Kinetics of partly diffusion controlled reactions

We propose a simple kinetic model in which two types of processes between two reactants occur: diffusional processes for distances greater than , the reaction distance, and static quenching for distances smaller than . This model might be useful in the studies of diffusion in membrane systems.

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An alternative model for anomalous water

An alternative model for the structure of anomalous water is proposed, in which all hydrogen atoms are involved in hydrogen bonds. Such a model seems to agree well with the main experimental properties of that substance. In particular, IR and Raman spectra can be estimated in agreement with the measurements of Lippincott and coworkers.     

Kinetics of partly diffusion controlled reactions. I. Transient and apparent transient effect in fluorescence quenching

Analysis of decay curves of electronically excited molecules A* versus time in presence of quencher B leads to the determination of kinetic data for the reaction (i.e. for diffusion limited reactions, the experimental collisional distance σ′ greater than the true collisional distance σ) and the sum D of diffusion coefficients of both reactants). Experimental fluorescence steady state measurements (Stern-Volmer representation) are inconsistent with calculated curves using classical Smoluchowski's model with σ′and D. The difference between this has been interpreted by complex formation in the ground state between A and B. But, this assumption is unnecessary if we take into account a “static” quenching arising from B molecules located at distances between σ and σ′ from A*. A theoretical model based upon this principle is described; good agreement between the model and experimental results was obtained.     

An electrodynamics-based model for ion diffusion in microbial polysaccharides

An electrodynamics-based model was formulated for simulation of ion diffusion in microbial polysaccharides. The fixed charges and electrostatic double layers that may associate with microbial polysaccharides and their effects on ion diffusion were explicitly built into the model. The model extends a common multicomponent ion diffusion formulation that is based on irreversible thermodynamics under a zero ionic charge flux condition, which is only applicable to the regions without fixed charges and electrostatic double layers. An efficient numerical procedure was presented to solve the differential equations in the model. The model well described key features of experimental observations of ion diffusion in negatively charged microbial polysaccharides including accelerated diffusive transport of cations, exclusion of anions, and increased rate of cation transport with increasing negative charge density. The simulated diffusive fluxes of cations and anions were consistent with a cation exchange diffusion concept in negatively charged polysaccharides at the interface of plant roots and soils; and the developed model allows to mathematically study such diffusion phenomena. An illustrative example was also provided to simulate dynamic behavior of ionic current during ion diffusion within a charged bacterial cell wall polysaccharide and the effects of the ionic current on the compression or expansion of the bacterial electrostatic double layer at the interface of the cell wall and bulk solution.     

Kinetics of partly diffusion controlled reactions. III. Competition between physical and chemical kinetics. Experimental results

In a previous paper (II in this series), we showed that the kinetic parameters of a partly diffusion controlled reaction could be calculated when the p Three examples of fluorescence quenching of electronically excited states were studied: pyrene by biacetyl; sulforhodamine G by I^?; and POPOP by fluorescence and fluorescence polarization measurements. They illustrate the kinetic models derived in papers I and II of this series.     

Kinetics of partly diffusion controlled reactions. II. Competition between physical and chemical kinetics. Theoretical treatment

We propose a quantitative method for computing the time-dependent value of the apparent rate constant of a bimolecular reaction in the liquid phase in The model proposed permits one to determine the time-dependence of the apparent rate constant of a bimolecular process in the case where a competition The results are necessary for the understanding of the deactivation of excited electronic states by various quenchers. As a matter of fact, it appears that in numerous experimental cases, such a competition is physically significant.     

Microviscosity and wavelength effects on radical cage pair recombination

This study probed two aspects of the reactivity of geminate radical cage pairs formed by photolysis of (Cp′ = η⁵-C₅H₄CH₃). The first aspect studied examined whether the bulk viscosity has any predictive power in determining the magnitude of the cage recombination efficiency (F_cP). Although there is a clear relationship between the magnitude of F_cP and viscosity for systems where the bulk viscosity of the solution is altered by the addition of a non-macroscopic viscosity enhancer, the relationship is unclear for systems where the bulk viscosity is altered using polymeric viscosity enhancers. For this investigation, F_cP values were measured using femtosecond pump-probe transient absorption spectroscopy. The results clearly indicate that bulk viscosity can change drastically without affecting F_cP in systems containing small amounts of added polymers. The bulk viscosity is thus a poor parameter for predicting the cage effect in such systems. The second investigation looked at the effect of the photochemical excitation energy on F_cP for the [Cp′(CO)₃Mo, MoCp′(CO)₃] cage pair in hexane. The results showed that F_cP increased when the wavelength of irradiation was changed from 546 nm to 436 nm, and then remained constant as the wavelength of irradiation was changed from 436 nm to 404 nm to 366 nm. These results are somewhat surprising because the recombination efficiencies for diatomic and triatomic molecules have been shown to decrease monotonically with increasing excitation energy. Two explanations are offered for the reverse wavelength dependence observed in this study. The first explanation invokes the different dynamic behaviors of the two excited states involved at the selected wavelengths, and the second invokes the different speeds of radical separation following irradiation at the selected wavelengths.     

Kinetics of partly diffusion controlled reactions. X. Use of a high pressure cell in fluorescence quenching experiments

The increase of the pressure over a liquid results in an increase of its viscosity, which makes possible the study of the mechanism of diffusion-controlled, or not, fluorescence quenching processes. This technique has been applied to the quenching of the fluorescence of two substituted anthraquinones by N,N-dimethyl-p-toluidine in acetonitrile solution.

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Kinetics of partly diffusion controlled reactions, XVIII. The use of liquid state physics concepts for bimolecular quenching models in bidimensional space

Theory of diffusion controlled reaction kinetics is reinvestigated, by taking into account results of pseudobidimensional liquid physics (as in the case in membranes), and leads to important changes on the apparent rate constant of reaction.

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Relevance of cage recombination in the plastic deformation of polymers

For a polymer in which permanent rupture of individual molecules is the rate-limiting process for plastic deformation, the kinetics of chain-end diffusion and secondary radical reactions should be compared with the kinetics of caged radical recombination in the calculation of activation parameters for plastic deformation. If mechanisms of cage escape are slower than those for cage recombination, the activation parameters for plastic deformation will differ from those for the initial bond-breaking process. For the case of polyethylene deformed in the vicinity of 250°K, the critical thermally activated event appears to involve scission of the polymer molecule near the site of an abstracted hydrogen atom. For this system the dominant cage-escape mechanism is diffusion, which is faster than either hydrogen abstraction or unzipping to the monomer. However, at low stresses the rate of cage recombination is expected to be higher than the rate of cage escape, so that the activation parameters for deformation should be the sum of those for chain scission and diffusion. The contribution of diffusion (ca. 15 kcal/mole) to the activation energy for deformation (E^*, extrapolated to zero stress conditions) is relatively modest. However, the calculated molar activation volume for deformation V^* increases by almost an order of magnitude, i.e., from ca. 10 to ca. 76 cm³/mole when diffusion is required. Consideration of experimental values of E^* and V^* for high molecular weight polyethylene indicates that, in the regime examined, chain scission plus chain-end diffusion is required to effect plastic deformation.     

Au42: an alternative icosahedral golden fullerene cage

We present a new icosahedral gold fullerene, Au42, based on density functional theory calculations. The Au42 fullerene has a nanoscale hollow space that can hold up to 13 Au atoms. The Au42 fullerene also has a larger HOMO-LUMO gap compared to the compact-filling geometries. However, unlike the known gold fullerene Au32, the Au42 fullerene does not satisfy the 2(N + 1)2 aromatic rule and has a positive NICS value at the center of the cage. These two nanometer-sized gold fullerenes can be used as golden cages to accommodate other atoms or molecules for the purpose of studying fundamental chemistry because of their apparently dissimilar chemical characteristics, or they can be used as structural motifs to build highly stable core-shell nanoclusters or novel cluster-assembled materials.     

CO cage recombination in hemoglobin: Picosecond photolysis and nanosecond observation

Carboxy hemoglobin in aqueous solution was photodissociated by laser pulses of 30 ps at 532 nm. Kinetic studies show that only upon complete photodissociation can the pure CO geminate binding process be revealed. The protein region of the iron cage, where the geminate ligand diffuses before reaching the heme, is smaller than a single subunit and larger than the heme pocket.     

A discrete stochastic model of diffusion controlled reactions. The rate constant

Diffusion controlled chemical reaction is modelled as random walk on a cubic lattice. The rate coefficient is shown to depend on the spatial distribution of the reactants.

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A push-and-pull model for allosteric anion binding in cage complexes

A series of electronic structure calculations has been carried out on an artificial anion binding host. The compound with four Pd(ii) cations and a total of eight bis-monodentate pyridyl ligands forms by self-assembly an interpenetrated double cage with three binding pockets. Through the use of a simple push-and-pull model connecting the potentials of the different sites, we are able to explain the allosteric effect observed in anion binding. Two factors seem to be particularly significant in the latter, namely the flatness of the potential in each binding pocket as well as the length of the ligand. Our results are found to be in excellent agreement with the experimentally observed structures.     

Two-dimensional diffusion model for monomolecular decomposition

We have considered the introduction of a two-dimensional diffusion equation in the theory of the monomolecular decomposition of complex molecules with a number of atoms N 3, the decomposition being described by the motion of the molecular system in the space of the reaction coordinate and the energy. An approximate solution of this equation has been used to obtain the equations of the classical theory of monomolecular decomposition and to estimate the rate constants in the limiting cases of strong and weak friction.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 1, pp. 20–26, January–February, 1989.     

Domain model for anomalously fast diffusion

Electrostatic calculations are performed in idealized β-Al₂O₃, containing domains with displaced sodium ions. The average additional energy per sodium ion is calculated for several sizes and shapes of the domains. The movement of the domain walls is considered to be essential in the process of anomalously fast conduction. The electrostatic energy contribution to the activation energy for wall movement is calculated. Some rules are formulated which might permit the selection of possible super ionic conductors by electrostatic calculations in ionic compounds.     

Kinetics of partly diffusion controlled reactions, IX. Cage effect in the photolysis of benzyl chloride in the liquid phase. First results

The cage recombination of free benzyl and chloride radicals after their formation via photolysis of benzyl chloride is interpreted using a model for partly diffusion controlled reactions. A new kinetic model is presented which takes into account the noncontact of the interacting species at the moment of their formation. There is good agreement between the results observed and the model presented.

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An expanded cavity hexaamine cage for copper(II)

The crystal structure of the bicyclic hexaamine complex [Cu(fac-Me5-tricosane-N6)](ClO4)2.H2O (fac-Me5-tricosane-N6 = facial-1,5,9,13,20-pentamethyl-3,7,11,15,18,22-hexaazabicyclo[7.7.7]tricosane) at 100 K defines an apparently tetragonally compressed octahedral geometry, which is attributed to a combination of dynamic interconversion and static disorder between two tetragonally elongated structures sharing a common short axis. This structure is fluxional at 60 K and above as shown by EPR spectroscopy. Aqueous cyclic voltammetry reveals that a remarkably stable Cu(I) form of the complex is stabilised by the encapsulating nature of the expanded cage ligand.     

An analysis of nonlinear ion transport model including diffusion and migration

The nonlocal identification problem related to nonlinear ion transport model including diffusion and migration is studied. Ion transport is assumed to be superposition of diffusion and migration under the influence of an electric field. Mathematical modeling of the experiment leads to an identification problem for a strongly nonlinear parabolic equation with nonlocal additional condition. Uniqueness of the nonlinear direct problem solution, and its continuity with respect to the total charge function is proved. An existence of a quasisolution of the identification problem is proved in the class of derived admissible coefficients. The nonlinear finite difference approximation of this problem, with an appropriate iteration algorithm, is derived. Numerical solutions of the identification problem are presented for various values of valences and diffusivities of oxidized and reduced oxidized species. The obtained results permits one to derive behaviour of the concentration and total charge depending on physical parameters.