Detailed equilibrium principle in reversible bimolecular diffusion-controlled reactions

The expressions for the observed rate constants of the forward bimolecular and back monomolecular reactions have been obtained using the boundary conditions imposed upon the reaction surfaces of each reagent particle. The rate, equilibrium and thermodynamical parameters of the back dissociation reactions of phenoxy radical dimers have been measured. The resultant values of k^obs_?1 as a function of solvent viscosity are accurately described by theoretical relationships obtained in the present work.     

Computation of equilibrium oxidation and reduction potentials for reversible and dissociative electron-transfer reactions in solution

Equilibrium free-energy cycles relating oxidation and reduction potentials in solution to ionization potentials and electron affinities in the gas phase are constructed and the utilities of various levels of theory for computing particular free-energy changes within these cycles are discussed within the context of several examples. Emphasis is placed on the use of quantum-mechanical continuum solvation models to compute free energies of solvation. Key systems discussed include quinones, substituted anilines, substituted phenols, and reductive dechlorination reactions.Dedicated to Prof. Jean-Louis Rivail, whose pioneering efforts in developing and exploiting continuum solvent models were critical in making quantum chemistry more applicable to solution phenomenaProceedings of the 11th International Congress of Quantum Chemistry satellite meeting in honor of Jean-Louis Rivail     

Long chain branching on linear polypropylene by solid state reactions

A method was developed for the long chain branching (LCB) of isotactic polypropylene (iPP) via modification in the solid state. PP long chains have been linked as branches to the original linear iPP chains using solid state reactions in the presence of a free radical initiator and a multifunctional monomer (co-agent). The modified samples of branched iPP were characterised by gel permeation chromatography (GPC) and rheological measurements. Several methods were applied in order to estimate indirectly the extent of branching. A ranking was made of the co-agents according to their ability in inducing LCB as opposed to cross-linking and degradation of iPP. The furfuryl sulphide (FS) showed the highest efficiency for the branching reaction, while the divinylbenzene (DVB) is not suitable for branching.     

Estimation of kinetic parameters of reversible chain reactions of quinoneimines with hydroquinones having self-acceleration periods

A new approach is suggested for determining the kinetic parameters and rate constants of the elementary steps of reversible chain reactions having self-acceleration periods due to the long time required for the concentrations of the chain-carrier radicals to reach their steady-state values. This approach is illustrated by the example of the reversible chain reaction between N,N′-diphenyl-1,4-benzoquinonediimine and 2,5-dichlorohydroquinone in chlorobenzene. The disappearance rate of one of the initial reactants, N,N′-diphenyl-1,4-benzoquinonediimine, at the inflection point of its disappearance curve, is considered as the basic kinetic characteristic of the reaction. The empirical function y = aexp(bt ^c ) + d, where a, b, c, and d are the fitted parameters (b < 0, c > 1), is suggested for approximating the S-shaped kinetic curves and for calculating the reaction rate. The rate constants of the elementary steps are preferably derived from experimental data obtained at equal concentrations of the initial reactants, and also product additions when their effect on the reaction rate is studied. The effective rate constant of chain termination is derived from the time to reach the steady state. The results obtained in this way are compared with earlier data obtained using the “initial” reaction rates calculated by means of exponential approximation of portions of N,N′-diphenyl-1,4-benzoquinonediimine disappearance curves after the inflection point.     

The future of reversible addition fragmentation chain transfer polymerization

We examine the reversible addition fragmentation chain transfer (RAFT) process with regard to its potential and limits in future industrial applications (including those conducted on a larger scale) as well as materials science. The outlook for the RAFT process is bright: Its unrivaled inherent process simplicity coupled with a wide tolerance to monomer classes and functionalities makes it a prime candidate for the use in large reactors. At the same time, it allows for ready access to complex macromolecular architectures of variable shape and size. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5715–5723, 2008     

Thermoanalytical studies of the kinetic of reversible reactions

A method is described for determination of the kinetic parameters of reversible reactions under isothermal and non-isothermal conditions in thermal analysis. Reversible reactions of the first and second orders and of mixed types are considered. The kinetic parameters of the forward and reverse reactions are determined on the basis of the initial integral data of the thermoanalytical experiment. The results of processing a computerized experiment demonstrated that the suggested method is satisfactorily applicable.

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Zusammenfassung Es wurde eine Methode zur Bestimmung kinetischer Parameter von reversiblen Reaktionen unter isothermen bzw. nicht-isothermen Umständen in der Thermoanalyse beschrieben. Umkehrbare Reaktionen erster Ordnung, zweiter Ordnung sowie Reaktionen vom Mischtyp wurden betrachtet. Die kinetischen Parameter der Hin- bzw. Rückreaktionen wurden auf der Grundlage der Anfangsintegrale der termoanalytischen Experimente bestimmt. Die Ergebnisse eines computermodellierten Experimentes zeigen, dass die vorgeschlagene Methode befriedigend angewendet werden kann.

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Living polymerization with reversible chain transfer and reversible deactivation: The case of cyclic esters

Polymerization of cyclic esters at the properly chosen conditions can be treated as living polymerization, in agreement with the tentative definition of the Nomenclature Commission of IUPAC (Macromolecular Division) requiring that no irreversible transfer or irreversible termination take place. For these processes the most kinetic or structural (end group) studies do not reveal any deviation. However, since in these polymerizations reversible transfer to backbones of macromolecules and/or reversible deactivation take place, the molar mass distribution can be Poissonian only at certain conditions. These processes have been studied quantitatively and the corresponding rate constants were determined. Thus, the importance of these processes could be established by comparing the rate constants of transfer and/or deactivation with rate constants of propagation. In this way, polymerizations of cyclic esters were used to illustrate the meaning and scope of the definition of “living polymerization”, a process from which irreversible transfer and deactivation are absent and in which living polymers are formed, i. e. composed of macromolecules that do not irreversibly loose their ability to grow.     

New regularities and an equation of state for liquids

Three regularities have been introduced for liquids (T < T_C and ρ > ρ_C) based on average potential energy. The experimental data have been used to show the validity of the regularities. First, there exists near-linearity relation between and ρ for all isotherms of a liquid, where P_i and ρ are internal pressure and density, respectively. Second, changes linearly with ρ for each isotherm of any liquid, where Z and V_m are compressibility factor and molar volume, respectively. Third, a new regularity using the definition of bulk modulus and our new equation of state between reduced bulk modulus and density has been introduced, that is versus ρ must be linear for all isotherms of a liquid where B_r is the reduced bulk modulus.

A new equation of state has been also derived. The density of some liquids in the extensive ranges of temperature and pressure has been calculated using the new equation of state. The densities calculated from this equation agree with experiment to better than 0.3%. The new equation of state can predict internal pressure, thermal expansion coefficient, and isothermal compressibility of liquids within experimental error.     

Polysulphides reversible faradaic reactions in supercapacitor application

The electrochemical performance of polysulphide compounds as an electrolyte in supercapacitor was studied. Numerous reversible faradaic reactions during which oxidation state of sulphur changes from − 2 to − 0.4 work as an efficient source of faradaic reactions. Polysulphides were synthesized by a simple, inexpensive method. This innovative system offers quite high values of specific capacitance, more than twice as high as electrolyte without polysulphides. In addition, cyclability (5000 cycles) does not influence the capacitance of the electrochemical capacitor when polysulphide compounds are used as the electrolyte.     

Studies of crotyl anion reactions and some regularities of copolymerization in hexamethylphosphortriamide

Cis-trans isomerization of butenes in free-anion systems proceeds through the α-butene intermediate. Direct cis- and trans-transformation of crotyl anions does not occur. Reactions of crotyl anion and its ion pairs with proton-donor compounds in hexamethylphosphortriamide (HMPTA) are accompanied by the formation of an equilibrium mixture of butenes containing 97 per cent β-butenes. The addition of these active species to alkylene oxides and sulphides, cyclohexadiene-1,3, styrene and 2,3-dimethylbutadiene occurs with the participation of the terminal carbon atom of the crotyl active centre, resulting in the predominant formation of 1,4-units. It has been shown that the reactivity ratios r₁ and r₂ for the pairs butadiene-styrene, butadiene-2,3-dimethylbutadiene and butadiene-cyclohexadiene-1,3 become very close in HMPTA.Studies of copolymer microstructure reveal an increase of butadiene 1,4-units content paralleling a rise of second monomer content.     

Corrections to the law of mass action and properties of the asymptotic t=infinity state for reversible diffusion-limited reactions

For diffusion-limited reversible A+A<==>B reactions we reexamine two fundamental concepts of classical chemical kinetics-the notion of "chemical equilibrium" and the "law of mass action." We consider a general model with distance-dependent reaction rates, such that any pair of A particles, performing standard random walks on sites of a d-dimensional lattice and being at a distance mu apart of each other at time moment t, may associate forming a B particle at the rate k+(mu). In turn, any randomly moving B particle may spontaneously dissociate at the rate k-(lambda) into a geminate pair of As "born" at a distance lambda apart of each other. Within a formally exact approach based on Gardiner's Poisson representation method we show that the asymptotic t=infinity state attained by such diffusion-limited reactions is generally not a true thermodynamic equilibrium, but rather a nonequilibrium steady state, and that the law of mass action is invalid. The classical concepts hold only in case when the ratio k+(mu)k-(mu) does not depend on mu for any mu.     

Experimental evidence for a highly reversible excited state equilibrium between s-cis and s-trans rotational isomers of 2-methoxynaphthalene in solution

Detailed studies on the kinetics and the thermodynamics of the excited-state torsional isomerization of the title molecule (1) relative to exocyclic C2-O bond, when dissolved in 3-methylpentane, are reported by means of nontime- and time-resolved fluorescence spectroscopy. Over the broad temperature range studied, 1 exists in spectrally distinct, thermally equilibrated s-cis and s-trans conformations in the ground state (S(0)). In the lowest excited singlet state (S(1)) and above 260 K a pure adiabatic interconversion channel is activated that interconverts s-cis* and s-trans* conformers through a nearly fully reversible isomerization pathway with an activation energy of about 29 kJ/mol. The excited-state equilibrium constant is found to be remarkably temperature-independent just barely exceeding 1 above 260 K. Contrary to the predominantly irreversible photoisomerization mechanism generally observed in related compounds, this work provides insights into the high reversibility of an excited-state rotameric equilibration in solution.