Two-step Consecutive Reaction Research Articles

An exploratory analysis of the electron donor–acceptor complex formation between risperidone and iodine by principal component analysis, soft- and second-order hard-modelling approaches

An exploratory analysis of the electron donor–acceptor complex formation between risperidone and iodine has been done by a strategy equipped with rank augmentation, principal component analysis (PCA) and soft-modelling approach, orthogonal projection approach (OPA), followed by second-order hard-modelling in two non-aqueous solvents with different polarities. From the results of PCA and OPA, the correct number of chemical components and the reactions were identified. In all cases the collected data were found to be rank deficient. In the case of electron donor–acceptor complex formation in acetonitrile, a single second-order reaction and for the process in chloroform, a consecutive two-step reaction was identified. Iodine participates in the second-order rate determining steps in the two solvents. Rank deficiency of the processes was broken through second-order global analysis of the multiple processes with different initial concentrations. Second-order global hard-modelling of the non-linear parameters (rate constants) as well as linear parameters (absorptivities) based on the model derived resulted in well-defined parameters with small standard deviations. Well-resolution of the parameters and the profiles indicated that the models were properly selected.

DME Dissociation Reaction on Platinum Electrode Surface: A Quantitative Kinetic Analysis by In Situ IR Spectroscopy

The kinetics of electrocatalytic dissociation reaction of dimethyl ether (DME) on a platinum (Pt) polycrystalline electrode in an acidic solution yielding carbon monoxide (CO) has been quantitatively analyzed by in situ IR spectroscopy in the potential region between 100 and 500 mV (vs reversible hydrogen electrode). A two-step consecutive reaction model, an initial dehydrogenation step followed by a CO formation step, is proposed for the dissociation process of the DME molecule. The mechanism of the DME dissociation on the Pt electrode surface is discussed based on the kinetic analysis.

Autooscillations in a perfectly stirred reactor with a two-step consecutive reaction

A mathematical model of the dynamic behavior of a perfectly stirred reactor in which a two consecutive exothermic reactions occurs is developed. Both stages are exothermic. In the Da-Se parametric space, the region of autooscillations (limiting cycles) is identified. The character of alternation of the stabilities of steady states is investigated. It is shown that, in the region of one steady state, two mechanisms of formation (disappearance) of limiting cycles are realized.

Consecutive Nucleation Events During Divetrification of Zr52.5Cu17.9Ni14.6Al10Ti5 Bulk Metallic Glass

Differential scanning calorimetry, x-ray, and in-situ synchrotron diffraction were used to study the divitrification of Zr52.5Cu17.9Ni14.6Al10Ti5 bulk metallic glass. Two consecutive exothermal peaks were identified by differential scanning calorimetry during both isochronal and isothermal scans. Examination of the X-ray and in-situ synchrotron diffraction patterns at various stages of annealing confirms that the exothermal peaks correspond to two nucleation events, with different local atomic structures. Modeling of the calorimetry data indicates that the devitrification in this alloy is adequately described by Johnson CMehl CAvrami theory with a two-step consecutive reaction model.

Mean centering of ratio spectra as a new method for determination of rate constants of consecutive reactions

Mean centering of ratio spectra is applied to resolve two-way kinetic-spectral data and acquire the rate constants and the spectrum of intermediate component. The proposed method can be applied for two-step consecutive reactions. The method is based on the mean centering of ratio spectra in order to obtain kinetic profile of intermediate component. Rate constants can be obtained using computational fitting. Given the kinetic parameters, the absorption spectrum of intermediate can be obtained through least squares regression. The proposed method can be applied to the reaction systems that reactant, intermediate or final product does not absorb in the investigation wavelength region. Also for very slow one-step reactions that pure spectrum for product is not available or overlaps completely with reactant spectrum, the proposed method can be used to resolve kinetic profile and consequently the pure spectrum of product. The applicability of the method has been evaluated by using model data. Alkaline hydrolysis of dimethyl phthalate as a real system was also studied by the proposed method.

Substrate- and Isoform-Specific Dioxygen Complexes of Nitric Oxide Synthase

Nitric oxide synthase (NOS) catalyzes the formation of NO via a consecutive two-step reaction. In the first step, L-arginine (Arg) is converted to N-hydroxy-L-arginine (NOHA). In the second step, NOHA is further converted to citrulline and nitric oxide (NO). To assess the mechanistic differences between the two steps of the reaction, we have used resonance Raman spectroscopy combined with a homemade continuous-flow rapid solution mixer to study the structural properties of the metastable dioxygen-bound complexes of the oxygenase domain of inducible NOS (iNOSoxy). We identified the O-O stretching frequency of the substrate-free enzyme at 1133 cm-1. This frequency is insensitive to the presence of tetrahydrobiopterin, but it shifts to 1126 cm-1 upon binding of Arg, which we attribute to H-bonding interactions to the terminal oxygen atom of the heme iron-bound dioxygen. In contrast, the addition of NOHA to the enzyme did not bring about a shift in the frequency of the O-O stretching mode, because, unlike Arg, there is no H-bond associated with the terminal oxygen atom of the dioxygen. The substrate-specific H-bonding interactions play a critical role in determining the fate of the key peroxy intermediate. In the first step of the reaction, the H-bonds facilitate the rupture of the O-O bond, leading to the formation of the active ferryl species, which is essential for the oxidation of the Arg. On the other hand, in the second step of the reaction, the absence of the H-bonds prevents the premature O-O bond cleavage, such that the peroxy intermediate can perform a nucleophilic addition reaction to the substrate, NOHA.

Micellar induced regioselectivity in the two-step consecutive reaction of SO 2−3 with Br [sbnd](CH 2CH 2) n[sbnd]Br ( [formula omitted

High field (800 MHz) 1H NMR was used to monitor the two-step consecutive reaction of excess SO 2− 3 with symmetrical bifunctional α , ω -dibromoalkanes with butane (DBB), hexane (DBH), octane (DBO), and decane (DBD) chains in CTAB micelles at 25 °C. The first-order rate constant for the first substitution step for DBB and DBH is about 5 times faster than for the second, but the kinetics for DBO and DBD were not cleanly first-order. After 40 min, the solution contained about 80% of the intermediate bromoalkanesulfonate from DBB and DBH and the remainder is alkanedisulfonate and unreacted starting material. The same reactions were carried out in homogeneous MeOH/D 2O solutions at 50 °C. The rate constants for all four α , ω -dibromoalkanes were first-order throughout the time course of the reaction and the same within ±10%. However, because micellar solutions are organized on the nanoscale and bring together lipophilic and hydrophilic reactants into a small reaction volume at the micellar interface, they speed this substitution reaction considerably compared to reaction in MeOH/D 2O. The CTAB micelles also induce a significant regioselectivity in product formation by speeding the first step of the consecutive reaction more than the second. The results are consistent with the bromoalkanesulfonate intermediates having a radial orientation within the micelles with the CH 2SO − 3 group in the interfacial region and the CH 2Br group directed into the micellar core such that the concentration of CH 2Br groups in the reactive zone, i.e., the micellar interface, is significantly reduced. These results provide the first example of self-assembled surfactant system altering the relative rates of the reaction steps of a consecutive reaction and, in doing so, enhancing monosubstitution of a symmetrically disubstituted species.

Study on pyrolysis kinetics of walnut shell

Slow pyrolysis of walnut shell which is a cheap and abundantly available solid waste was carried out using thermogravimetric analysis. The effects of raw material heating rate on the pyrolysis properties and kinetic parameters were investigated. A two-step consecutive reaction model were used to simulate the pyrolysis process. The kinetic parameters were established by using the pattern search method. Comparison between experimental data and the model prediction indicated that the two-step consecutive reaction model can better describe the slow pyrolysis of walnut shell as the formation of an intermediate during the pyrolysis process was taken into account.

Kinetics and Mechanism of Cyclodextrin Inclusion Complexation Incorporating Bidirectional Inclusion and Formation of Orientational Isomers

The kinetics of cyclodextrin (CD) inclusion complexation has been usually analyzed in terms of a one-step reaction or a consecutive two-step reaction involving intracomplex structural transformation as a second step. These schemes presume the inclusion of guest molecules through only one side of the CD cavity and the formation of unidirectional CD complexes. However, there has been increasing experimental evidence for the inclusion of guests through both sides of the CD cavity and the formation of orientational isomers for noncentrosymmetric guest molecules. This article presents a novel parallel reaction scheme for CD inclusion complexation, incorporating bidirectional inclusion and the formation of orientational isomers into the scheme. It is shown that the parallel reaction scheme gives the same concentration versus reaction time relationship as the consecutive two-step reaction scheme. The experimental methods for determining the microscopic directional rate constants are presented. The kinetic parameters of the two-step reaction scheme are expressed as functions of the directional rate constants. The ratios of orientational isomers of alpha-CD-based [2]-pseudorotaxanes and the microscopic directional rate constants of the threading and dethreading reactions are estimated from the reported thermodynamic and kinetics data obtained by using either the one-step or two-step reaction scheme. It is shown that the thermodynamic preference of an isomer over the other is mainly due to the slow dethreading rate of the isomer.

Preparation, thermal decomposition and lifetime of europium tris(dibenzoylmethanato)phenanthroline complex doped xerogel

The Eu tris(dibenzoylmethanato)phenanthroline complex doped xerogel has been synthesized by a catalyst-free sol-gel roure. The non-isothermal kinetic analysis is calculated by Friedman isoconversional method and multivariate non-linear regression method. The overall decomposition process below 600°C is fitted by an Fn model (n order reaction), corresponding to the dehydration of the matrix, and a two-step consecutive reaction of Cn model (n order autocatalytic reaction), corresponding to the decomposition of organic complex. Correlation coefficient is 0.99986. The lifetime values of xerogel, defined as the 5% decomposition of europium organic complex, indicate that the xerogel can find application at near room temperature.

Biphasic kinetics in the reaction between amino acids or glutathione and the chromium acetate cluster, [Cr 3O(OAc) 6] +

Kinetics for the breakdown of the trinuclear chromium acetate cluster with a series of monoprotic and diprotic amino acid ligands and with glutathione in aqueous media have been investigated spectrophotometrically at pH 3.5–5.5 and in a temperature range of 45–60 °C. Under pseudo-first-order conditions, reactions with these ligands exhibited biphasic kinetic behavior that can be accounted for by a consecutive two-step reaction, A → B → C, where A is assumed to be a forced ion pair, B an intermediate and C is the product; experimental data fit to a biexponential equation for the transformation. Rates for k short, k long, and k obs were determined by manual extrapolation of absorbance data or curve-fitting routines; associated activation parameters for each step of the reaction were calculated using the Eyring equation. Rates for the first and second steps of the reaction are on the order of ∼10 −4 and ∼10 −5 s −1, respectively. The large negative values of Δ S ‡ and smaller Δ H ‡ in the first step indicate an associative step, while high positive values of Δ S ‡ in the second step indicate dissociation. To account for the results mechanistically, the results are interpreted to be a first step of ligand exchange with a pseudo-axial aqua ligand, followed by a dissociative step involving acetate or oxo ligand displacement. The dissociative step is the rate determining step, with k obs ≈ k long. The results demonstrate reaction pathways that are available to the Cr(III) metal centers that may be physiologically relevant in the ligand-rich environment of biological systems. Under general conditions Cr(III) clusters may be expected to be broken down, unless some unique biological environment stabilizes the cluster. The present study has application to the processes related to Cr(III) transport and excretion, to potential mechanisms of Cr(III) action in a biological setting, and to the pharmacokinetics of Cr(III) supplements for animal and human consumption.

Two-step Consecutive Reaction Model of Biomass Thermal Decomposition by DSC

The thermal decomposition of one kind of biomass in air has been investigated by differential scanning calorimetry (DSC) analyzer. The results indicate that the heating process of samples from ambient temperature to 923 K at low heating rates shows two obvious exothermic peaks. According to the decomposition mechanism, the first exothermic step is attributed to oxidative degradation of hemicellulose and cellulose, and the second exothermic step is attributed to lignin degradation and char oxidation. The reaction model of the studied biomass thermal decomposition has been studied by iso-conversional methods and optimization computation. The results suggest that the two-step consecutive reaction model is suitable to describe the exotherm of biomass thermal decomposition in air.

Regulation of the Monomer-Dimer Equilibrium in Inducible Nitric-oxide Synthase by Nitric Oxide

The oxygenase domain of inducible nitric-oxide synthase exists as a functional tight homodimer in the presence of the substrate L-arginine and the cofactor tetrahydrobiopterin (H4B). In the absence of H4B, the enzyme is a mixture of monomer and loose dimer. We show that exposure of H4B-free enzyme to NO induces dissociation of the loose dimer into monomers in a reaction that follows single exponential decay kinetics with a lifetime of approximately 300 min. It is followed by a faster autoreduction reaction of the heme iron with a lifetime of approximately 30 min and the concurrent breakage of the proximal iron-thiolate bond, forming a five-coordinate NO-bound ferrous species. Mass spectrometry revealed that the NO-induced monomerization is associated with intramolecular disulfide bond formation between Cys104 and Cys109, located in the zinc-binding motif. The regulatory effect of NO as a dimer inhibitor is discussed in the context of the structure/function relationships of this enzyme.

Precise Investigation of the Axial Ligand Substitution Mechanism on a Hydrogenphosphato-Bridged Lantern-Type Platinum(III) Binuclear Complex in Acidic Aqueous Solution

Detailed equilibrium and kinetic studies on axial water ligand substitution reactions of the "lantern-type" platinum(III) binuclear complex, [Pt(2)(mu-HPO(4))(4)(H(2)O)(2)](2)(-), with halide and pseudo-halide ions (X(-) = Cl(-), Br(-), and SCN(-)) were carried out in acidic aqueous solution at 25 degrees C with I = 1.0 M. The diaqua Pt(III) dimer complex is in acid dissociation equilibrium in aqueous solution with -log K(h1) = 2.69 +/- 0.04. The consecutive formation constants of the aquahalo complex () and the dihalo complex () were determined spectrophotometrically to be log = 2.36 +/- 0.01 and log = 1.47 +/- 0.01 for the reaction with Cl(-) and log = 2.90 +/- 0.04 and log = 2.28 +/- 0.01 for the reaction with Br(-), respectively. In the kinetic measurements carried out under the pseudo-first-order conditions with a large excess concentration of halide ion compared to that of Pt(III) dimer (C(X)()- >> C(Pt)), all of the reactions proceeded via a one-step first-order reaction, which is a contrast to the consecutive two-step reaction for the amidato-bridged platinum(III) binuclear complexes. The conditional first-order rate constant (k(obs)) depended on C(X)()- as well as the acidity of the solution. From kinetic analyses, the rate-limiting step was determined to be the first substitution process that forms the monohalo species, which is in rapid equilibrium with the dihalo complex. The reaction with 4-penten-1-ol was also kinetically investigated to examine the reactivity of the lantern complex with olefin compounds.

The kinetics of thermal dehydration of copper(II) acetate monohydrate in air

The thermal decomposition of copper(II) acetate monohydrate was studied in air using TG-DTG/DTA, DSC, XRD techniques. TG-DTA curves show that the decomposition occurs in four steps. TG and XRD data indicate the reduction of Cu(II) during the decomposition of CuAc 2·H 2O. DTG and DSC data imply at least two steps contained in dehydration reaction, which is verified by the activation energy values estimated with Friedman and Flynn–Wall–Ozawa (FWO) methods. The dependence of activation energy on conversions indicates the dehydration reaction contains an additional initial reversible step. The most-probable kinetic model has been estimated with multivariate non-linear regression method assuming a two-step consecutive reaction. Bna (expanded Prout–Tompkins equation) → Cn ( n order autocatalytic reaction) model fits the original data best with a high correlation coefficient of 0.9998, and the calculated apparent activation energies of the fitted models are consistent with those calculated by isoconversional methods using original data. The corresponding function f( α), activation energy E and preexponential factor A of Bna, are (1 − α) 0.7593 α 0.2867, 128.5 kJ/mol and 1.6 × 10 15, respectively. Those of Cn, are (1 − α) 1.0534(1 + 2.712 α), 80.5 kJ/mol and 6.9 × 10 7, respectively. The combination of model free isoconversional methods and multivariate non-linear regression can give more reasonable and applicable models than commonly used model fitting methods.

Comparative performance of fixed-film biological filters: Application of reactor theory

Nitrification is classified as a two-step consecutive reaction where R1 represents the rate of formation of the intermediate product NO2-N and R2 represents the rate of formation of the final product NO3-N. The relative rates of R1 and R2 are influenced by reactor type characterized hydraulically as plug-flow, plug-flow with dispersion and mixed-flow. We develop substrate conversion models for fixed-film biofilters operating in the first-order kinetic regime based on application of chemical reactor theory. Reactor type, inlet conditions and the biofilm kinetic constants Ki (h−1) are used to predict changes in NH4-N, NO2-N, NO3-N and BOD5. The inhibiting effects of the latter on R1 and R2 were established based on the γ relation, e.g.:γ=KiKi,max=1−[BOD5][BOD5,max]Nwhere BOD5,max is the concentration that causes nitrification to cease and N is a variable relating Ki to increasing BOD5. Conversion models were incorporated in spreadsheet programs that provided steady-state concentrations of nitrogen and BOD5 at several points in a recirculating aquaculture system operating with input values for fish feed rate, reactor volume, microscreen performance, make-up and recirculating flow rates. When rate constants are standardized, spreadsheet use demonstrates plug-flow reactors provide higher rates of R1 and R2 than mixed-flow reactors thereby reducing volume requirements for target concentrations of NH4-N and NO2-N. The benefit provided by the plug-flow reactor varies with hydraulic residence time t as well as the effective vessel dispersion number, D/μL. Both reactor types are capable of providing net increases in NO2-N during treatment but the rate of decrease in the mixed-flow case falls well behind that predicted for plug-flow operation. We show the potential for a positive net change in NO2-N increases with decreases in the dimensionless ratios K2,(R2)/K1,(R1) and [NO2-N]/[NH4-N] and when the product K1,(R1)t provides low to moderate NH4-N conversions. Maintaining high levels of the latter reduces the effective reactor utilization rate (%) defined here as (RNavg/RNmax)100 where RNavg is the mean reactive nitrogen concentration ([NH4-N] + [NO2-N]) within the reactor, and RNmax represents the feed concentration of the same. Low utilization rates provide a hedge against unexpected increases in substrate loading and reduce water pumping requirements but force use of elevated reactor volumes. Further γ effects on R1 and R2 can be reduced through use of a tanks-in-series versus a single mixed-flow reactor configuration and by improving the solids removal efficiency of microscreen treatment.

Base promoted etherification reaction of a non-polar reactive dye and methanol by zeolite KA

An unexpected base promoted etherification reaction of a non-polar 4,6-dichloro-1,3,5-triazine type reactive dye and methanol was observed when methanol was dried over zeolite KA. A dye conversion of 100% was measured after 3 min reaction time at room temperature, while, when methanol was not pre-contacted with zeolite KA the dye conversion was only 31% after 24 h. The rate constants of the two-step consecutive reaction between dye and methanol were calculated in order to determine the accelerating effect. At 293 K, k 1 increased a factor of 3000 and k 2 a factor of 26,000 compared to these values observed with undried methanol. It has been found that the ion-exchange properties of the zeolite KA are responsible for the base promoted reaction. Elemental analyses of the zeolite KA, pellets and powder, and methanol extracts showed an exchange of cations between the two phases. In the case of pellets the binder material contributes to the cation content of the methanol. The potential of zeolite KA as a base promoter is discussed.

Heme Distortion Modulated by Ligand-Protein Interactions in Inducible Nitric-oxide Synthase

The catalytic center of nitric-oxide synthase (NOS) consists of a thiolate-coordinated heme macrocycle, a tetrahydrobiopterin (H4B) cofactor, and an l-arginine (l-Arg)/N-hydroxyarginine substrate binding site. To determine how the interplay between the cofactor, the substrates, and the protein matrix housing the heme regulates the enzymatic activity of NOS, the CO-, NO-, and CN(-)-bound adducts of the oxygenase domain of the inducible isoform of NOS (iNOS(oxy)) were examined with resonance Raman spectroscopy. The Raman data of the CO-bound ferrous protein demonstrated that the presence of l-Arg causes the Fe-C-O moiety to adopt a bent structure because of an H-bonding interaction whereas H4B binding exerts no effect. Similar behavior was found in the CN(-)-bound ferric protein and in the nitric oxide (NO)-bound ferrous protein. In contrast, in the NO-bound ferric complexes, the addition of l-Arg alone does not affect the structural properties of the Fe-N-O moiety, but H4B binding forces it to adopt a bent structure, which is further enhanced by the subsequent addition of l-Arg. The differential interactions between the various heme ligands and the protein matrix in response to l-Arg and/or H4B binding is coupled to heme distortions, as reflected by the development of a variety of out-of-plane heme modes in the low frequency Raman spectra. The extent and symmetry of heme deformation modulated by ligand, substrate, and cofactor binding may provide important control over the catalytic and autoinhibitory properties of the enzyme.

Kinetics and mechanism of base-catalysed degradations of substituted aryl-N-hydroxycarbamates, their N-methyl and N-phenyl analogues.

The kinetics and mechanism of the degradation reactions of substituted phenyl N-hydroxycarbamates and their N-methyl and N-phenyl analogues have been studied at pseudo-first-order reaction conditions in aqueous buffers and sodium hydroxide solutions at 20 [degree]C and 60 [degree]C and at I= 1 mol[middle dot]l(-1). The dependence of log k(obs) on pH for phenyl N-hydroxycarbamates at pH < 9 and pH > 13 is linear with the unit slope; at pH 10-12 log k(obs) is pH independent. The Bronsted coefficient [small beta](lg) is about -1 (pH 7-13) and -1.53 (pH > 13) indicating that the degradation reaction of phenyl N-hydroxycarbamates follows an E1cB mechanism giving the corresponding phenol/phenolate and HO-N[double bond, length as m-dash]C[double bond, length as m-dash]O. The latter species undergoes further decomposition to give carbonate, nitrogen and ammonia as final products. In contrast to the phenyl N-hydroxycarbamates the N-methyl derivatives at pH 7-9 undergo degradation to the corresponding phenol/phenolate, carbonate and methylamine via a concerted mechanism ([small beta](lg) is about -0.75). The only exception is 4-nitrophenyl N-hydroxy-N-methylcarbamate in which the predominant break down pathway proceeds via the Smiles rearrangement to give sodium N-methyl-(4-nitrophenoxy)carbamate. At pH > 9 the reaction of N-hydroxy-N-methylcarbamates is kinetically complex: the dependence of absorbance on time is not exponential and it proceeds as a consecutive two-step reaction. N-Hydroxy-N-phenylcarbamate under the same conditions undergoes degradation to phenol, carbonate, aniline and azoxybenzene.

Iterative target transformation factor analysis for the resolution of kinetic–spectral data with an unknown kinetic model

The resolution of two-way kinetic–spectral data obtained in the process of spectroscopic reaction with an unknown kinetic model must rely on chemometric algorithms with a non-model-based procedure. Iterative target transformation factor analysis (ITTFA), a type of self-modeling curve resolution (SMCR), is applied to resolve this kind of kinetic–spectral data matrix and the kinetic spectrum of each component is obtained. The construction of the initial iterative vector is the key to ITTFA. A novel approach is proposed to construct the initial vectors based on spectral features like the isoabsorptive points between components in the system to ensure that ITTFA can converge to the real solution without any constraints. The effect of the different initial vectors on the results is discussed and it is shown that the initial vector based on the isoabsorptive points can converge accurately and rapidly. To estimate the scale coefficients, which contribute to the conversion of the iterative results into the real kinetic spectra, a method is improved based on the mass equilibrium principle or isoabsorptive points of the relevant components. With the proposed method, the simulated data matrix for a two-step consecutive reaction with an unknown kinetic model, where all components are absorbing, has been successfully resolved. The data matrix measured for the electrodegradation process of salicylic acid, where the final product has no absorption, can also be resolved to obtain a reliable result.