Bimolecular Reaction Rate Constants Research Articles

Photocatalytic degradation of three amantadine antiviral drugs as well as their eco-toxicity evolution

Abstracts Advanced oxidation processes (AOPs) relying on in situ generated highly reactive OH are successfully applied to water purification. The absolute reaction rate constants for OH with three antiviral drugs were first reported through pulsed radiolysis experiments. Results found that OH reacted quickly with these substrates, with bimolecular reaction rate constants of 6.31 × 10 9 , 5.13 × 10 9 , and 7.05 × 10 9 M −1 s −1 for 1-amantadine, 2-amantadine, and rimantadine, respectively. The photocatalytic degradation kinetics of substrates were followed pseudo-first-order kinetics according to Langmuir–Hinshelwood model in TiO 2 suspensions, and the apparent rate constants were obtained as 0.076, 0.084, and 0.102 min −1 for three antiviral drugs, respectively. Scavenger experiments revealed that OH was the major reactive species involved in antiviral drugs degradation. To probe the photocatalytic degradation mechanism, the fate of nitrogen elements and the change of total organic carbon were also examined, and the data showed that all three drugs could be completely mineralized into CO 2 , H 2 O, and inorganic ions (NO 3 − and NH 4 + ) without generating any detectable products with enough degradation time. To further insight into the potential adverse effect of three antiviral drugs and their degradation products, the acute aquatic toxicity of degradation solutions were evaluated at three different trophic levels, and the toxicities first increased slightly and then decreased rapidly as the total organic carbon decreased.

Role of the chemical mechanism in the temperature dependence of the rate of gas combustion reactions

The determining role of the kinetic mechanism in the temperature dependence of reaction rates is demonstrated. New evidence is presented to prove that the branched chain nature of gas combustion at low pressures and under isothermal conditions, discovered by N.N. Semenov, persists in combustion processes at elevated pressures under any temperature and macrokinetic conditions. It is noted that the temperature dependence of the rate constants of bimolecular reactions weakens with an increasing activation energy.

Photochemical properties of gemifloxacin: A laser flash photolysis study

The photochemical properties of gemifloxacin (GEFX), a fluoroquinolone antibacterial drug that exhibits phototoxicity toward biological substrates, were studied in aqueous solutions by laser flash photolysis (LFP) and pulse radiolysis. GEFX triplet state (3GEFX∗) absorption spectra showed maximum absorption at 510nm. 3GEFX∗ was quenched by naproxen (NAP) via energy transfer with a rate constant of 1.2×108dm3mol−1s−1. The energy of 3GEFX∗ was 266kJmol−1 and the transient absorption spectra showed direct evidence of electron transfer from 2′-deoxyguanosine-5′-monophosphate, N,N,N′,N′-tetramethyl-p-phenylenediamine, and tryptophan to 3GEFX∗ with bimolecular reaction rate constants of 4.1×106, 2.0×107, and 2.2×107dm3mol−1s−1, respectively. The rate constants for reactions of GEFX with OH, eaq− were found to be 1.5×1010 and 1.4×1010dm3mol−1s−1, respectively. The mechanisms and products of the photosensitive damage of lysozyme were related to the GEFX concentration, irradiation time, and ambient conditions.

Photosensitive damage of lysozyme caused by pazufloxacin and the protective effect of ferulic acid

Laser flash photolysis and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) experiments were carried out to study the photosensitive damage induced by pazufloxacin (PAX) and the protection of lysozyme (Lyso) by ferulic acid (FCA), an active antioxidant. Transient absorption spectra revealed electron transfer from Lyso and FCA to (3)PAX*, with respective bimolecular reaction rate constants of 6.3x10(7) and 1.4x10(10) dm(3)/(mol s). A pulse radiolysis study was also performed to investigate the formation of Lyso radical cations. Results showed that FCA effectively inhibits the cross-linking of protein induced by fluoroquinolones. Finally, a mechanism of the protective effect of FCA on Lyso was proposed.

Concentration-dependent photodegradation kinetics and hydroxyl-radical oxidation of phenicol antibiotics

Thiamphenicol and florfenicol are two phenicol antibiotics widely used in aquaculture and are ubiquitous as micropollutants in surface waters. The present study investigated their photodegradation kinetics, hydroxyl-radical (OH) oxidation reactivities and products. Firstly, the photolytic kinetics of the phenicols in pure water was studied as a function of initial concentrations (C0) under UV–vis irradiation (λ>200nm). It was found that the kinetics was influenced by C0. A linear plot of the pseudo-first-order rate constant vs C0 was observed with a negative slope. Secondly, the reaction between the phenicol antibiotics and OH was examined with a competition kinetic method under simulated solar irradiation (λ>290nm), which quantified their bimolecular reaction rate constants of (2.13±0.02)×109M−1s−1 and (1.82±0.10)×109M−1s−1 for thiamphenicol and florfenicol, respectively. Then the corresponding OH oxidated half-lives in sunlit surface waters were calculated to be 90.5–106.1h. Some main intermediates were formed from the reaction, which suggested that the two phenicols underwent hydroxylation, oxygenation and dehydrogenation when OH existed. These results are of importance to assess the phenicol persistence in wastewater treatment and sunlit surface waters.

Bimolecular Photoinduced Electron Transfer Beyond the Diffusion Limit: The Rehm–Weller Experiment Revisited with Femtosecond Time Resolution

To access the intrinsic, diffusion free, rate constant of bimolecular photoinduced electron transfer reactions, fluorescence quenching experiments have been performed with 14 donor/acceptor pairs, covering a driving-force range going from 0.6 to 2.4 eV, using steady-state and femtosecond time-resolved emission, and applying a diffusion-reaction model that accounts for the static and transient stages of the quenching for the analysis. The intrinsic electron transfer rate constants are up to 2 orders of magnitude larger than the diffusion rate constant in acetonitrile. Above ∼1.5 eV, a slight decrease of the rate constant is observed, pointing to a much weaker Marcus inverted region than those reported for other types of electron transfer reactions, such as charge recombination. Despite this, the driving force dependence can be rationalized in terms of Marcus theory.

Primary Photochemical Properties of Difloxacin in Neutral Aqueous Solution

The photochemical properties of difloxacin (DFX) were investigated in neutral aqueous solution. DFX aqueous solution showed intense absorption with one peak at 273 nm (molar absorption coefficient epsilon=33000 dm(3).mol(-1).cm(-1)) and two other peaks at 323 and 335 nm (epsilon=15500 dm(3).mol(-l).cm(-1)) with the same molar absorption coefficient. Both the absorption and emission properties of DFX were pH-dependent. The acid dissociation constant (pK(a)) for the protonation equilibria of the ground state (5.9 and 9.8) were determined spectroscopically. DFX fluoresces weakly, and its maximum quantum yield for fluorescence emission was 0.06 at pH 3. Laser flash photolysis and pulse radiolysis studies were carried out to characterize the transient species of DFX aqueous solution. Triplet-triplet absorption reached a maximum at 620 nm with a molar absorption coefficient of 7900 dm(3).mol(-1).cm(-1). The energy transfer method was used to estimate the triplet energy of DFX, which was 263.5 kJ.mol(-1). The quantum yield of triplet formation was determined to be 0.21. Furthermore, DFX showed monophotonic photoionization with a quantum yield of 0.02. Pulse radiolysis indicated that DFX could react with e(aq)(-); and .OH, and the bimolecular rate constants for these reactions were 1.72x10(10) and 1.0x10(10) dm(3).mol(-1).s(-1), respectively. It is expected that this research may be helpful in determining the phototoxicity mechanism of DFX.

Free radical destruction of haloacetamides in aqueous solution

Haloacetamides are disinfection byproducts (DBPs) formed through chlorine/chloramine disinfection processes in drinking waters and have been recently highlighted in the US national Reconnaissance Survey. These species occur at low concentrations, but have been determined to have high cytotoxicity and mutagenicity and therefore may represent a human health hazard. Advanced oxidation/reduction processes (AO/RPs) which utilize free radical reactions are new alternatives to degrade these species. This study reports the absolute bimolecular reaction rate constants for 12 haloacetamides with •OH radical and hydrated electron. The •OH radical reaction rates varied from 5.18 × 107 to 1.14 × 1010 M−1 s−1, and hydrated electron reaction rates varied from 5.00 × 109 to 3.64 × 1010 M−1 s−1. Results obtained using an ion chromatograph indicated that ARPs are suitable for dehaloliazation of haloacetamides. These data are required for both evaluating AO/RPs for the destruction of these compounds and for studies of their fate and transport in surface waters where radical chemistry may be important in assessing their lifetime.

Electron Transfer by Excited Benzoquinone Anions: Slow Rates for Two-Electron Transitions

Electron transfer (ET) rate constants from the lowest excited state of the radical anion of benzoquinone, BQ(-•)*, were measured in THF solution. Rate constants for bimolecular electron transfer reactions typically reach the diffusion-controlled limit when the free-energy change, ΔG°, reaches -0.3 eV. The rate constants for ET from BQ(-•)* are one-to-two decades smaller at this energy and do not reach the diffusion-controlled limit until -ΔG° is 1.5-2.0 eV. The rates are so slow probably because a second electron must also undergo a transition to make use of the energy of the excited state. Similarly, ET, from solvated electrons to neutral BQ to form the lowest excited state, is slow, while fast ET is observed at a higher excited state, which can be populated in a transition involving only one electron. A simple picture based on perturbation theory can roughly account for the control of electron transfer by the need for transition of a second electron. The picture also explains how extra driving force (-ΔG°) can restore fast rates of electron transfer.

A model for the nitric oxide producing nitrite reductase activity of hemoglobin as a function of oxygen saturation

The production of nitric oxide by the nitrite reductase activity of hemoglobin has been proposed to play a major role in hypoxic vasodilation. The bimolecular reaction rate constant for nitric oxide formation is a complex function of hemoglobin oxygenation stemming from the intrinsic allosteric character of hemoglobin, resulting in an unsymmetrical inverted U shape profile of activity versus oxygen saturation. We present an analysis of the hemoglobin nitrite reductase activity based on the Monod Wyman Changeux (MWC) allosteric model and derive a set of equations that enabled us to express the rate constant of bimolecular reaction of nitrite with hemoglobin as a function of hemoglobin saturation and use this expression to explore the factors controlling the shape of the nitrite reductase activity versus hemoglobin saturation curve. From the value of the maximum reductase activity, we derive equations to calculate microscopic nitrite reductase reaction rate constants for the R and T quaternary states. We have also developed two methods to parameterize the MWC model based on the Hill equation, with its parameters, h and P50, and the knowledge that these two descriptions of the binding curve coincide in the region of the curve where h is defined. This has allowed the calculation of the hemoglobin nitrite reductase activity rate profiles for the human hemoglobin and for bovine hemoglobin. The properties of these rate profiles are discussed.

Radiation chemistry of salicylic and methyl substituted salicylic acids: Models for the radiation chemistry of pharmaceutical compounds

Salicylic acid and its derivatives are components of many medications and moieties found in numerous pharmaceutical compounds. They have been used as models for various pharmaceutical compounds in pharmacological studies, for the treatment of pharmaceuticals and personal care products (PPCPs), and, reactions with natural organic matter (NOM). In this study, the radiation chemistry of benzoic acid, salicylic acid and four methyl substituted salicylic acids (MSA) is reported. The absolute bimolecular reaction rate constants for hydroxyl radical reaction with benzoic and salicylic acids as well as 3-methyl-, 4-methyl-, 5-methyl-, and 6-methyl-salicylic acid were determined (5.86±0.54)×109, (1.07±0.07)×1010, (7.48±0.17)×109, (7.31±0.29)×109, (5.47±0.25)×109, (6.94±0.10)×109 (M−1s−1), respectively. The hydrated electron reaction rate constants were measured (3.02±0.10)×109, (8.98±0.27)×109, (5.39±0.21)×109, (4.33±0.17)×109, (4.72±0.15)×109, (1.42±0.02)×109 (M−1s−1), respectively. The transient absorption spectra for the six model compounds were examined and their role as model compounds for the radiation chemistry of pharmaceuticals investigated.

Reaction Rate and Isomer-Specific Product Branching Ratios of C2H + C4H8: 1-Butene, cis-2-Butene, trans-2-Butene, and Isobutene at 79 K

The reactions of C2H radicals with C4H8 isomers 1-butene, cis-2-butene, trans-2-butene, and isobutene are studied by laser photolysis-vacuum ultraviolet mass spectrometry in a Laval nozzle expansion at 79 K. Bimolecular-reaction rate constants are obtained by measuring the formation rate of the reaction product species as a function of the reactant density under pseudo-first-order conditions. The rate constants are (1.9 ± 0.5) × 10(-10), (1.7 ± 0.5) × 10(-10), (2.1 ± 0.7) × 10(-10), and (1.8 ± 0.9) × 10(-10) cm(3) s(-1) for the reaction of C2H with 1-butene, cis-2-butene, trans-2-butene, and isobutene, respectively. Bimolecular rate constants for 1-butene and isobutene compare well to values measured previously at 103 K using C2H chemiluminescence. Photoionization spectra of the reaction products are measured and fitted to ionization spectra of the contributing isomers. In conjunction with absolute-ionization cross sections, these fits provide isomer-resolved product branching fractions. The reaction between C2H and 1-butene yields (65 ± 10)% C4H4 in the form of vinylacetylene and (35 ± 10)% C5H6 in the form of 4-penten-1-yne. The cis-2-butene and trans-2-butene reactions yield solely 3-penten-1-yne, and no discrimination is made between cis- and trans-3-penten-1-yne. Last, the isobutene reaction yields (26 ± 15)% 3-penten-1-yne, (35 ± 15)% 2-methyl-1-buten-3-yne, and (39 ± 15)% 4-methyl-3-penten-1-yne. The branching fractions reported for the C2H and butene reactions indicate that these reactions preferentially proceed via CH3 or C2H3 elimination rather than H-atom elimination. Within the experimental uncertainties, no evidence is found for the formation of cyclic species.

An accurate expression for the rates of diffusion-influenced bimolecular reactions with long-range reactivity

By using the recently developed method for solving the Fredholm integral equations of the second kind, we derive a very accurate expression for the steady-state rate constant of diffusion-influenced bimolecular reactions involving long-range reactivity. We consider the general case in which the reactants interact via an arbitrary central potential and hydrodynamic interaction. The rate expression becomes exact in the two opposite limits of small and large reactivity, and also performs very well in the intermediate regime.

Degradation of Diclofenac by Advanced Oxidation and Reduction Processes: Kinetic Studies, Degradation Pathways and Toxicity Assessments

Many pharmaceutical compounds and metabolites are found in surface and ground waters suggesting their ineffective removal by conventional wastewater treatment technologies. Advanced oxidation/reduction processes (AO/RPs), which utilize free radical reactions to directly degrade chemical contaminants, are alternatives to traditional water treatment. This study reports the absolute rate constants for reaction of diclofenac sodium and model compound (2, 6-dichloraniline) with the two major AO/RP radicals: the hydroxyl radical (•OH) and hydrated electron (eaq−). The bimolecular reaction rate constants (M−1 s−1) for diclofenac for •OH was (9.29 ± 0.11) × 109, and for e−aq was (1.53 ± 0.03) ×109. To provide a better understanding of the decomposition of the intermediate radicals produced by hydroxyl radical reactions, transient absorption spectra are observed from 1 – 250 μs. In addition, preliminary degradation mechanisms and major products were elucidated using 60Co γ-irradiation and LC-MS. The toxicity of products was evaluated using luminescent bacteria. These data are required for both evaluating the potential use of AO/RPs for the destruction of these compounds and for studies of their fate and transport in surface waters where radical chemistry may be important in assessing their lifetime.

Pathology development stage and its influence on the delayed fluorescence kinetics of molecular probes

This paper describes regularity of the delayed fluorescence kinetics changes of xanthene dyes in the cancer cells received from tumors of different diameter. It shows that the rate constant of bimolecular reactions (singlet–triplet T−1Δg(O2) and triplet–triplet annihilations) as well as thermoactive fluorescence decrease with increasing pathological process development stage. It could be a result of both more expressed cancer cell hypoxia and a change in cytoplasm viscosity.

The behavior of effective rate constants for bimolecular reactions in an asymptotic transport regime

Previous research has shown that rate constants measured in batch tests (κ) may over-predict the amount of product formation when used in continuum models, and that these rate constants are often much greater than effective ones (κ(eff)) determined from upscaling studies. However, there is evidence that mixing is more important than the rate constants when using upscaled models. We use a numerical two-dimensional pore-scale porous medium with an approach similar to an experimental column test, and focus on the scenario of the displacement and mixing of two solutions with irreversible bimolecular reactions. Break-through curves of multiple cross-sectional averaged concentrations are analyzed for conservative and reactive transport, as well as the segregation of reactant species along the cross-sections. We compute effective parameters for the continuum scale in order to better understand the impact of using intrinsic rate constants in upscaled models. For a range of Damköhler numbers (Da), we compute effective reaction rate parameters and a reaction effectiveness factor; the latter is described by an empirical formula that depends on the Damköhler number and captures the upscaled system behavior. Our pore-scale results also confirm the segregation concept advanced by Kapoor et al. (1997). We find that for Da>1, κ(eff)<<κ, and yet the relative difference in total mass transformation between the pore-scale simulation and what is predicted by the upscaled continuum model using κ is about 10%. The explanation for this paradox is that the early transition of the regime from rate-limited to mixing-limited results in a model that is relatively insensitive to the rate constant because mixing controls the availability of reactants. Thus, the reaction-rate parameter used in the model has limited influence on the rate of product computed.

Investigation of the Consecutive Electron Transfer of Metalloporphyrin Species Containing Different Substituents at the Liquid/Liquid Interface by Thin-Layer Cyclic Voltammetry

The cyclic voltammetric behavior of metalloporphyrin species containing different substituents was investigated by thin-layer electrochemistry. In this paper, the two-step electron transfer between zinc porphyrin species containing different substituents in nitrobenzene and Fe(CN)64– in the aqueous phase was studied. The bimolecular rate constants for the two-step electron transfer reactions were obtained simultaneously by thin-layer theory. When we use the thin-layer theory to calculate the kinetics constants, the process of consecutive electron transfer is considered to be complete. However, this was not the case; by comparing the data that are calculated by Faraday’s law of electrolysis and the theory based on thin-layer cyclic voltammetry for consecutive electron transfer across the interface between two immiscible electrolyte solutions, the correction coefficient was obtained. The kinetics constants of multistep interfacial bimolecular reactions can be further calculated with more precision.

Correlating DFT‐Calculated Energy Barriers to Experiments in Nonheme Octahedral FeIVO Species

The experimentally measured bimolecular reaction rate constant, k(2), should in principle correlate with the theoretically calculated rate-limiting free energy barrier, ΔG(≠), through the Eyring equation, but it fails quite often to do so due to the inability of current computational methods to account in a precise manner for all the factors contributing to ΔG(≠). This is further aggravated by the exponential sensitivity of the Eyring equation to these factors. We have taken herein a pragmatic approach for C-H activation reactions of 1,4-cyclohexadiene with a variety of octahedral nonheme Fe(IV)O complexes. The approach consists of empirically determining two constants that would aid in predicting experimental k(2) values uniformly from theoretically calculated electronic energy (ΔE(≠)) values. Shown in this study is the predictive power as well as insights into energy relationships in Fe(IV)O C-H activation reactions. We also find that the difference between ΔG(≠) and ΔE(≠) converges at slow reactions, in a manner suggestive of changes in the importance of the triplet spin state weight in the overall reaction.

Reactivity of cycloxydim toward singlet oxygen in solution and on wax film

The reactivity of the herbicide cycloxydim (CD) toward singlet oxygen was studied in organic solution and on wax films to mimic the leaf surface. Experiments in solution were conducted in acetonitrile using phenalenone as a sensitizer. For the experiments in the solid state, phenalenone was included in paraffinic wax films and CD deposited at the film surface. By laser flash photolysis we observed that the triplet of phenalenone reacts with CD with a bimolecular rate constant of (9.6±1)×106M−1s−1. However, scavenging experiments using β-carotene as a singlet oxygen quencher showed that the observed CD degradation in aerated acetonitrile involves singlet oxygen essentially. The bimolecular rate constant of reaction of CD with singlet oxygen was evaluated to (1.0±0.2)×107M−1s−1. Phenalenone included in the wax films also significantly increased the rate of CD photodegradation, the involvement of singlet oxygen being very likely. The photoproducts formed via the singlet oxygen pathway resulted from the oxidation of the thiol group and/or the cyclohexene ring. This study should help to better understand the fate of CD after its spraying on crops.

Photochemical fate of beta-blockers in NOM enriched waters

Beta-blockers, prescribed for the treatment of high blood pressure and for long-term use after a heart attack, have been detected in surface and ground waters. This study examines the photochemical fate of three beta-blockers, atenolol, metoprolol, and nadolol. Hydrolysis accounted for minor losses of these beta-blockers in the pH range 4–10. The rate of direct photolysis at pH 7 in a solar simulator varied from 6.1 to 8.9h−1 at pH 7. However, the addition of a natural organic matter (NOM) isolate enhanced the photochemical loss of all three compounds. Indirect photochemical fate, generally described by reactions with hydroxyl radical (OH) and singlet oxygen (1ΔO2), and, the direct reaction with the triplet excited state, 3NOM⁎, also varied but collectively appeared to be the major loss factor. Bimolecular reaction rate constants of the three beta-blockers with 1ΔO2 and OH were measured and accounted for 0.02–0.04% and 7.2–38.9% of their loss, respectively. These data suggest that the 3NOM⁎ contributed 50.6–85.4%. Experiments with various 3NOM⁎ quenchers supported the hypothesis that it was singly the most important reaction. Atenolol was chosen for more detailed investigation, with the photoproducts identified by LC–MS analysis. The results suggested that electron-transfer could be an important mechanism in photochemical fate of beta-blockers in the presence of NOM.