Experimental verification of the Smoluchowski theory for a bimolecular diffusion-controlled reaction in liquid phase

Why Should the Reaction Order of a Bimolecular Reaction be 2.33 Instead of 2?

Predicting the reaction kinetics, that is, how fast a reaction can happen in a solution, is essential information for many processes, such as industrial chemical manufacturing, refining, synthesis and separation of petroleum products, environmental processes in air and water, biological reactions in cells, biosensing, and drug delivery. Collision theory was originally developed to explain the reaction kinetics of gas reactions with no dilution. For a reaction in a diluted inert gas solution or a diluted liquid solution, diffusion often dominates the collision process. Thus, it is necessary to include diffusion in such a calculation. Traditionally, the classical Smoluchowski rate is used as a starting point to predict the collision frequency of two molecules in a diluted solution. In this report, a different collision model is derived from the adsorption of molecules on a flat surface. A surprising result is obtained, showing that the reaction order for bimolecular reactions should be 2 and 1/3 instead of 2, following a fractal reaction kinetics.

Acetaminophen degradation by hydroxyl and organic radicals in the peracetic acid-based advanced oxidation processes: Theoretical calculation and toxicity assessment

The research on the mechanisms and kinetics of radical oxidation in peracetic acid-based advanced oxidation processes was relatively limited. In this work, HO^• and organic radicals mediated reactions of acetaminophen (ACT) were investigated, and the reactivities of important organic radicals (CH₃COO^• and CH₃COOO^•) were calculated. The results showed that initiated reaction rate constants of ACT are in the order: CH₃COO^• (5.44 × 10¹⁰ M^-1 s^-1) > HO^• (7.07 × 10⁹ M^-1 s^-1) > CH₃O^{• (}1.57 × 10⁷ M^-1 s^-1) > CH₃COOO^• (3.65 × 10⁵ M^-1 s^-1) >> ^•CH₃ (5.17 × 10² M^-1 s^-1) > CH₃C^•O (1.17 × 10² M^-1 s^-1) > CH₃OO^• (11.80 M^-1 s^-1). HO^•, CH₃COO^• and CH₃COOO^• play important roles in ACT degradation. CH₃COO^• is another important radical in the hydroxylation of aromatic compounds in addition to HO^•. Reaction rate constants of CH₃COO^• and aromatic compounds are 1.40 × 10⁶ - 6.25 × 10¹⁰ M^-1 s^-1 with addition as the dominant pathway. CH₃COOO^• has high reactivity to phenolate and aniline only among the studied aromatic compounds, and it was more selective than CH₃COO^•. CH₃COO^•-mediated hydroxylation of aromatic compounds could produce their hydroxylated products with higher toxicity.

Diffusional Encounter Rate Constants for Xanthone and 2-Naphthoic Acid by Flash Photolysis Experiments and Brownian Dynamics Simulations: Substantial Effects of Polarizability of the Triplet State

Diffusional encounter rate constants, for xanthone and 2-naphthoic acid molecules in their triplet states with xanthone or 2-naphthoic acid molecules in their triplet or singlet states, were determined using nanosecond laser flash photolysis spectroscopy. Simultaneously, Brownian dynamics simulations were used to compute these rate constants for assumed models of encountering molecules. Altogether, a global fit to transient absorption progress curves, reporting populations of triplet state xanthone and triplet state 2-naphthoic acid molecules, allowed us to determine six diffusional encounter rate constants from our experiments. The most important result of this study is the detection of substantial effects of the electric polarizability of molecules in their triplet state, visible for xanthone triplet and 2-naphthoic acid ground states, a homo triplet-triplet annihilation of 2-naphthoic acid, and a hetero triplet-triplet annihilation for xanthone and 2-naphthoic acid.

How Protic Solvents Determine the Reaction Mechanisms of Diphenylcarbene in Solution

We investigate the effects of small admixtures of protic solvent molecules, such as water and alcohols, on the ultrafast dynamics of diphenylcarbene in acetonitrile at room temperature. Broadband transient absorption measurements and quantum mechanics/molecular mechanics molecular dynamics simulations allow elucidating the dominant reaction mechanism of an intermediate hydrogen-bonded complex between singlet diphenylcarbene and a protic solvent molecule, thus competing with intersystem crossing. Analysis of the data indicates that complex formation is a diffusion-controlled process with orientational requirements. The reaction path involving a benzhydryl cation is less likely in neat bulkier alcohols, as it requires the interaction of the carbene with a protic solvent molecule being part of a hydrogen-bonded network. The simulations indicate a further reaction path toward O-H insertion and two side reactions depending on the involved protic solvent species. Thus, we established that not only the number but also the chemical nature of the protic solvent molecule determine which reaction path is pursued.

Effect of Hydrodynamic Interactions on Reaction Rates in Membranes

The Brownian motion of two particles in three dimensions serves as a model for predicting the diffusion-limited reaction rate, as first discussed by von Smoluchowski. Deutch and Felderhof extended the calculation to account for hydrodynamic interactions between the particles and the target, which results in a reduction of the rate coefficient by about half. Many chemical reactions take place in quasi-two-dimensional systems, such as on the membrane or surface of a cell. We perform a Smoluchowski-like calculation in a quasi-two-dimensional geometry, i.e., a membrane surrounded by fluid, and account for hydrodynamic interactions between the particles. We show that rate coefficients are reduced relative to the case of no interactions. The reduction is more pronounced than the three-dimensional case due to the long-range nature of two-dimensional flows.

Intermolecular Electron Transfer from Excited Benzophenone Ketyl Radical

The electron transfer from the benzophenone ketyl radical in the excited state (BPH(.-)(D(1))) to several quenchers (Qs) was investigated using nanosecond/picosecond two-color two-laser flash photolysis and nanosecond/nanosecond two-color two-laser flash photolysis. The electron transfer from BPH(.-)(D(1)) to Qs was confirmed by the transient absorption and fluorescence quenching measurements. The intermolecular electron-transfer rate constants were determined using the Stern-Volmer analysis. The driving force dependence of the electron-transfer rate was revealed.

Remarkable Reactivities of the Xanthone Ketyl Radical in the Excited State Compared with That in the Ground State

The properties and reactivities of the xanthone (Xn) ketyl radical (XnH*) in the doublet excited state (XnH*(D1)) were examined by using two-color two-laser flash photolysis. The absorption and fluorescence of XnH*(D1) were observed for the first time. Several factors governing the deactivation processes of XnH*(D1) such as interaction and reaction with solvent molecules were discussed. The remarkable change of reactivity of XnH*(D1) compared with that in the ground state (XnH*(D0)) was indicated from the experimental results. The rapid halogen abstraction of XnH*(D1) from some halogen donors such as carbon tetrachloride (CCl4) was found to occur. The halogen abstraction occurred more efficiently in the polar solvents than in the nonpolar solvents. It is suggested that the polar solvents promote the spin distribution of XnH*(D1) of the phenyl ring favorable to the halogen abstraction.