Corrected Rate Constants Research Articles

Influence of Season on Biodegradation Rates in Rivers.

Biodegradation plays a key role in the fate of chemicals in the environment. The variability of biodegradation in time can cause uncertainty in evaluating the environmental persistence and risk of chemicals. However, the seasonality of biodegradation in rivers has not yet been the subject of environmentally relevant testing and systematic investigation for large numbers of chemicals. In this work, we studied the biodegradation of 96 compounds during four seasons at four locations (up- and downstream of WWTPs located on two Swedish rivers). Significant seasonality (ANOVA, p < 0.05) of the first-order rate constant for primary biodegradation was observed for most compounds. Variations in pH and total bacterial cell count were not the major factors explaining the seasonality of biodegradation. Deviation from the classical Arrhenius-type behavior was observed for most of the studied compounds, which calls into question the application of this relationship to correct biodegradation rate constants for differences in environmental temperature. Similarities in magnitude and seasonality of biodegradation rate constants were observed for some groups of chemicals possessing the same functional groups. Moreover, reduced seasonality of biodegradation was observed downstream of WWTPs, while biodegradation rates of most compounds were not significantly different between up- and downstream.

Primary Antioxidant Power and Mpro SARS-CoV-2 Non-Covalent Inhibition Capabilities of Miquelianin.

The antioxidant power of quercetin-3-O-glucuronide (miquelianin) has been studied, at the density functional level of theory, in both lipid-like and aqueous environments. In the aqueous phase, the computed pKa equilibria allowed the identification of the neutral and charged species present in solution that can react with the ⋅OOH radical. The Hydrogen Atom Transfer (HAT), Single Electron Transfer (SET) and Radical Adduct Formation (RAF) mechanisms were considered, and the individual, total and fraction corrected rate constants were obtained. Potential non-covalent inhibition of Mpro from SARS-CoV-2 by miquelianin has been also evaluated.

An improved TRF mechanism for a new turbulent premixed combustion model with application to engine combustion CFD

In the present work, an existing TRF (toluene reference fuel) chemical kinetic mechanism has been improved. In the improvement, the existing TRF mechanism was found to have unphysically higher laminar flame speeds at temperatures greater than 750 K which are typical in engine compression-combustion operating conditions. Eight dominant reactions which mainly control the laminar flame speeds under higher temperatures ( T &gt; 750 K) have been identified. Based on a recently published power-law laminar flame speed correlation which is able to provide good predictions under a wide range of engine conditions, correct reaction rate constants for the eight high-temperature dominant reactions have been determined for physical laminar flame speeds. The identification and improvement of the eight high-temperature dominant reactions are the first novelty of this work. In order to simulate ethanol and gasoline blends, a latest ethanol sub-mechanism has been implemented into the TRF mechanism. The improved TRF mechanism was validated using available experimental data in the literature. Based on the improved TRF mechanism, a new mechanism library has been generated, which can be used for an improved mechanism-dynamic-selection turbulent premixed combustion model in which turbulent diffusivity is constructed as a function of local turbulence/thermodynamics conditions. The dynamic turbulent diffusivity sub-model is the second novelty of this work. The improved TRF mechanism and its combination with the improved mechanism-dynamic-selection turbulent premixed combustion model were successfully applied to the prediction of combustion and emissions of GTDI (gasoline turbocharged direct injection) engines under both warm-up and transient cold start operating conditions, indicating that the improved models in this work are beneficial for predictive engine combustion CFD (computational fluid dynamics).

Dynamic orientation control of bimolecular electron transfer at charged micelle surfaces.

Visible light excitation of the neutral complex [RuII(phen)2(bps)]0 (phen = 1,10-phenanthroline, bps = 4,7-biphenylsulfonate-1,10-phenanthroline) results in the formation of a triplet metal-to-ligand charge transfer excited state with a lifetime, τo, of 4.6 µs, where the promoted electron is localized on the bps ligand, 3*[RuIII(phen)2(bps•-)]0. The complex is dynamically quenched by di-n-heptyl-viologen, C7C7V2+, in solution and when the acceptor is embedded into negatively charged and neutral micelles. Addition of NaCl to solutions containing C7C7V2+ bound to negatively charged dodecyl sulfate sodium dodecyl sulfate micelles results in a monotonic increase in the quenching rate constant from kq = 6.0 × 107 to 1.7 × 109 M-1 s-1. In contrast, kq was independent of [NaCl] and diffusion limited in water and neutral micellar solution. Activated rate constants, kact, revealed that electron transfer was slowed by a factor of 450 when occurring in negatively charged micelle solution relative to neutral octaethylene glycol monododecyl ether (C12E8) micelles. In the 3*[RuIII(phen)2(bps•-)]0 excited state, the bps ligand is oriented away from the anionic micelle surface potential, -141 ≤ ψ ≤ -67 mV, due to a Frumkin effect operative in the deceleration of kact. Frumkin corrected rate constants were within a factor of three of those measured in C12E8 solution. Distance-dependent reorganization energies resulting from the orientation vary from 0.47 eV to 0.35 eV, while electronic coupling decreases by a factor of 10. The collective data show that orientation control over bimolecular rate constants in micellar solution can be achieved by screening micellar surface charges.

Quantum dynamics study of isotope effects of the OD/OH + CH3 reactions

ABSTRACT A six-degrees-of-freedom, time-dependent quantum dynamics calculation is employed to study the integral cross sections, full-dimensional cumulative reaction probabilities and full-dimensional rate constants for the isotopic reactions of the OD and OH with CH3 reactions. The full-dimensional cumulative reaction probabilities and full-dimensional rate constants are obtained using the energy and J-K shifting approaches based on the six-degrees-of-freedom calculations. The comparison of integral cross sections shows that the OD + CH3 reaction has a larger energy threshold and a smaller tunnelling effect than the OH + CH3 reaction. The corrected rate constants using the experimental zero-point energy have a very good agreement with the experimental results. The comparison of the rate constants shows that the OD + CH3 has smaller rate constants than the OH + CH3 reaction, which indicates a smaller reactivity due to the isotope substitution.

Spatiotemporal coupling of excited state dynamics in time-resolved microscopies

In the high-density excitation limit, as is often probed with ultrafast spectroscopies, spatial and temporal evolution of photogenerated excited states are strongly coupled, giving rise to artifacts that influence experimentally-determined material parameters. The interplay between spatial and temporal degrees of freedom is especially pronounced in pump-probe microscopy, where small laser spot sizes amplify the effects of spatiotemporal coupling on spectroscopic observables. To quantitatively model these effects, a continuum model is developed that accounts for laser spot size as well as nonlinear excited state decay and diffusion. It is shown that effective excitation densities cannot be used to determine quantitatively correct rate constants. Significant error is introduced unless experimental data is fit with a numerical model that accounts for spatial anisotropy in the excitation density. Furthermore, the quantitative determination of material diffusion coefficients is shown to be highly sensitive to experimental parameters.

Comment on “Analysis of Crossover-Induced Capacity Fade in Redox Flow Batteries with Non-Selective Separators” [J. Electrochem. Soc., 165 A3144 (2018)

The particular overpotential definition used in expressing the kinetic rate constants given by Marcus-Hush-Chidsey (MHC) theory was incorrect in our original article [J. Electrochem. Soc., 165, A3144 (2018)] and has been corrected here to be expressed in terms of the standard overpotential. Implementation of the corrected rate constants is necessary to reproduce near-equilibrium variations of cell voltage with state-of-charge, a property that is assured by the de Donder relation. However, we find that the RFB performance metrics presented in our original article (discharge capacity utilization, capacity fade, and time-averaged polarization) change insignificantly after correction of this theory and its implementation in our numerical model. While future studies should acknowledge this subtle point, no conclusions from our original article were changed by the error in the theory originally presented.

Testing Transition State Theory on Kac-Zwanzig Model

A variant of the Kac-Zwanzig model is used to test the prediction of transition state theory (TST) and variational transition state theory (VTST). The model describes the evolution of a distinguished particle moving in a double-well external potential and coupled to N free particles through linear springs. While the Kac-Zwanzig model is deterministic, under appropriate choice of the model parameters the evolution of the distinguished particle can be approximated by a two-state Markov chain whose transition rate constants can be computed exactly in suitable limit. Here, these transition rate constants are compared with the predictions of TST and VTST. It is shown that the application of TST with a naive (albeit natural) choice of dividing surface leads to the wrong prediction of the transition rate constants. This is due to crossings of the dividing surface that do not correspond to actual transition events. However, optimizing over the dividing surface within VTST allows one to eliminate completely these spurious crossings, and therefore derive the correct transition rate constants for the model. The reasons why VTST is successful in this model are discussed, which allows one to speculate on the reliability of VTST in more complicated systems.

Photophysics of anthracene–indole systems in unilamellar vesicles of DMPC and POPC: Exciplex formation and temperature effects

The quenching of anthracene fluorescence by indole (IN), 1,2-dimethylindole (DMI), tryptophan (Trp) and indole 3-acetic acid (IAA) in dimiristoylphophatidylcholine (DMPC) and palmitoyloleoylphosphatidylcholine (POPC) lipid bilayers was investigated. The studies were carried out at 25 °C in POPC vesicles and below (15 °C) and above (35 °C) the phase transition temperature (24 °C) of DMPC. A very efficient quenching of the anthracene fluorescence by IN and DMI in the lipid membrane is observed in all cases. It is less efficient in the case of Trp and IAA. Stern–Volmer plots are linear for DMI but present a downward curvature for the other quenchers. This was interpreted as an indication of the presence of an inaccessible fraction of anthracene molecules. By a modified Stern–Volmer analysis the fraction accessible to the quenchers and the quenching constant were determined. Partition constants of the quenchers were obtained from the changes in the fluorescence emission of the indole moiety caused by the presence of the phospholipid. Using the partition constants bimolecular quenching rate constants were determined in terms of the local concentration of quencher in the lipid bilayer. These corrected rate constants are lower than those in homogeneous solvents. In the case of DMPC values the gel phase are higher than in the liquid-crystalline phase. In the quenching by IN and DMI a new, red shifted, emission band appears which could be assigned to an exciplex emission. The exciplex band is absent in the quenching by IAA and Trp.

Architectural Effects of Poly(ε-caprolactone)s on the Crystallization Kinetics

A series of hyperbranched poly(ε-caprolactone)s (HPCLs) with molecular architectural variation, which refers to the different lengths of the linear backbone segments consisting of 5, 10, and 20 ε-caprolactone monomer units (thereby referred to as HPCL-5, HPCL-10, and HPCL-20, respectively) and the different numbers of branching points, were synthesized without significant variation in the molecular weights, and linear poly(ε-caprolactone) (LPCL) of the same chemical structure and similar molecular weight was used as the linear counterpart to HPCLs for comparison. The specific molecular architectures of these samples were characterized by 1H NMR end-group analyses. The nonisothermal crystallization exotherms of HPCLs and LPCL were measured by DSC and further analyzed by the modified Avrami method. Regardless of the samples, the Avrami exponents ranged from 2.4 to 3.3, indicating that the nucleation and growth mechanisms of the samples are apparently identical. On the other hand, the crystallization kinetics was found to be fairly affected by the molecular architectures. All the kinetic parameters estimated from DSC exotherms such as the initial slope, Si, and from the modified Avrami analyses such as the corrected rate constant, Kc, and crystallization half-time, t1/2, indicated that HPCLs with longer linear segments and fewer branches showed the faster crystallization, whereas LPCL exhibited intermediate crystallization rate between HPCL-10 and HPCL-20, i.e., HPCL-5 < HPCL-10 < LPCL < HPCL-20. The slower crystallization was linked with the frequent presence of heterogeneous branching points, hindering the regular chain packing, in the backbone of HPCLs with shorter linear segments. In addition, the faster crystallization of HPCL-20 compared with LPCL was attributed to the higher cooperative chain mobility in their melt state as evaluated by activation energy of flow. A number of spherulites developed during isothermal process from the melt were observed for all samples by POM, and the radial growth rates of these spherulites were evaluated to correspond well with the results of Si, Kc, and t1/2 from the nonisothermal crystallization analyses.

Cloth catalysts in water denitrification: III. pH inhibition of nitrite hydrogenation over Pd/ACC

Nitrite hydrogenation over Pd supported on an activated carbon cloth (ACC) has been investigated in an isothermal semi-batch reactor. The reaction is accompanied by increasing pH, which inhibits the reaction. The rate was corrected for pH changes and the reaction was found to be first-order with respect to nitrite concentration and zero-order with respect to hydrogen pressure. The corrected rate constant is of the form keff=kexp[(pH−pzc)/(pH0−pzc)], where k is the intrinsic constant and the pzc is the point of zero charge of catalyst surface. While Pd/ACC catalysts yield better rates than those of Pd supported on glass fiber cloth (GFC), they exhibit much poorer selectivity (in the unbuffered reaction conditions) due to ACC surface enrichment by hydrogen.

Influence of temperature on the reduction kinetics of Zn 2+ at a mercury electrode

The reduction of Zn 2+ at a mercury electrode has been studied by the a.c. impedance technique in the 263–318 K temperature range. Kinetic data have been analyzed in terms of a mechanism consisting of two consecutive charge transfer steps. The apparent activation enthalpies and charge transfer coefficients of both steps are found to be temperature independent. Differential capacity curves and potentials of zero charge of the NaClO 4 supporting electrolyte solutions were measured at each temperature of interest. Double layer effects on the Zn 2+ reduction rate constants were analyzed within the framework of the unequal distances of closest approach theory, and the corrected rate constants were found to be independent of supporting electrolyte concentration when allowance for changes in the Zn 2+ activity coefficient was made. The implications of the activation parameter values on the nature of the rate determining steps are also discussed.

Modeling Cage-to-Cage Dynamics of Adsorbates at Arbitrary Loadings with Dynamically Corrected Transition-State Theory

Transition state theory (TST) formalisms have allowed researchers to extend the time and length scales accessible to the modeling of diffusion in microporous materials. In a previous paper (Tunca, C.; Ford, D. M. J. Chem.Phys. 1999, 111, 2751), we used multidimensional TST to obtain the escape rate of adsorbate molecules from an a cage in zeolite ZK4 as a function of loading. A physically reasonable approximation was applied to make the TST partition functions tractable, and they were evaluated with a nested Widom insertion scheme. However, that study was limited to the case of empty neighboring cages and nondynamically corrected rate constants. This paper extends the work in several ways. First, we consider the effects of adsorbate occupancy in the neighboring cages. Our results show that the TST escape rates increase with loading in the original cage but show nonmonotonic behavior with respect to loading in the destination cage. Furthermore, we employ an expanded ensemble method (EEM) to obtain the partition functions, thus avoiding the limitations associated with the Widom insertions used previously. Even at high cage loadings, the results have small statistical errors due to the use of the EEM. Finally, we calculate and present dynamical corrections to the rate constants. The dynamical corrections are rather modest in magnitude for this model system, typically representing changes of about 10% relative to the TST values.

Preceding chemical reactions in dc polarography:: different approaches to the analysis of the carbonyl compounds dehydration kinetics

It is shown that many dehydration rate constants of carbonyl compounds (acetaldehyde, pyruvic and glyoxylic acids, formaldehyde) found by various kinetic approaches to dc polarography are overestimated. This anomaly is connected neither with the adsorption effect, nor with neglecting the spherical diffusion. It is caused by the influence of the diffusion contribution to the total limiting kinetic current of the depolarizer from the solution bulk. Quantitative consideration of this effect has allowed us to obtain the correct rate constants under steady-state conditions, independently of the degree of the diffusion contribution.

Corrected Rate Constants for Collision-Induced Electronic Transitions from the N2 aΠg (υ = 0 and 1) Levels.

Corrected Rate Constants for Collision-Induced Electronic Transitions from the N₂ aΠ_g (υ = 0 and 1) Levels.

Corrected Rate Constants for Collision-Induced Electronic Transitions from the N2 aΠg (υ = 0 and 1) Levels

Corrected Rate Constants for Collision-Induced Electronic Transitions from the N₂ aΠ_g (υ = 0 and 1) Levels

Faradaic Impedance Study of Mn(II) Reduction Mechanism in NaClO4 and NaCl Solutions on Mercury Electrode

The electrochemical reaction of the Mn(II)/Mn(Hg) system on mercury electrode was studied in 1 M NaClO4 and 1 M NaCl as supporting electrolytes of different complexing and adsorptive properties. The impedance measurements confirmed the two-stage electroreduction of the Mn(II) in investigated solutions. Both the apparent and the true rate constants of the second electron transfer in both supporting electrolytes are lower by one order of magnitude than the rate constant of the first electron transfer. Similar values of corrected rate constants in both electrolytes suggest the similarity in mechanism of the Mn(II) electroreduction.

On the (im)possibility of evaluating correct individual rate constants of chain propagation kp and chain termination kt by combining kp2/kt and kp/kt data for chain-length dependent termination

On the (im)possibility of evaluating correct individual rate constants of chain propagation kp and chain termination kt by combining kp2/kt and kp/kt data for chain-length dependent termination

On the (im)possibility of evaluating correct individual rate constants of chain propagationkp and chain terminationkt by combiningkp2/kt andkp/kt data for chain-length dependent termination

On the basis of simulated data two ways of evaluating individual rate constants by combining kp2/kt and kp /kt (kp , kt = rate constants of chain propagation and termination, respectively) were checked considering the chain-length dependence of kt. The first way tried to make use of the fact that pseudostationary polymerization yields data for kp2/kt as well as for kp /kt referring to the very same experiment, in the second way kp2/kt (from steady state experiments) and kp/kt data referring to the same mean length of the terminating radical chains were compared. In the first case no meaningful data at all could be obtained because different averages of kt are operative in the expressions for kp /kt and kp2/kt. In spite of the comparatively small difference between these two averages (≈15% only) this makes the method collapse. The second way, which can be regarded as an intelligent modification of the “classical” method of determining individual rate constants, at least succeeded in reproducing the correct order of magnitude of the individual rate constants. However, although stationary and pseudostationary experiments independently could be shown to return the same kt for the same average chain-length of terminating radicals within extremely narrow limits no reasonable chain-length dependence of kt could be derived in this way. The reason is an extreme sensitivity of the pair of equations for kp/kt and kp2/kt towards small errors and inconsistencies which renders the method unsuccessful even for the high quality simulation data and most probably makes it even collapse for real data. This casts a characteristic light on the unsatisfactory situation with respect to individual rate constants determined in the classical way, regardless of a chain-length dependence of termination. As a consequence, all efforts of establishing the chain-length dependence of kt are recommended to avoid this way and should rather resort to methods based on inserting a directly determined kp into the equations characteristic of kp2/kt or kp/kt, properly considering the chain-length dependent character of kt.

Electron self-exchange kinetics between 2,3-dicyano-5,6-dichloro-p-benzoquinone (DDQ) and its radical anion. Part 1. Solvent dynamical effects †

Rate constants of the electron self-exchange between 2,3-dicyano-5,6-dichloro-p-benzoquinone (DDQ) and its radical anion (DDQ˙–) are measured by means of EPR-line-broadening effects in different solvents at T = 293 K. Solvents of different polarity like CHCl3, CH2Cl2, CH3CN, benzonitrile and acetone were used. No correlation is found in the sense of the classical Marcus theory, where ln ket should depend linearly on the solvent parameter γ = 1/n2 – 1/es (n = refractive index, es = static relative permittivity of the solvent). The investigated γ-range was 0.270 ≤ γ ≤ 0.530. The diffusion corrected rate constants ket cover a range of 8.9 to 36.7 × 108 M–1 s–1 and clearly show a solvent-dynamical effect, expressed by the longitudinal relaxation time τL dependence of the solvents used. The observed dynamical friction solvent effects clearly indicate an adiabatic reaction behaviour of this electron self-exchange reaction.