Ratio Of Rate Constants Research Articles

Tropospheric Oxidation of 1,1,2,3-Tetrafluoropropene (CF2=CF–CH2F) Initiated by ·OH Radical and Aerial Degradation of Its Product Radicals

It is essential to understand the tropospheric chemistry and environmental impact of 1,1,2,3-tetrafluoropropene (CF2=CF–CH2F) before its wide-scale applications. So, in this present work, we have performed the quantum chemical calculations of the ·OH radical initiated oxidation of CF2=CF–CH2F. First, all the reaction species of the title reaction are optimized at the M06-2X/6-311+G(d,p) level of theory and then the energies are refined at the CCSD(T) level with the same basis set. The relative energy and thermochemistry estimations indicate that the addition reactions of the ·OH radical to the carbon atoms of the double bond of CF2=CF–CH2F are energetically and thermodynamically more dominant rather than the direct H-abstraction reactions by the ·OH radical. The rate constant and branching ratio analysis also indicate the favorability of the ·OH-addition reactions over the H-abstraction reactions. The estimated overall rate constant at 298.15 K for this reaction is found to be 9.27 × 10–13 cm3 molecule–1 s–1 computed by variational transition state theory. The atmospheric lifetime of CF2 = CF–CH2F is calculated as 12.5 days with respect to ·OH reaction. The radiative efficiency is calculated as 0.016 W m–2 ppb–1, and global warming potentials of CF2=CF–CH2F for the different time horizons are significantly low. Further, photochemical ozone creation potential is estimated as 5.55. Finally, aerial degradation of the ·OH-addition product radicals was studied in the presence of the NO radical using the same quantum chemical method. From this degradation study, we have observed that CF2(OH)COF, FCOCH2F, FCFO, FCOOH, and ·CH2F are formed as end products.

Influence of Brønsted Acid-Site Density on Reaction-Diffusion Phenomena that Govern Propene Oligomerization Rate and Selectivity in MFI Zeolites

Brønsted acid zeolites catalyze propene oligomerization to higher-molecular-weight alkenes. Previous studies that only consider kinetic factors have reached contradictory conclusions regarding the dependence of oligomerization rates on H+-site density in MFI zeolites. Herein, we account for both the reaction and diffusion phenomena that govern the rates and selectivity of propene oligomerization and interrogate suites of MFI samples synthesized with widely varying H+-site density (0.3–5.7 H+/unit cell; Si/Al 13–250) and crystallite size (0.03–2.65 μm). Heavier alkene products become occluded within zeolitic micropores during reaction and impose intrazeolite diffusional restrictions for all MFI samples over wide ranges of reaction conditions (483–523 K, 7–615 kPa C3H6). This occluded organic phase becomes heavier in composition at higher propene pressures and lower reaction temperatures, which favor the growth of higher-molecular-weight oligomers, resulting in effective diffusivities of propene and product alkenes that systematically decrease with increasing propene pressure. Analysis of product selectivity reveals that MFI samples of lower H+-site density possess higher rate constant ratios for trimerization relative to dimerization, leading to the formation of heavier alkenes within their micropores and, in turn, to a greater decrease in the effective diffusivity of propene and product alkenes with increasing propene pressure. As a result, measured dimerization rates on MFI samples, which convolve the influences of kinetic factors and intrazeolite diffusional constraints imposed by the occluded organic phase, exhibit an increasingly negative-order dependence on propene pressure with decreasing H+-site density. The combined influences of kinetic factors and intrazeolite diffusional constraints result in different dependences of oligomerization rate on H+-site density when measured at different propene pressures, reconciling apparent contradictions among previous reports. Overall, these findings reveal the strong influences of H+-site density in MFI zeolites on both the kinetics of propene oligomerization reactions and the composition of the product-derived intrazeolite organic phase that influences effective diffusivities, and consequently measured propene oligomerization rates and product selectivity.

Theoretical studies on the kinetics and dynamics of the BeH+ + H2O reaction: comparison with the experiment.

The reaction of BeH+ with background gaseous H2O may play a role in qubit loss for quantum information processing with Be+ as trapped ions, and yet its reaction mechanism has not been well understood until now. In this work, a globally accurate, full-dimensional ground-state potential energy surface (PES) for the BeH+ + H2O reaction was constructed by fitting a total of 170 438 ab initio energy points at the level of RCCSD(T)-F12/aug-cc-pVTZ using the fundamental invariant-neural network method. The total root-mean-square error of the final PES was 0.178 kcal mol-1. For comparison, quasi-classical trajectory calculations were carried out on the PES at an experimental temperature of 150 K. The obtained thermal rate constant and product branching ratio of the BeD+ + H2O reaction agreed quite well with experimental results. In addition, the vibrational state distributions and energy disposals of the products were calculated and rationalized using the sudden vector projection model.

Rate constant and branching ratio of the reaction of ethyl peroxy radicals with methyl peroxy radicals.

The cross-reaction of ethyl peroxy radicals (C2H5O2) with methyl peroxy radicals (CH3O2) (R1) has been studied using laser photolysis coupled to time resolved detection of the two different peroxy radicals by continuous wave cavity ring down spectroscopy (cw-CRDS) in their AÃ-X̃ electronic transition in the near-infrared region, C2H5O2 at 7602.25 cm-1, and CH3O2 at 7488.13 cm-1. This detection scheme is not completely selective for both radicals, but it is demonstrated that it has great advantages compared to the widely used, but unselective UV absorption spectroscopy. Peroxy radicals were generated from the reaction of Cl-atoms with the appropriate hydrocarbon (CH4 and C2H6) in the presence of O2, whereby Cl-atoms were generated by 351 nm photolysis of Cl2. For different reasons detailed in the manuscript, all experiments were carried out under excess of C2H5O2 over CH3O2. The experimental results were best reproduced by an appropriate chemical model with a rate constant for the cross-reaction of k = (3.8 ± 1.0) × 10-13 cm3 s-1 and a yield for the radical channel, leading to CH3O and C2H5O, of (ϕ1a = 0.40 ± 0.20).

α-нуклеофилы – основа супернуклеофильных организованных микрогетерогенных систем для разложения фосфорорганических соединений

Peroxyhydrolysis of 4-nitrophenyl esters of phosphoric and phosphonic acids has been studied in organized microheterogeneous systems based on dimeric cationic imidazole-containing surfactants (AlkIm+-(CH2)m-Im+Alk ∙ 2Br- , m = 2,3,4, Alk = C12H25, C14H29). Micellar effects of the surfactants (at pH = const and [surfactant]0 = const) reach ~ 10–100 times. Physicochemical parameters of the peroxyhydrolysis process (such as substrate binding constants, hydroperoxide anion nucleophilicity in micellar pseudophase) are described in terms of the pseudophase distribution model. Observed rate enhancement mainly depends on reagent concentration increasing directly connected to hydrophobicity of the substrate and surfactant. Changes of HOO- nucleophilicity under the displacement of the peroxyhydrolysis from water to surfactant micelles also contributed to the micellar effects. Comparative analysis of the regularities of peroxyhydrolysis and alkaline hydrolysis testifies for generality of such features in OMS for nucleophilic substitutions. In this regard, the α-effect as rate constants ratio of peroxyhydrolysis (𝑘𝑘2,𝐻𝐻𝐻𝐻𝑂𝑂− 𝑚𝑚 ) to base hydrolysis (𝑘𝑘2,𝐻𝐻𝑂𝑂− 𝑚𝑚 ) in micellar pseudophase 𝑘𝑘2,𝐻𝐻𝐻𝐻𝑂𝑂− 𝑚𝑚 /𝑘𝑘2,𝐻𝐻𝑂𝑂− 𝑚𝑚 ) reaches ~ 80–100 times. Therefore, the α-effect appears not only in water, but in OMS, too.

Chemically reacting mixing in coaxial miscible liquid jets under variable viscosities and reaction rates

We study the mixing and reactions between two fluids with disparate viscosities in a co-axial flow at high Schmidt number, through numerical simulations. The chemical reactions chosen in this work are competitive-consecutive in nature. Numerical implementation of the reactions proceeds as given by Li and Toor (1986), in a Large-Eddy Simulation (LES) framework. The aim of the present study is to investigate the effect of reaction rate constant ratio on the product and byproduct formations under distinct viscosity ratios between the annular and central jets. Flow features promoting mixing and reactions such as turbulence, engulfment, jet instability, waves and break-up are described. The effect of viscosity difference between the jet and co-flow on the mixing and reactions are also delineated. It is found that selectivities of the undesirable product decrease with reducing viscosity ratio as well as increasing reaction rate constant ratio.

Synthesis and Photophysical Properties of Novel Meta-Conjugated Organic Molecules with 1,3,5-Benzene Branching Units

The synthesis and photophysical investigation of three novel meta-conjugated molecules based on 3,1,2-benzothiadiazole and thiophene-2,5-diyl derivatives linked through 1,3,5-benzene branching units are described. Each of them is a symmetrical molecule with two branching units, four identical lateral thiophene-containing fragments, and one central benzothiadiazole-containing fragment. To study the effect of the chemical structure on their photophysical properties, the molecules with different linearly conjugated lateral and central fragments due to incorporation of additional thiophene rings were synthesized and compared. It was shown that absorption spectra of the meta-conjugated molecules can be represented as a sum of absorption bands of model compounds for their peripheral and central fragments containing a common benzene ring being branched at the 1,3,5-benzene unit in the meta-conjugated molecules. Therefore, they cannot be considered simply as isolated π-conjugated systems of their peripheral and central fragments. Instead, DFT calculations showed that several transitions between the orbitals located in different regions of the meta-conjugated molecule are responsible for the formation of their absorption spectra, and they strongly depend on the degree of their overlapping. Theoretical absorption spectra reconstructed from the DFT data demonstrated a good agreement with the experimental results: the transitions with larger oscillator strength correspond to the bands with higher molar extinction coefficients and vice versa. It was shown that luminescence spectral maxima of the meta-conjugated molecules monotonically shift to the lower energy from 489 to 540 and 613 nm with increasing the number of thiophene rings in the peripheral and central fragments, respectively. However, luminescence quantum yield of the meta-conjugated molecules critically depends on the length of linearly conjugated fragments in its structure decreasing from 24% to 1.3% with increasing the number of thiophene rings in the lateral fragments but increasing to 90% in the molecule with more thiophene rings in both types of the fragments. The results obtained are well correlated to the ratio of radiative and nonradiative deactivation rate constants of the meta-conjugated molecules that indicates a high rate of internal conversion between the excited states corresponding to different fragments of the molecule. The CV measurements allowed estimating the HOMO, LUMO, and bandgap values of the target and model compounds, which confirm the presence of meta-conjugation within the molecules investigated. Thus, connection of linearly conjugated fragments through meta-positions (meta-conjugation) of a benzene ring leads to an intermediate option between fully conjugated and nonconjugated molecules due to partial delocalization of electron density through the 1,3,5-substituted benzene branching center.

Multiconformer transition state theory rate constant and branching ratios for the OH‐initiated reaction of CH3OCF2CHF2 and its primary product, HC(O)OCF2CHF2

AbstractObtaining rate constants and branching ratios for the atmospheric degradation of organic compounds is crucial for their environmental assessment and for a deeper understanding of their atmospheric chemistry and reactivity. The theoretical prediction of such quantities is a challenging endeavor that grows with conformational complexity. In this work, we have theoretically calculated the rate constants and branching ratios for the tropospheric OH‐initiated oxidation reactions of the hydrofluoroether and its primary product, the fluorinated ester. The calculations were performed with a cost‐effective multiconformer transition state theory protocol employing the recently developed constrained transition state randomization method. The calculated rate constant lies within the error bars of the recommended experimental value, while the calculated rate coefficient of represents, to our knowledge, the first determination of this reaction's rate constant. The results provide new insight and elucidate the main mechanistic issues discussed in recent experimental work while they also encourage the use of this protocol in investigations of the tropospheric reactivity of other fluorinated organic compounds.

Ozonation/UV irradiation of dispersed Ag/AgI nanoparticles in water resources: stability and aggregation.

Proliferation of nanoparticles (NPs) as aqueous pollutants is a matter of growing concern today. The aggregation kinetics of colloidal bare silver (Ag, 20.5nm) and silver iodide (AgI, 15.3nm) NPs were investigated during ozone/ultraviolet (O3/UV) oxidation. Dynamic light scattering was applied to monitor the aggregation of NPs, and the z-average of treated samples was considered aggregate diameter. The effect of temperature, pH, and initial concentration of NPs was investigated on the aggregation rate constant and stability ratio. At a short oxidation period of approximately 1min, the lower stability ratio was achieved for Ag NPs (< 50) than AgI NPs (> 100). Under acidic conditions, the negative surface charge of both NPs was neutralized that resulted in faster aggregation. In contrast, the impact of temperature and initial concentration of NPs on the aggregation rate was different for both NPs, which was due to the type of O3/UV interaction with the surface of NPs and the thickness of the electrical double layer surrounding the NPs. The aggregation behavior of Ag NPs obeyed diffusion-limited regime, while an intermediate regime between diffusion- and reaction-limited was observed for AgI NP aggregation. The resulting aggregate morphologies showed that the clusters were ramified for Ag and compressed for AgI NPs. Applying the O3/UV oxidation process for water treatment purposes leads to a significant reduction in aggregation time for inherently unstable Ag and stable AgI toxic NPs from several hours or days to several minutes.

Chemical engineering kinetics of p-xylene chlorination in a gas-liquid agitated reactor

This study aims at determining both the rate constants of chlorination of p-xylene with molecular chlorine and FeCl3 as a catalyst at 70 °C and 90 °C and the related chemical engineering parameters to be used in the design of an effective chemical reactor.This was accomplished by a novel approach of constructing a linear mathematical function for a gas–liquid semi-batch agitated reactor correlating all engaged physical and chemical factors. Collecting values for all the necessary parameters for the application of the function, or in the absence of them calculating them, and exploiting the intercept and slope of the linear function became possible the apparent and the catalytic reaction rate constants to be calculated. It was found that the calculated values were of the same order of magnitude as values obtained by other researchers.The parameters needed to be calculated were Henry’s constant, the diffusion coefficient of the solute A (chlorine) to the solvent B (p = xylene), DAB, and the mean liquid phase mass transfer coefficient, kl. Other parameters of chemical engineering interest were also determined, namely, the interface per unit volume of liquid, a, the thickness of the liquid film, δl, and the Hatta number, MH.Evaluating the Hatta number estimated, it was concluded that the chlorination of p-xylene with molecular chlorine is very slow at both temperatures investigated, and almost no reaction occurs in the film compared to the bulk of the liquid. Thus, to increase the overall reaction rate, a large volume of liquid is needed while the increase of agitation intensity is of no value here.Combining the rate constant ratios obtained recently for the four consecutive second-order irreversible chlorinations of p-xylene with molecular chlorine with FeCl3 as a catalyst at 70 °C and under identical conditions it was possible to determine the values of all rate constants up to the fourth chlorinated product.

Kinetics of chromium(V)-oxo and chromium(IV)-oxo porphyrins: Reactivity and mechanism for sulfoxidation reactions

In this work, chromium(IV)-oxo porphyrins [CrIV(Por)(O)] (2) (Por = porphyrin) were produced either by oxidation of [CrIII(Por)Cl] (1) with iodobenzene diacetate or visible light photolysis of porphyrin‑chromium(III) chlorates. Subsequent oxidation of 2 with silver perchlorate gave chromium(V)-oxo porphyrins [CrV(Por)(O)](ClO4) (3) in three porphyrin ligands, including 5,10,15,20-tetramesitylporphyrin(TMP, a), 5,10,15,20-tetrakis(2,6-difluorophenyl)porphyrin(TDFPP, b), and 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (TPFPP, c). Complexes 2 and 3 reacted with thioanisoles to produce the corresponding sulfoxides, and their kinetics of sulfoxidation reactions with a series of aryl methyl sulfides(thioanisoles) were studied in organic solutions. Chromium(V)-oxo porphyrins are several orders of magnitudes more reactive than chromium(IV)-oxo species, and representative second-order rate constants (kox) for the oxidation of thioansole are (0.40 ± 0.01) M−1 s−1 (3a), and (2.82 ± 0.20) × 102 M−1 s−1 (3b), and (2.20 ± 0.01) × 103 M−1 s−1 (3c). The order of reactivity for 2 and 3 follows TPFPP > TDFPP > TMP, in agreement with the electrophilic nature of metal-oxo complexes. Hammett analyses indicate significant charge transfer in the transition states for oxidation of para-substituted thioanisoles by [CrV(Por)(O)]+. The ρ+ constants are −1.69 for 3a, −2.63 for 3b, and − 2.89 for 3c, respectively, mirror values found previously for related metal-oxo species. A mechanism involving the electrophilic attack of the CrV-oxo at sulfides to form a sulfur cation intermediate in the rate-determining step is suggested. Competition studies with chromium(III) porphyrin chloride and PhI(OAc)2 gave relative rate constants for oxidations of competing thioanisoles that closely match ratios of absolute rate constants from chromium(V)-oxo species, which are true oxidants under catalytic conditions.

Ethylene Polymerization over Supported Vanadium-Magnesium Catalysts with Different Vanadium Content: The Effect of Hydrogen on Molecular Weight Characteristics of the Produced Bimodal Polyethylene

Data are presented on the activity of supported vanadium-magnesium catalysts (VMCs) with different vanadium content in ethylene polymerization and the molecular weight characteristics of the produced polyethylene. The VC1 catalyst, with a very low vanadium content (0.12 wt.%), showed a sixfold higher activity per unit weight of vanadium than the VC2 catalyst with a high-vanadium content (4.0 wt.%). Additionally, the total activity of VC2 (kg PE/g cat·h) was fivefold higher when compared to VC1. The introduction of hydrogen in polymerization leads to a considerable decrease in the activity of both catalysts. The polyethylene obtained in the presence of hydrogen over both catalysts has a broad bimodal molecular weight distribution (MWD) with a distinct shoulder in the high-molecular region (Mw ≥ 106 g/mol). Decomposition of the MWD curves of bimodal polyethylene into two fractions (high- and low-molecular fractions) made it possible to determine for the first time the ratio of the reaction rate constants of chain transfer with hydrogen (KtrH) and polymer chain propagation (Kp) for two groups of the VMC active sites producing low- and high-molecular fractions of bimodal polyethylene.

Insight into the mechanism of DNA synthesis by human terminal deoxynucleotidyltransferase.

Terminal deoxynucleotidyltransferase (TdT) is a member of the DNA polymerase X family that is responsible for random addition of nucleotides to single-stranded DNA. We present investigation into the role of metal ions and specific interactions of dNTP with active-site amino acid residues in the mechanisms underlying the recognition of nucleoside triphosphates by human TdT under pre-steady-state conditions. In the elongation mode, the ratios of translocation and dissociation rate constants, as well as the catalytic rate constant were dependent on the nature of the nucleobase. Preferences of TdT in dNTP incorporation were researched by molecular dynamics simulations of complexes of TdT with a primer and dNTP or with the elongated primer. Purine nucleotides lost the "summarised" H-bonding network after the attachment of the nucleotide to the primer, whereas pyrimidine nucleotides increased the number and relative lifetime of H-bonds in the post-catalytic complex. The effect of divalent metal ions on the primer elongation revealed that Me&lt;sup&gt;2+&lt;/sup&gt; cofactor can significantly change parameters of the primer elongation by strongly affecting the rate of nucleotide attachment and the polymerisation mode.

Effective dose rate conversion coefficients for photons from radionuclides calculated using the Monte Carlo method and adult voxel phantoms

In radiation exposure situations, knowledge of the equivalent or effective dose is required to determine whether the individuals are working within safe limits. However, in most situations, doses cannot be measured directly, and tissue or organ absorbed doses and effective doses are determined computationally to help assess stochastic risks. This study incorporates voxel-based whole body models of ICRP (adult reference male and female phantoms) into the Monte Carlo code MCNP 6.1 to simulate radiation interactions inside the human body due to the photons emitted from 1179 different radioisotopes. Absorbed doses in tissues or organs were computed from simulations where a radioactive point source was placed 1 m away at 1 m above the ground for eight different source orientations with respect to each phantom. The results were subsequently multiplied by the radiation weighting factors (WR = 1 for photons) and the tissue weighting factors in accordance with the methodology of ICRP 103 to obtain organ equivalent doses and effective doses. The calculated effective dose for each radioisotope and source orientation was later divided by unit source activity and unit exposure time to convert to effective dose rate conversion coefficient (EDCC). This novel quantity is proposed in this study for more realistic dose assessments. The results show that the anterior-posterior sources led to the highest EDCC values for all the radioisotopes. In addition, the right posterior orientations yield the lowest EDCCs for most of the radioisotopes. Under all source orientations, EDCC values were highest for Cf-254 isotope and lowest for Sm-151. EDCC to traditional air kerma rate constant (AKRC) ratios were also calculated to compare these two quantities. For all source orientations and most radioisotopes, this ratio was less than one and its largest value was observed for the anterior-posterior source. The results obtained in this study are based on effective doses from photon emitting radioisotopes developed for whole body models under various source orientations, thus should more accurately represent the stochastic risk to individuals in an external exposure situations.

Inactivation of plant pathogenic bacterium Ralstonia solanacearum in drainage solution from hydroponic system by a rotating advanced oxidation contactor equipped with TiO2/zeolite composite sheets

A rotating advanced oxidation contactor (RAOC) equipped with TiO2/zeolite composite sheets was developed to inactivate the plant pathogenic bacterium Ralstonia solanacearum in drainage solution (DS) from a hydroponic system. The inactivation efficiency of R. solanacearum in DS and pure culture solution (PS) by the RAOC was compared with that achieved by a submerged composite sheet photocatalysis reactor (SSPR). The initial number of living bacteria (N0) was adjusted to around 106–107 CFU mL−1. The inactivation efficiency of R. solanacearum by the SSPR at 4.0 × 107 CFU mL−1 of N0 significantly decreased compared with that at 1.8 × 106 CFU mL−1 of N0 owing to the attenuation of UV intensity by light absorption and scattering by solids derived from R. solanacearum, while the RAOC achieved >2-log inactivation during 24 h of treatment regardless of N0. The inactivation of R. solanacearum by the RAOC decelerated after 6 h, possibly because of competition for reactive oxygen species between R. solanacearum and products accumulated by inactivation of R. solanacearum. The ratio of rate constants for inactivation of R. solanacearum in DS to that in PS by the RAOC was 8 times that for the SSPR. This shows that the RAOC greatly mitigates the light attenuation and inhibitory effects of coexisting substances on inactivation of R. solanacearum in the DS. The RAOC is therefore a promising and upscalable photocatalytic reactor for efficient inactivation of R. solanacearum in DS.

Analysis of reaction path and different lumped kinetic models for asphaltene hydrocracking

The product distribution of asphaltenes hydrocracking was studied in an autoclave with decalin as the liquid-phase system at different reaction temperatures and times to elucidate the relevant reaction mechanism. The results showed that the reaction products might be reversely condensed to generate secondary asphaltenes, resulting in a trend of first increasing and then decreasing the yields of some products at 420 °C. The significant yield increase of gas and gasoline at 420 °C might be due to the cracking of saturated and aromatic fractions to form light fractions. The conclusion was drawn the reaction behavior of asphaltenes hydrocracking under the research conditions followed the first-order reversible reaction kinetics by the reaction kinetic analysis, which further supported the inference that some products could recondense to form secondary asphaltenes. The four-lumped, five-lumped, and six-lumped models were constructed to study in depth the reaction behavior of asphaltene hydrocracking in the current system, and the results showed that the more similar the structure and composition of the reaction products to asphaltenes, the faster the reaction rate of their condensation to asphaltenes. Furthermore, the higher the temperature was more favorable to improve the selectivity and yield of transportation fuel diesel under the temperature of 380 °C-420 °C by the analysis of the ratio of rate constant. Although the applicability of lumped model was not positively correlated with the number of lumps, it is concluded that the six lumped model was the best model among the three models from the combined analysis of the applicability and the richness of the reaction information of the model.

Ti3+ self-doped TiO2 nanotube arrays revisited as Janus photoelectrodes for persulfate activation and water treatment

This study investigates the binary role of Ti3+ self-doped TiO2 nanotubes (bl-TNA) as an anode as well as a cathode in the photoelectrochemical (PEC) oxidative treatment of organic contaminants. Compared to the Ti cathode-based asymmetric system, the symmetric PEC system with Janus bl-TNA photoelectrodes demonstrated higher interfacial charge transfer, photoresponsive activity, and kinetic constant for bisphenol-A (BPA) degradation. The bl-TNA photocathode enhanced the formation of radicals by providing additional reaction sites for photocatalytic reactions and cathodic peroxydisulfate (PDS) activation. Additionally, the rate constant ratio for BPA degradation between the anode and cathode in the symmetric system changed from 7.5:1–1.3:1 under PEC and PEC/PDS conditions, indicating that PDS addition enhanced the photoactivity of the electrodes, with a higher rate at the cathode. Finally, only 2.4% performance decline was observed after 64 h with the periodic polarity reversal operation, demonstrating the long-term stability of the proposed PEC system.

Rate constant ratios in the consecutive chlorination of liquid‐phase p‐xylene with Cl2 and antimony pentachloride as a catalyst

AbstractThe rate constant ratios of the consecutive chlorination of p‐xylene at 70oC in a gas–liquid semibatch reactor using molecular chlorine, dichloroethane as a solvent, and antimony pentachloride as a catalyst was investigated up to the fourth successive reaction (tetrachloro‐p‐xylene production). The rate ratios were determined with both mathematical expressions and a graphical method proposed recently in the literature by use of the maxima of the successive products. In addition, to this simple method that concerns only one pair of values of products (single‐point maximum approach), another novel method is proposed and successfully applied concerning many pairs of corresponding values of intermediate products (multiple points maximum approach). This latter method permits the estimation of uncertainties and propagation of uncertainties that proved the usefulness and the possibilities of determining the parameters based on special points of the kinetic dependencies. The rate ratios found for monochloro‐p‐xylene (2‐chloro‐p‐xylene), dichloro‐p‐xylene (the sum of 2,3‐dichloro‐p‐xylene and 2,5‐dichloro‐p‐xylene), trichloro‐p‐xylene (2,3,5‐trichloro‐p‐xylene), and tetrachloro‐p‐xylene (2,3,5,6‐tetrachloro‐p‐xylene) are k2/k1 = 0.09741, k3/k1 = 0.002685, and k4/k1 = 0.000499. The ratio of the dichloro‐isomers produced was also determined in the initial stages as 3.3 in favor of 2,5‐dichloro‐p‐xylene, which is reasonable since 2,3‐dichloro‐p‐xylene is highly hindered by the adjacent groups on the aromatic nucleus. Nevertheless, as the production of trichloro‐p‐xylene increased considerably, the proportion of 2,3‐dichloro‐p‐xylene gradually diminished down to complete disappearance. This is attributed to the ease of this isomer to be further chlorinated to trichloro‐product since its available free sites on the nucleus are less hindered compared to 2,5‐dichloro‐p‐xylene. The standard uncertainties of the rate constant ratios, the standard deviation of the means, as well as the expanded uncertainties of the means were calculated and proved statistically accepted. Finally, the propagation of uncertainties for the trichloro‐p‐xylene was estimated using the partial derivatives of this product for each of the rate constant ratios.

"Derivation of Kinetic Equations for Free-Radical Nonbranched-Chain Processes of Hydrocarbon and Hydrogen Oxidation"

The derivation of kinetic equations for the oxidation processes by the free-radical nonbranched-chain mechanism is shown. This derivation is based on the proposed reaction scheme for the initiated addition of free radicals to the multiple bond of the molecular oxygen includes the addition reaction of the peroxyl free radical to the oxygen molecule to form the tetraoxyl free radical. This reaction competes with chain propagation reactions through a reactive free radical. The chain evolution stage in this scheme involves a few of free radicals, one of which (tetraoxyl) is relatively low-reactive and inhibits the chain process by shortening of the kinetic chain length. The rate equations (containing one to three parameters to be determined directly) are deduced using the quasi-steady-state treatment. The kinetic description with use the obtained rate equations is applied to the γ-induced nonbranched-chain processes of the free-radical oxidation of liquid o-xylene at 373 K and hydrogen dissolved in water containing different amounts of dioxygen at 296 K. The ratios of rate constants of competing reactions and rate constants of addition reactions to the molecular oxygen are defined. In these processes the oxygen with the increase of its concentration begins to act as an oxidation autoinhibitor (or an antioxidant), and the rate of peroxide formation as a function of the dissolved oxygen concentration has a maximum. It is shown that a maximum in these curves arises from the competition between hydrocarbon (or hydrogen) molecules and dioxygen for reacting with the emerging peroxyl 1:1 adduct radical. From the energetic standpoint possible nonchain pathways of the free-radical oxidation of hydrogen and the routes of ozone decay via the reaction with the hydroxyl free radical in the upper atmosphere (including the addition yielding the hydrotetraoxyl free radical, which can be an intermediate in the sequence of conversions of biologically hazardous UV radiation energy) were examined. The energetics of the key radical-molecule gas-phase reactions is considered.

Quantitative parameters of magnetic resonance imaging cannot predict human epidermal growth factor receptor 2 (HER2) status in rectal cancer

PurposeTo retrospectively investigate whether magnetic resonance imaging (MRI) quantitative parameters can differentiate human epidermal growth factor receptor 2 (HER2) status in rectal cancer. Materials and methodsThis study included 89 patients with surgically confirmed rectal cancer who underwent preoperative MRI from June 2014 to May 2019. Patients were divided into three groups: HER2 negative (HER2-Neg); HER2-low expression (HER2-L); and HER2 positive (HER2-Pos). Quantitative perfusion parameters derived from dynamic contrast-enhanced MRI (DCE-MRI) Tofts model (pharmacokinetic blood dual compartment model) were listed as follows: volume transfer constant (Ktrans), rate constant (Kep), and extracellular volume ratio (Ve). The mean, minimum, and maximum apparent diffusion coefficient (ADC) values at standard (800 s/mm2) b-values were obtained with diffusion-weighted imaging (DWI). Clinicopathologic characteristics and quantitative parameters were compared by Fisher's exact test and one-way analysis of variance (ANOVA), respectively. ResultsThe 89 patients included 52 (58.4%) with HER2-Neg, 31 (34.8%) with HER2-L, and 6 (6.8%) with HER2-Pos states. Fisher's exact test showed that clinicopathologic characteristics among the three groups were not significantly different (p = 0.281 to 1.000). Likewise, there were no associations between HER2 status and any quantitative parameters, including Ktrans (p = 0.296), Kep (p = 0.290), Ve (p = 0.184), ADCmean (p = 0.181), ADCmin (p = 0.143), or ADCmax (p = 0.058). ConclusionQuantitative perfusion parameters (Ktrans, Kep, Ve) and ADC values were not able to discriminate HER2 status in patients with rectal cancer or evaluate treatment response in real time.