Finding of remarkable synergistic effect on the aroxyl-radical-scavenging rates under the coexistence of α-tocopherol and catechins

Measurements of aroxyl radical (ArO^•)-scavenging rate constants () of antioxidants (AOHs) (α-tocopherol (α-TocH) and three catechins (CatHs) (ie, epicatechin (EC), epigallocatechin (EGC), and epigallocatechin gallate (EGCG)) were performed in ethanol solution, using stopped-flow spectrophotometry. values were measured not only for each AOH, but also for the mixtures of two AOHs (α-TocH and CatH). A notable synergistic effect that the value of α-TocH increases 1.29, 1.84, and 1.65 times under the coexistence of constant concentrations of EC, EGC, and EGCG, respectively, was observed for the solutions including α-TocH and CatH. Similarly, values of CatHs (EC, EGC, and EGCG) increased 1.72, 2.25, and 2.34 times under the coexistence of constant concentrations of α-TocH, respectively. UV-Vis absorption of α-tocopheroxyl radical (α-Toc^•) (λ_max = 428 nm), which had been produced by reaction of α-TocH with ArO^•, decreased remarkably under the coexistence of α-TocH and CatHs due to the fast α-TocH-regeneration reaction by CatHs. The result suggests that the prooxidant reaction due to α-Toc^• is suppressed by the coexistence of CatHs. By analyzing the formation and decay curves of α-Toc^•, it has been ascertained that one molecule of EGCG having three OH groups at B-ring may rapidly regenerate three molecules of α-Toc^• to α-TocH.     

Kinetic model for Ostwald's rule of stages with applications to Boc-diphenylalanine self-assembly

A kinetic model which describes Ostwald's rule of stages, during the process of crystal growth from solution, is reported here. Reaction equations for stages are given where the stages convert from one to another. The final stage reacts to release a portion of solute back into solution, while the remainder converts to the final equilibrium form. Additionally, a remnant of the solute that was not consumed by any of the transitional stages, ultimately is converted into the final product. This particular model was motivated by a recent report for Boc-diphenylalanine self-assembly where the dissolved peptide was observed to go through two polymorphic stages before reaching the equilibrium supramolecular assembly [A. Levin et al., Nat. Commun. 5, 5219, (2014)]. Kinetic data for the concentration of solute present during the process are listed in the above-mentioned report. We show here how the model, for , describes the time-dependent behavior of the solute decay during the growth process. After comparing the model to the experimental data, we are able to report values for all of the rate constants and propose a rule whereby the relative magnitudes of these constants can be used to predict whether a supersaturated substance will noticeably pass through transitional stages or simply convert from solute to the equilibrium solid form.     

Thermal rate constant for the C(3P) + OH(X2Π) → CO(X1Σ) + H(2S) reaction using stochastic energy grained master equation method

In the present work, the kinetic mechanism of the reaction is studied. The rate constants were determined using the Master Equation Solver for Multi-Energy Well Reactions (MESMER). The master equation modeling was also employed to examine the pressure dependence for each pathway involved. The theoretical analysis shows that the overall rate coefficient is practically independent of pressure up to 100 Torr for the temperature range 125-500 K. The unusual dependence of the overall rate constant with temperature was fit with the d-Arrhenius expression , where cm³molecule⁻¹s⁻¹, , and  kJ·mol⁻¹, for 125⩽ T ⩽ 500 K. The thermal rate constant results are in relatively good agreement with other theoretical studies.     

Oxidation of methylamine

A detailed chemical kinetic model for oxidation of methylamine has been developed, based on theoretical work and a critical evaluation of data from the literature. The rate coefficients for the reactions of CHNH + O CHNH / CHNH + HO, CHNH + H CH + NH, CHNH CHNH, and CHNH + O CHNH + HO were calculated from ab initio theory. The mechanism was validated against experimental results from batch reactors, flow reactors, shock tubes, and premixed flames. The model predicts satisfactorily explosion limits for CHNH and its oxidation in a flow reactor. However, oxidation in the presence of nitric oxide, which strongly promotes reaction at lower temperatures, is only described qualitatively. Furthermore, calculated flame speeds are higher than reported experimental values; the model does not capture the inhibiting effect of the NH group in CHNH compared to CH. More work is desirable to confirm the products of the CHNH + NO reaction and to look into possible pathways to NH in methylamine oxidation.     

Kinetics of two redox reactions in 2-butoxyethanol and water near its critical point of solution

The rate constants of two redox reactions and in the critical solution of 2-butoxyethanol and water have been measured by using the UV spectrophotometry at the initial reaction stage. It was found that the rate constants at various temperatures for two reactions were well described by the Arrhenius equation in the noncritical region. The critical slowing down effect was detected in the critical region. The critical slowing down exponents were determined to be 0.044 ± 0.004 and 0.046 ± 0.005 for reactions and , respectively. The values of the critical slowing down exponents showed that only dynamic critical slowing down effect, and no thermodynamic singularity could be observed for the two reactions.     

Multiconformer transition state theory rate constants for the reaction between OH and -dimethoxyfluoropolyethers

In this work, we have calculated rate constants for the tropospheric reaction between the OH radical and -dimethoxyfluoropolyethers. The latter are a specific class of the hydrofluoropolyethers family with the general formula , from which we have selected three case studies: , , and . The calculations were performed by applying a cost-effective protocol developed for bimolecular hydrogen-abstraction reactions and based on multiconformer transition state theory relying on computationally accessible M08-HX/apcseg-2//M08-HX/pcseg-1 calculations. Within the protocol's uncertainties and approximations, the results show that (1) the calculated rate constants have the same order of magnitude and (2) if observed together with previous experimental and theoretical investigations, the chain length (that varies with q and p) is seen to have a small effect on the rate constant, which is consistent with the “no discernible effect” reported in the experimental work.     

Twenty-species and 15-species chemical kinetic mechanisms for cyclohexane using the local self-similarity tabulation method

The 1081 species cyclohexane-oxidation elementary reaction mechanism of Silke et al. (DOI: 10.1021/jp067592d ) is reduced in the number of species by a factor using the local self-similarity tabulation (LS2T) method. Reduced-species mechanisms of both 20 (R20) and 15 (R15) species are created in the high-pressure combustion regime typical of diesel engines. To evaluate the performance of R20 and R15 against the elementary kinetics, simulations are performed for cyclohexane/air mixtures at initial temperatures of 1150, 900, 750, and 680 K and constant pressures of 20 and 40 bar for a variety of equivalence ratios (, 1.0, and 2 for 1150 and 900 K; for 750 K; for 680 K). Very good agreement between R20 and R15 with the elementary kinetics mechanism is demonstrated at 1150 and 900 K for which the self-similarity is very well obeyed; however, only fair agreement is obtained at 750 and 680 K, a fact which is traced to the less faithful adherence to the self-similarity due to the one order of magnitude increase in ignition time over the range 750-680 K. These results are found to be quasi-independent of the tabulation grid. Future work is proposed to improve the reduction in the cold-ignition, high-pressure regime.     

Direct measurements of C3F7I dissociation rate constants using a shock tube ARAS technique

This work presents the first direct experimental study on the thermal unimolecular decomposition of n-C₃F₇I. Experiments were performed behind incident and reflected shock waves using the atomic resonance absorption spectroscopy (ARAS) technique on a resonant line of atomic iodine at 183.04 nm. The reaction C₃F₇I + Ar → C₃F₇ + I + Ar (1) was studied at specific temperature (800–1200 K) and pressure (0.6–8.3 bar) ranges. Under experimental conditions, the obtained values of the rate constant at temperatures below 950 K are close to the high-pressure limit; however, considering theoretical calculations, the influence of pressure on the rate constant at elevated temperatures remains noticeable. The resulting value of the experimental rate constant of reaction 1 is presented in the following Arrhenius form: Experimental data were found to correlate with the results of the Rice–Ramsperger–Kassel–Marcus –master equation analysis based on quantum-chemical calculations. The following low- and high-pressure limiting rate coefficients were obtained over the temperature range T = 300–3000 K: with the center broadening factor F_c = 0.119.     

Predicting polycyclic aromatic hydrocarbon formation with an automatically generated mechanism for acetylene pyrolysis

Using Reaction Mechanism Generator (RMG), we have automatically constructed a detailed mechanism for acetylene pyrolysis, which predicts formation of polycyclic aromatic hydrocarbons (PAHs) up to pyrene. To improve the data available for formation pathways from naphthalene to pyrene, new high‐pressure limit reaction rate coefficients and species thermochemistry were calculated using a combination of electronic structure data from the literature and new quantum calculations. Pressure‐dependent kinetics for the CH potential energy surface calculated by Zádor et al. were incorporated to ensure accurate pathways for acetylene initiation reactions. After adding these new data into the RMG database, a pressure‐dependent mechanism was generated in a single RMG simulation which captures chemistry from C to C. In general, the RMG‐generated model accurately predicts major species profiles in comparison to plug‐flow reactor data from the literature. The primary shortcoming of the model is that formation of anthracene, phenanthrene, and pyrene are underpredicted, and PAHs beyond pyrene are not captured. Reaction path analysis was performed for the RMG model to identify key pathways. Notable conclusions include the importance of accounting for the acetone impurity in acetylene in accurately predicting formation of odd‐carbon species, the remarkably low contribution of acetylene dimerization to vinylacetylene or diacetylene, and the dominance of the hydrogen abstraction CH addition (HACA) mechanism in the formation pathways to all PAH species in the model. This work demonstrates the improved ability of RMG to model PAH formation, while highlighting the need for more kinetics data for elementary reaction pathways to larger PAHs.     

Theoretical investigation of the formic acid decomposition kinetics

Decomposition of formic acid (HCO₂H) proceeds via three unimolecular channels: dehydration, decarboxylation, and dissociation, the latter expected to be of minor contribution to the overall kinetics. In addition, despite the similar values reported for the individual activation energies for the dehydration and decarboxylation reactions, experimental works have shown that the former is dominant in the reaction mechanism. These reactions show pressure-dependent rate coefficients, and the high-pressure condition is not yet verified at atmospheric pressure. This work aims to investigate the influence of temperature and pressure on the rate coefficients. Hence, theoretical calculations at the CCSD(T)/CBS level have been performed to accurately describe the unimolecular reaction and Rice-Ramsperger-Kassel-Marcus (RRKM) rate coefficients have been calculated and integrated for the prediction of k(T,P) rate coefficients, adopting both strong and weak collision models, over the intervals 0.5-10 atm and 298-2200 K. Our results suggest that the isomerization path is important and explains the preference for the (CO + H₂O) channel. Rate coefficients for the (CO₂ + H₂) and (CO + H₂O) formations are given, in s⁻¹, as exp(−34404/T) and exp(−33785/T), respectively. The dissociation limit of 107.29 kcal mol^–1, with respect the Z-HCO₂H conformer, leading to OH + HCO, via a barrierless potential curve, with rate coefficients, in s⁻¹, expressed as k_HCO+OH(T) = 1.68 × 10¹⁷ exp(−56018/T). Temperature and pressure dependence for the HCO + OH → CO₂ + H₂ and HCO + OH → CO + H₂O reactions have also been estimated.     

Formation of phenanthrene via H-assisted isomerization of 2-ethynylbiphenyl produced in the reaction of phenyl with phenylacetylene

Model chemistry G3(MP2,CC)//B3LYP/6-311G(d,p) calculations of the potential energy surface for the reaction of phenyl radical (C₆H₅) with phenylacetylene (C₈H₆) have been carried out and combined with Rice-Ramsperger-Kassel-Marcus/Master Equation calculations of temperature- and pressure-dependent rate constants. The results showed that the reaction can serve as a viable source for the formation of phenanthrene via an indirect route involving a primary reaction of phenyl addition to the ortho carbon in the ring of phenylacetylene and H elimination producing 2-ethynylbiphenyl followed by secondary H-assisted isomerization of 2-ethynylbiphenyl to phenanthrene. In the secondary reaction, the H atom adds to the α carbon of the ethynyl side chain, then a six-member ring closure takes place followed by aromatization via an H loss. The channel of H addition to the side chain of 2-ethynylbiphenyl appears to be much faster than H addition to the ortho carbon in the ethynyl-substituted ring leading back to the initial C₆H₅ + C₈H₆ reactants. Rate constants for the primary C₆H₅ + C₈H₆2-ethynylbiphenyl ( p1 ) + H and secondary p1  + Hphenanthrene ( p2 ) + H reactions have been computed in the temperature range of 500-2500 K at pressures of 30 Torr, 1, 10, and 100 atm and fitted to modified Arrhenius expressions. The suggested kinetic scheme and rate constants are proposed as a prototype for the modeling of the growth of polycyclic aromatic hydrocarbons via the phenyl addition-dehydrocyclization (PAC) mechanism involving an addition of a PAH radical to an ethynyl-substituted PAH molecule.     

Size Evolution of Photoabsorption Spectra of Small Clusters: A Computational Study

Photoabsorption spectra of clusters, N=5–9, have been calculated using a diatomics-in-molecules like electronic structure model and a path-integral Monte Carlo sampling method. A qualitative change in the calculated spectra has been observed at N=9, which has been interpreted in terms of a structural transformation in the clusters consisting in a transition from trimer-like ionic cores observed for N≤7 to dimer-like ionic cores prevailing in through an intermediate state (comparable abundances of both types of ionic cores) observed in . The calculated spectra have been thoroughly compared with an earlier calculation on , , and reported from our group and data available for the same cluster sizes from an experiment.     

Leaching kinetics of electric arc furnace dust in nitric acid solutions

Electric arc furnace dust contains mainly ZnO, ZnFe₂O₄, and iron oxides. In this study, chemical composition of ZnO, ZnFe₂O₄, and Fe₂O₃ and leaching kinetics of ZnO, ZnFe₂O₄, and Fe₂O₃ in HNO₃ solutions were investigated. It was seen that the dissolution of ZnO is very fast, therefore the leaching kinetics of ZnO cannot be determined. Kinetic parameters and model equations were derived for the leaching of ZnFe₂O₄ and Fe₂O_3. Leaching kinetics of ZnFe₂O₄ was explained by the pseudohomogeneous reaction model. Activation energy and order of HNO₃ concentration were found to be as 37.5 kJ mol⁻¹ and 0.37, respectively. The model equation was derived as . It was determined that experimental data for the leaching kinetics of Fe₂O₃ best fit with the shrinking core model (SCM). Activation energy and order of HNO₃ concentration were found to be as 51.5 kJ mol⁻¹ and 0.67, respectively The model equation was derived using SCM as _.     

Fluorine Substitution of TCNQ Alters the Redox-Driven Catalytic Pathway for the Ferricyanide-Thiosulfate Reaction

Mechanistic variation in catalysis through substituent-based redox tuning is well established. Fluorination of TCNQ (TCNQ=tetracyanoquinodimethane) provides ~850 mV variation in the redox potentials of the and (n=0, 2, 4) processes. With , catalysis of the kinetically very slow ferrocyanide-thiosulfate redox reaction in aqueous solution occurs via a mechanism in which the catalyst is reduced to when reacting with which is oxidised to . Subsequently, reacts with to form and reform the catalyst, in another thermodynamically favoured process. An analogous mechanism applies with as a catalyst. In contrast, since the reaction of with is thermodynamically unfavourable, an alternative mechanism is required to explain the catalytic activity observed in this non-fluorinated system. Here, upon addition of , reduction of to occurs with concomitant oxidation of to , which then acts as the catalyst for oxidation. Thermodynamic data explain the observed differences in the catalytic mechanisms. (n=0, 4) also act as catalysts for the ferricyanide-thiosulfate reaction in aqueous solution. The present study shows that homogeneous pathways are available following addition of these dissolved materials. Previously, these (n=0, 4) coordination polymers have been regarded as insoluble in water and proposed as heterogeneous catalysts for the ferricyanide-thiosulfate reaction. Details and mechanistic differences were established using UV-visible spectrophotometry and cyclic voltammetry.     

А kinеtiс аnаlysis fоr prоduсtiоn of саlсium bоrоgluсоnаtе frоm colemanite

In this paper, the kinetic model of colemanite dissolution in gluconic acid solutions was carried out in a batch reactor. The effects of the particle size, reaction temperature, stirring speed, gluconic acid concentration, and solid/liquid ratio on colemanite dissolution were experimentally studied. The empirical parameters were the gluconic acid concentration (0.05-0.2 M), the temperature (20-50°C), the solid/liquid ratio (0.05/500-1.5/500 g⋅L⁻¹), particle size (193.5-1000 μm), and stirring speed (400-700 rpm). The kinetic models for heterogeneous solid-liquid reactions were used with the dissolution data in evaluating the kinetic. The dissolution of colemanite in gluconic acid solutions was controlled by diffusion through the product layer. The activation energy was found to be 8.39 kJ⋅mol⁻¹. The rate expression associated with the dissolution rate of colemanite depending on the parameters chosen may be summarized as follows:      

On the normalized Laplacian of Möbius phenylene chain and its applications

Let denote a molecular graph of linear [n] phenylene with n hexagons and n squares, and let the Möbius phenylene chain be the graph obtained from the by identifying the opposite lateral edges in reversed way. Utilizing the decomposition theorem of the normalized Laplacian characteristic polynomial, we study the normalized Laplacian spectrum of , which consists of the eigenvalues of two symmetric matrices ℒ _R and ℒ _Q of order 3n. By investigating the relationship between the roots and coefficients of the characteristic polynomials of the two matrices above, we obtain an explicit closed-form formula of the multiplicative degree-Kirchhoff index as well as the number of spanning trees of . Furthermore, we determine the limited value for the quotient of the multiplicative degree-Kirchhoff index and the Gutman index of .     

Complexes Featuring a cis-[M U M] Core (M=Rh,Ir): A New Route to Uranium-Metal Multiple Bonds

Although examples of multiple bonds between actinide elements and main-group elements are quite common, studies of the multiple bonds between actinide elements and transition metals are extremely rare owing to difficulties associated with their synthesis. Here we report the first example of molecular uranium complexes featuring a cis-[M U M] core (M=Rh, Ir), which exhibits an unprecedented arrangement of two M U double dative bond linkages to a single U center. These complexes were prepared by the reactions of chlorine-bridged heterometallic complexes [{U{N(CH₃)(CH₂CH₂NPⁱPr₂)₂}(Cl)₂[(μ-Cl)M(COD)]₂}] (M=Rh, Ir) with MeMgBr or MeLi, a new method for the construction of species with U−M multiple bonds. Theoretical calculations including dispersion confirmed the presence of two U M double dative bonds in these complexes. This study not only enriches the U M multiple bond chemistry, but also provides a new opportunity to explore the bonding of actinide elements.     

A study of some kinetic aspects of the CCl4 interaction with oxide ions in KCl-SrCl2 melts with different content of SrCl2

A comparative investigation of a complex process of the interaction between CCl₄ vapor and oxide ions O^2– (carbochlorination) in K₂SrCl₄ and KSr₂Cl₅ melts at 973 K was performed by the potentiometric method using Pt(O₂)|ZrO₂(Y₂O₃) membrane oxygen electrode as reversible to oxide ion. The analysis of the limiting stages of this process was made on the basis of van't Hoff diagrams. The entire process can be divided into three stages with corresponding limiting processes: the rate of CCl₄ dissolution in the melts for stage 1, the chemical reaction in the melts for stage 2, and the rate of the contamination of the melts with oxygen-containing admixtures for the stage 3. The rate constants of the carbochlorination process in both melts at 973 K were calculated using the data corresponding to stage 2 as (4.4 ± 0.25) × 10⁵ kg mol⁻¹ min⁻¹ for K₂SrCl₄ and (1.83 ± 0.5) × 10⁵ kg mol⁻¹ min⁻¹ for KSr₂Cl₅. The final concentration of oxide ions after the treatment is higher ( = (1.6 ± 0.7) × 10⁻⁷ mol kg⁻¹ for KSr₂Cl₅ and  = (2.5 ± 1.3) × 10⁻⁸ mol kg⁻¹ for K₂SrCl₄ melt, respectively). This corresponds to the difference in the oxoacidic properties of the studied melts.     

Photoionization Bands of Cyanogen: Multi-Mode Vibronic Coupling and Renner-Teller Effects

Multi-mode vibronic coupling in the , , and electronic states of Cyanogen radical cation (C N ) is investigated with the aid of ab initio quantum chemistry and first principles quantum dynamics methods. The electronic degenerate states of Π symmetry of C N undergo Renner-Teller (RT) splitting along degenerate vibrational modes of π symmetry. The RT split components form symmetry allowed conical intersections with those from nearby RT split states or with non-degenerate electronic states of Σ symmetry. A parameterized vibronic Hamiltonian is constructed using standard vibronic coupling theory in a diabatic electronic basis and symmetry rules. The parameters of the Hamiltonian are derived from ab initio calculated adiabatic electronic energies. The vibronic spectrum is calculated, assigned and compared with the available experimental data. The impact of various electronic coupling on the vibronic structure of the spectrum is discussed.     

On the Mechanical Properties of Popgraphene-Based Nanotubes: a Reactive Molecular Dynamics Study

Carbon-based tubular materials have sparked a great interest in future electronics and optoelectronics device applications. In this work, we computationally studied the mechanical properties of nanotubes generated from popgraphene (PopNTs). Popgraphene is a 2D carbon allotrope composed of 5-8-5 rings. We carried out fully atomistic reactive (ReaxFF) molecular dynamics for PopNTs of different chiralities ( and ) and/or diameters and at different temperatures (from 300 up to 1200 K). Results showed that the tubes are thermally stable (at least up to 1200 K). All tubes presented stress/strain curves with a quasi-linear behavior followed by an abrupt drop of stress values. Interestingly, armchair-like PopNTs ( ) can stand a higher strain load before fracturing when contrasted to the zigzag-like ones ( ). Moreover, it was obtained that Young's modulus (Y_Mod) (750–900 GPa) and ultimate strength (σ_US) (120–150 GPa) values are similar to the ones reported for conventional armchair and zigzag carbon nanotubes. Y_Mod values obtained for PopNTs are not significantly temperature-dependent. While the σ_US values for the showed a quasi-linear dependence with the temperature, the exhibited no clear trends.