Rate Constants Of Diffusion-controlled Reactions Research Articles

A Molecular Dynamics simulation study on nitrobenzene and [rad]OH radical in supercritical water

With Molecular Dynamics simulation method, the diffusion of nitrobenzene and OH radical in supercritical water was studied at the temperature of 643.2K–843.2K and under the pressure of 23MPa–29MPa. The diffusion-controlled reaction rate constant was calculated. Thus, guidance for reaction kinetics study on the nitrobenzene oxidation in supercritical water process can be provided. Additionally, the microstructure of supercritical water-nitrobenzene system under different conditions was investigated. The deviation of the calculation results of infinite dilute diffusion coefficients obtained from the data processing method in this work is within 9%, compared with experimental values in the literatures. With MD simulation method, at the temperature of 643.2K–823.2K and under the pressure of 23MPa–29MPa, the infinite dilute diffusion coefficients of nitrobenzene and OH radical in supercritical water were obtained. Based on the infinite dilute diffusion coefficients of nitrobenzene and OH radical in supercritical water, the rate constant of the diffusion-controlled reaction was calculated.

Mobility of Molecules and Ions Solubilized in Protein Gels: Diffusion in the Thick Fraction of Hen Egg White

The thick fraction of hen egg white is a protein hydrogel with an immeasurably high viscosity composed of ∼90% water that can serve as a model system for mammalian mucous membrane. Measurements of the rate constants of diffusion-controlled reactions occurring within the gel (and corresponding activation energies) and electric conductivity revealed that the thick fraction of egg white can be envisioned as a 3D network comprising hydrated protein molecules (held by intermolecular S-S bridges) surrounded by water pools and channels (of nonuniform diameters) that have a microviscosity that is very similar to that of bulk water. This was corroborated by differential scanning calorimetry measurements that revealed that 16% of water is bound to proteins. The melting kinetics of ice crystallites (produced from the freezable water) indicates nonhomogeneous water pool size.

Kinetics of diffusion-controlled enzymatic reactions with charged substrates

The Debye-Hückel limiting law (DHL) has often been used to estimate rate constants of diffusion-controlled reactions under different ionic strengths. Two main approximations are adopted in DHL: one is that the solution of the linearized Poisson-Boltzmann equation for a spherical cavity is used to estimate the excess electrostatic free energy of a solution; the other is that details of electrostatic interactions of the solutes are neglected. This makes DHL applicable only at low ionic strengths and dilute solutions (very low substrate/solute concentrations). We show in this work that through numerical solution of the Poisson-Nernst-Planck equations, diffusion-reaction processes can be studied at a variety of conditions including realistically concentrated solutions, high ionic strength, and certainly with non-equilibrium charge distributions. Reaction rate coefficients for the acetylcholine-acetylcholinesterase system are predicted to strongly depend on both ionic strength and substrate concentration. In particular, they increase considerably with increase of substrate concentrations at a fixed ionic strength, which is open to experimental testing. This phenomenon is also verified on a simple model, and is expected to be general for electrostatically attracting enzyme-substrate systems.PACS Codes: 82.45.Tv, 87.15.VvMSC Codes: 92C30

A variable reaction order model for prediction of curing kinetics of thermosetting polymers

Based on the study of the curing reaction of an fluorinated aliphatic cyanate ester resin using an isothermal Differential Scanning Calorimetric (DSC) method, a new kinetics model with variable reaction order was proposed to describe the curing of thermosetting resins in both the chemical controlled and diffusion controlled regions. α–Tg relationship was used to calculate the diffusion controlled reaction rate constant during cure. Then the chemical and diffusion rate constants were combined in the Simon–Gillham equation to model the overall rate constant for the entire reaction. By using the overall rate constant combined with the variable reaction order, a simple model can be built up for prediction of curing kinetics of thermosetting polymer in both the chemical and the diffusion controlled regions. The new model is simple, easy to use and has very good agreement with the experimental results.

Proton Reaction with a Mild Base in Ice Studied by Proton−Photoacid Dynamics

We have studied the kinetics of the reaction of a proton with a mild base in ice. The proton was injected to the ice crystal by a proton-transfer reaction from an electronically excited photoacid. Mild bases react with the transferred proton with a large intrinsic rate constant. We used our diffusion assisted geminate recombination model based on the Debye−Smolochowski equation with an additional term to account for the proton scavenging by the base. We find that in liquid water the measured scavenging effect is slightly smaller than expected from a diffusion controlled reaction rate constant. In ice, the scavenging effect is rather large, and the diffusion controlled reaction rate constant underestimates the measured proton scavenging effect in a wide range of concentrations and temperature. We explain the large effective proton scavenging in ice by the tendency of ice formation to concentrate the impurities to confined volumes in order to minimize the ice crystal energy.

On effective reactant radii in ortho-positronium oxidation and spin-exchange reactions

The ortho-positronium, o-Ps, quenching reactions promoted by (NH4)2(Ce(NO3)6) and (Gd(H2O)6)Cl3 were studied in aqueous solutions at 278, 288, 298 and 308 K. The results obtained, together with those reported in a previous paper of the authors', suggest that the effective reaction radii of 3d and 4f complexes in o-Ps quenching reactions depend on the reaction types. In particular, the effective reaction radii are longer for o-Ps oxidation (Ox) reactions (Ce4+, Fe3+, CrO2-4) than for Ps spin-exchange (SE) reactions (Gd3+, Fe2+, Cr3+). As a consequence it is suggested that the Ox reactions occur by 'hopping mechanism', so that effective and crystallographic radii of these ions in Ox reactions are practically equal even if the Ps electron orbital and metal-accepting orbital do not significantly overlap at their closest distance. On the other hand the effective reaction radii of SE reactions, shorter than the crystallographic ones, seem to imply that the magnetic interaction promoting the SE reactions studied here occurs between distant localised electrons. The results are discussed with reference to the Smoluchowski equation which relates the rate constants of diffusion-controlled reactions to reactant radii.

Simulation of biomolecular diffusion and complex formation

Diffusion is a phenomenon of very widespread importance in molecular biophysics. Diffusion can determine the rates and character of the assembly of multisubunit structures, the binding of ligands to receptors, and the internal motions of molecules and assemblies that involve solvent surface displacements. Current computer simulation techniques provide much more detailed descriptions of diffusional processes than have been available in the past. Models can be constructed to include such realistic features as structural subunits at the submolecular level (domains, monomers, or atoms); detailed electrostatic charge distributions and corresponding solvent-screened inter- and intramolecular interactions; and hydrodynamic interactions. The trajectories can be analyzed either to provide direct information on biomolecular function (e.g., the bimolecular rate constant for formation of an electron-transfer complex between two proteins), or to provide or test models for the interpretation of experimental data (e.g., the time dependence of fluorescence depolarization for segments of DNA). Here, we first review the theory of diffusional simulations, with special emphasis on new techniques such as those for obtaining transport properties of flexible assemblies and rate constants of diffusion-controlled reactions. Then we survey a variety of recent applications, including studies of large-scale motion in DNA segments and substrate "steering" in enzyme-substrate binding. We conclude with a discussion of current work (e.g., formation of protein complexes) and possible areas for future work.

Radiation chemistry of polyethylene. XIV. Allyl radical decay kinetics in different types of polyethylene

AbstractThe decay kinetics of the chain allyl free radical has been studied in the following morphological forms of polyethylene (PE): Marlex bulk film, hydrogenated PE, and extended‐chain PE. Coupled with previous work on single‐crystalline PE it can be seen that the decay rate is greater the more amorphous the sample. In the Marlex bulk film and hydrogenated PE the decay can be interpreted in terms of a simultaneous fast and slow decay process by means of our Q‐function equation, but with rising temperature the decay approximates a single rate process. With extended‐chain PE the allyl decay rate does not become appreciable until the melting range is approached. The fraction of allyl radicals decaying by the slow process is 2 to 10 times greater than the fraction of fast decaying radicals. The ratio of the fast decay rate constant to that of the slow rate constant is greater for the bulk Marlex film than for the hydrogenated PE. All ratios decrease with rising temperature. For times up to about 150 min the allyl decay in the extended‐chain PE accurately follows a single second‐order decay law with a time‐independent diffusion controlled reaction rate constant.