph of second-order rate constants against pH is a sigmoid curve showing that the rate of photolysis increases with an increase in pH. The results suggest the involvement of excited singlet state, in addition to excited triplet state, in the formation of LC.     

Internal Surface Coating of a Capillary Microreactor for the Selective Hydrogenation of 2-Methyl-3-Butyn-2-Ol Using a PdZn/TiO<Subscript>2</Subscript> Catalyst. The Effect of the Catalyst’s Activation Conditions on Its Catalytic Properties

Finely divided polymer-stabilized PdZn bimetallic nanoclusters are prepared by the polyol method. TiO₂ matrix-impregnated nanoclusters coated on the inner surface of a capillary microreactor are used as catalysts for the selective hydrogenation of 2-methyl-3-butyn-2-ol. The effect of the activation conditions (duration, temperature, and gas medium composition) on the physicochemical and catalytic properties of the coatings is studied. It is shown that their catalytic activities decrease and the reaction’s selectivity increases with an increase in the reaction time and the time of hydrogen reduction at 573 K. The highest selectivity (96.5% at a conversion rate of 99%) is reached on the coatings reduced with hydrogen for 6 h. The coatings remain highly active and stable for 1 month in the continuous flow mode of the reaction. Kinetic simulation showed that a high selectivity level is ensured by a decrease in the rate constants of hydrogenation of 2-methyl-3-buten-2-ol and the ratio of the alkene/alkyne adsorption constants after reductive treatment.     

光致噻吨酮与胺类、酚类、醇类的电子转移和氢转移反应

利用激光闪光光解方法研究了一系列胺类、酚类、醇类在脱氧乙腈中猝灭噻吨酮(TX)三重态的反应,得到了相应的瞬态吸收光谱和猝灭速率常数(kq).通过对光谱演变特性的分析,推断出三重态噻吨酮与不含有活泼氢的胺发生了电子转移反应,与含有活泼氢的胺发生了电子-质子转移反应.三重态噻吨酮与酚类、醇类反应中观察到噻吨酮加氢自由基的生成,据此推断出三重态噻吨酮与酚类、醇类发生了氢转移反应.胺类的猝灭速率常数随着反应自由能变(ΔG)的增大而减小,说明电子转移影响了噻吨酮三重态的猝灭.酚类的猝灭速率常数先随ΔG增大而减小,后随酚阳离子的酸性增强逐渐增大,可能是猝灭过程中电子转移影响减弱的同时氢转移影响逐渐增强.醇类的猝灭速率常数随着醇的α-C—H键能的增大而减小,说明α-C—H键能是影响噻吨酮三重态猝灭的关键因素.比较以前研究的胺类、酚类、醇类与三重态呫吨酮(XT)、芴酮(FL)反应的结果可知,由于分子结构差异性的影响,相关的猝灭速率常数按照呫吨酮、噻吨酮、芴酮的顺序逐渐减小.     

A laser photolysis study of the triplet exciplexes of quinones with 4-phenylaniline

Conclusions The rate constants were measured for the annihilation of the triplet exciplexes of quinones with 4-phenylaniline in various solvents and the prototropic equilibrium constants in the primary exciplex were also determined. Hydrogen bonding between the radicals in the exciplex leads to an acceleration of radiationless deactivation of the exciplex to the ground state and retardation of the dissociation of the exciplexes into radical-ions. The solvation of the exciplexes of an alcohol is accompanied by a decrease in the rate of deactivation of the exciplexes to the ground state.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2587–2590, November, 1986.     

Activation parameters of electron-transfer reactions with synchronous cleavage of a bond

The temperature dependence of the rate constants of the disproportionation reactions of verdazyl radicals (R') in their complexes with zinc ions and of the inner-sphere oxidation of copper(I) by tetraethylthiuram disulfide (tds) in acetonitrile and mixtures of the latter with benzene has been measured. The activation parameters of the reactions studied have been determined according to the Arrhenius equation. In these reactions electron transfer is accompanied by the synchronous cleavage of a chemical bond [Zn-R in the complex Zn(R')₂ ²⁺ or S-S in tds], which is manifested by a decrease in the value of the preexponential factor. It has been established that the dependence of the rate constants on the polarity of the medium does not conform to the framework of Marcus' theory, but the activation energy and the preexponential factor decrease symbatically and approximately exponentially as the mole fraction of the polar molecules of acetonitrile in its mixtures with benzene is increased.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 4, pp. 501–504, July–August, 1988.     

Kinetics of N-Acylation of Glycine and L-Proline with 4-Nitrophenyl 4-Nitrobenzoate in Aqueous-Organic Solutions

Kinetic characteristics (rate constants and activation energies and entropies) are determined for N-acylation of glycine and L-proline with 4-nitrophenyl 4-nitrobenzoate in water-acetonitrile, water-2-propanol, and water-2-methyl-2-propanol systems. With increasing water content in the system, the N-acylation rate constants increase and the activation energy and entropy decrease, which is due to features of solvation of the reagents and activated complex with aqueous-organic solvents.     

Kinetics of ethylbenzene oxy-chemiluminescence in the presence of antioxidants from tissues of the marine invertebrate Eupentacta fraudatrix: Estimating the concentration and reactivity of the natural antioxidants

Extracts from tissues of the holothuria Eupentacta fraudatrix suppress the oxy-chemiluminescence of ethylbenzene in chlorobenzene initiated by azobisisobutyronitrile thermolysis and enhanced by energy transfer to 9,10-dibromoanthracene. The decrease of the chemiexcitation rate in the reaction between peroxyl radicals is caused by radical interception by the antioxidants contained in the extracts. The concentrations of the antioxidants in the extracts and the rate constants of their reactions with peroxyl radicals have been estimated from chemiluminescence kinetics.     

Kinetics and equilibria of dialkylthallium(III) complexes of some thiacrown-ethers in acetonitrile

The rate constants k₁ and activation enthalpies H of complex formation of dialkylthallium(III) perchlorates with some thia-18-crown-6 ethers were determined in acetonitrile. The equilibrium constants K were also obtained. More symmertrical thiacrown-ethers give smaller k₁ and K values, which are mainly regulated by the entropy term rather than the enthalpy term. Diethylthallium(III) ion gives smaller rate constants than those of dimethylthallium(III) ion. The K values of these complexes decrease as oxygen of the 18-crown-6 was replaced by sulfur.     

Quantum state-to-state rate constants for the rotationally inelastic collision of CH(B2Sigma(-), nu=0, N-->N') with Ar

We have calculated the state-to-state integral cross sections and rate constants for the rotationally inelastic collision of CH(B (2)Sigma(-), nu=0, N-->N') with Ar using the quantum coupled-state and close-coupling methods on an ab initio potential-energy surface constructed by Alexander et al. [J. Chem. Phys. 101, 4547 (1994)]. Overall the calculated rate constants are in good agreements with the three available experimental results. The rate constants are comparable to the usual gas kinetic and decrease with increasing N and DeltaN. For the multiquantum transition cases, the theory underestimates the experiment. We discuss some possible causes to the discrepancies among the theory and the experiments.     

Kinetics of radical polymerization. XXX. Polymerization of styrene in solution

A study was made on the AIBN-initiated polymerization of styrene in benzene and dimethyl formamide at 50°C. The overall rate and the rate of initiation of the polymerization were determined and the number-average and weight-average molecular weights of the polymers formed were measured. The decrease in the rate and degree of polymerization with the decrease in styrene concentration was caused by the corresponding decreases in the rate constants of chain propagation and initiation steps. In the systems studied no chain transfer process occurred with an experimentally measurable rate.