Monoaurated vs. diaurated intermediates: causality or independence?

Diaurated intermediates of gold-catalysed reactions have been a long-standing subject of debate. Although diaurated complexes were regarded as a drain of active monoaurated intermediates in catalytic cycles, they were also identified as the products of gold–gold cooperation in dual–activation reactions. This study shows investigation of intermediates in water addition to alkynes catalysed by [(IPr)Au(CH₃CN)(BF₄)]. Electrospray ionisation mass spectrometry (ESI-MS) allowed us to detect both monoaurated and diaurated complexes in this reaction. Infrared photodissociation spectra of the trapped complexes show that the structure of the intermediates corresponds to α-gold ketone intermediates protonated or aurated at the oxygen atom. Delayed reactant labelling experiments provided the half life of the intermediates in reaction of 1-phenylpropyne (∼7 min) and the kinetic isotope effects for hydrogen introduction to the carbon atom (KIE ∼ 4–6) and for the protodeauration (KIE ∼ 2). The results suggest that the ESI-MS detected monoaurated and diaurated complexes report on species with a very similar or the same kinetics in solution. Kinetic analysis of the overall reaction showed that the reaction rate is first-order dependent on the concentration of the gold catalyst. Finally, all results are consistent with the reaction mechanism proceeding via monoaurated neutral α-gold ketone intermediates only.

Reaction kinetics and detected α-gold ketone intermediates reveal that gold-mediated hydration of alkynes does not rely on dual activation.     

Reactant subspaces and kinetics of chemical reaction networks

This paper studies a chemical reaction network’s (CRN) reactant subspace, i.e. the linear subspace generated by its reactant complexes, to elucidate its role in the system’s kinetic behaviour. We introduce concepts such as reactant rank and reactant deficiency and compare them with their analogues currently used in chemical reaction network theory. We construct a classification of CRNs based on the type of intersection between the reactant subspace R and the stoichiometric subspace S and identify the subnetwork of S-complexes, i.e. complexes which, when viewed as vectors, are contained in S, as a tool to study the network classes, which play a key role in the kinetic behaviour. Our main results on new connections between reactant subspaces and kinetic properties are (1) determination of kinetic characteristics of CRNs with zero reactant deficiency by considering the difference between (network) deficiency and reactant deficiency, (2) resolution of the coincidence problem between the reactant and kinetic subspaces for complex factorizable kinetics via an analogue of the generalized Feinberg–Horn theorem, and (3) construction of an appropriate subspace for the parametrization and uniqueness of positive equilibria for complex factorizable power law kinetics, extending the work of Müller and Regensburger.     

Single-phase and heterophase solid-state chemical kinetics of thermally induced methyl transfer in tetraglycine methyl ester

To better understand the general interrelationships between chemical transformations and physical transformations in solid-state reactions, we have studied the kinetics of methyl transfer in polycrystalline samples of tetraglycine methyl ester [TGME] over the temperature range of 83°C–115°C. Changes in the concentrations of the reactant and various intermediates (sarcosyltriglycine methyl ester METGME, and tetraglycine TG) and products (sarcosyltriglycine METG and N N-dimethyl glycyl triglycine Me₂TG) were measured over the entire time course of the reaction using HPLC. Corresponding measurements of physical transformations occurring during the course of the reactions were made using X-ray powder diffractometry and differential scanning calorimetry. Kinetic curves for the loss of TGME in the range of 83°C–115°C have a sigmoldal shape and collapse into one curve when plotted in terms of reduced time. t/t_0.5, as do plots of intermediate and product concentration plotted in the same manner. The first 25% of the reaction proceeds homogeneously through what is believed to be the formation of a crystalline solid solution of the intermediates and products in the reactant. The acceleratory character of the kinetic curves in the single-phase portion of the reaction has been described by a kinetic scheme that contains a concentration-dependent rate constant. The apperance of a new crystalline phase beyond 35% of the reaction changes the reaction mechanism from a bulk reaction to an interface-controlled process that causes further acceleration of the methyl transfer. The apparent activation energies for both single-phase and heterophase stages of the reaction are about 100–130 kJ/mole © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 339–348, 1997     

A computational approach to linear conjugacy in a class of power law kinetic systems

This paper studies linear conjugacy of PL-RDK systems, which are kinetic systems with power law rate functions whose kinetic orders are identical for branching reactions, i.e. reactions with the same reactant complex. Mass action kinetics (MAK) systems are the best known examples of such systems with reactant-determined kinetic orders (RDK). We specify their kinetics with their rate vector and T matrix. The T matrix is formed from the kinetic order matrix by replacing the reactions with their reactant complexes as row indices (thus compressing identical rows of branching reactions of a reactant complex to one) and taking the transpose of the resulting matrix. The T matrix is hence the kinetic analogue of the network’s matrix of complexes Y with the latter’s columns of non-reactant complexes truncated away. For MAK systems, the T matrix and the truncated Y matrix are identical. We show that, on non-branching networks, a necessary condition for linear conjugacy of MAK systems and, more generally, of PL-FSK (power law factor span surjective kinetics) systems, i.e. those whose T matrix columns are pairwise different, is \(T = T'\), i.e. equality of their T matrices. This motivated our inclusion of the condition \(T = T'\) in exploring extension of results from MAK to PL-RDK systems. We extend the Johnston–Siegel Criterion for linear conjugacy from MAK to PL-RDK systems satisfying the additional assumption of \(T = T'\) and adapt the MILP algorithms of Johnston et al. and Szederkenyi to search for linear conjugates of such systems. We conclude by illustrating the results with several examples and an outlook on further research.     

What can we learn about the mechanisms of thermal decompositions of solids from kinetic measurements?

Aspects of the theories that are conventionally and widely used for the kinetic analyses of thermal decompositions of solids, crystolysis reactions, are discussed critically. Particular emphasis is placed on shortcomings which arise because reaction models, originally developed for simple homogeneous reactions, have been extended, without adequate justification, to represent heterogeneous breakdowns of crystalline reactants. A further difficulty in the mechanistic interpretation of kinetic data obtained for solid-state reactions is that these rate measurements are often influenced by secondary controls. These include: (i) variations of reactant properties (particle sizes, reactant imperfections, nucleation and growth steps, etc.), (ii) the effects of reaction reversibility, of self-cooling, etc. and (iii) complex reaction mechanisms (concurrent and/or consecutive reactions, melting, etc.). A consequence of the contributions from these secondary rate controls is that the magnitudes of many reported kinetic parameters are empirical and results of chemical significance are not necessarily obtained by the most frequently used methods of rate data interpretation. Insights into the chemistry, controls and mechanisms of solid-state decompositions, in general, require more detailed and more extensive kinetic observations than are usually made. The value of complementary investigations, including microscopy, diffraction, etc., in interpreting measured rate data is also emphasized. Three different approaches to the formulation of theory generally applicable to crystolysis reactions are distinguished in the literature. These are: (i) acceptance that the concepts of homogeneous reaction kinetics are (approximately) applicable (assumed by many researchers), (ii) detailed examination of all experimentally accessible aspects of reaction chemistry, but with reduced emphasis on reaction kinetics (Boldyrev) and (iii) identification of rate control with a reactant vaporization step (L’vov). From the literature it appears that, while the foundations of the widely used model (i) remain unsatisfactory, the alternatives, (ii) and (iii), have not yet found favour. Currently, there appears to be no interest in, or discernible effort being directed towards, resolving this unsustainable situation in which three alternative theories remain available to account for the same phenomena. Surely, this is an unacceptable and unsustainable situation in a scientific discipline and requires urgent resolution?     

Redoxkinetik von Metallkomplexen in nichtwäßrigen Lösungen,XI. Die Reduktion von Thallium(III)oxinat mit Eisen(II) in schwach koordinierenden Lösungsmitteln

The kinetics of the iron(II) reduction of thallium(III) oxinate does not differ essentially from that of the oxinate-transfer from thallium(III)-oxinate to iron(III), described before: formation of a binuclear intermediate, rearrangements within, and subsequent reaction with the excess reactant to the final products. As for the redox process, these intermediates are binuclear Tl(II)-Fe(III) complexes which, with initial reactants, form further complexes in which the second electron is transferred. In the cases of excess Tl(ox)₃ and of equimolar reactants, disproportionations are likely involved.     

Solvent-free,catalyst-free aza-Michael addition of cyclohexylamine to diethyl maleate: Reaction mechanism and kinetics

The aza-Michael reaction is the addition of an amine to an electron deficient C=C double bond. This reaction is also used in the synthesis of precursors of polymeric networks. In this study, we paid attention to the kinetics and mechanism of the aza-Michael addition of cyclohexylamine (CHXA) to diethyl maleate (DEM) performed as a solvent-free, catalyst-free reaction and to concurrent reactions. In situ Raman spectroscopy, NMR spectroscopy and gas chromatography/mass spectrometry have shown the occurrence of three simultaneous reactions: (i) the aza-Michael addition of CHXA to DEM leading to diethyl 2-(cyclohexylamino)succinate, (ii) isomerization of DEM to diethyl fumarate (DEF), and (iii) the aza-Michael addition of CHXA to DEF formed by the reaction (ii). All of these reactions proceed with third order kinetics, first order in DEM or DEF and second order in CHXA. We propose a kinetic model that allows kinetic constants to be estimated. Furthermore, a numerical solution of the set of differential equations confirms the expected kinetic equations of reactions (i) and (ii) and gives values of rate constants comparable to the estimated ones. A DFT mechanistic study illustrates the structure of the reaction intermediates and transition states of all reactions and explains the contribution of the second amine molecule in the reaction mechanism.     

Kinetic Studies on Interaction of Platinum(II) Complexes with an ‘S’ Containing Ligand in Aqueous Medium

The kinetics of the substitution reactions of [Pt(dach)(H₂O)₂]²⁺ and [Pt(en)(H₂O)₂]²⁺ (where ‘dach’ and ‘en’ are cis-1,2-diaminocyclohexane and ethylenediamine, respectively) with excess N,N′-diethylthiourea have been studied in aqueous solution by UV–Vis spectrophotometry. The effect of different N–N spectator ligands on the reactivity of platinum(II) complexes was investigated by studying the water lability of the reactant complexes. The kinetic study has been substantiated by product isolation, IR, NMR and ESI-MS spectral analysis and DFT calculations. The reactions follow normal square-planar substitution mainly in an associative way. Rate parameters have been evaluated under different conditions. The substitution rates of the complexes studied can be tuned through the nature of the N–N chelates, which is important in the development of new active compounds for cancer therapy.     

Kinetics and mechanism of aqueous oxidation of H2S by Fe+3

The reaction kinetics of aqueous oxidation of H₂S by Fe⁺³ is investigated at 25°C by spectrophotometric method. The study conducted at various reactant concentrations and pH revealed that the reaction proceeds according to complex‐series reactions involving polysulfides as intermediates. The reaction of each step is first order with respect to Fe⁺³ and hydrogen sulfide or polysulfide. A mechanism is proposed, involving sulfido and polysulfido radicals. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 331–335, 1999     

Kinetics of Condensation Reactions between Ethanolamine (Amino‐Alcohol) and Dicarboxylic Acids at 413 K

The kinetics of condensation polymerization reactions between amino‐alcohol and various dibasic acids ranging from oxalic acid (n = 0 number of –CH₂ groups) to sebacic acid (n = 8) is studied at 413 K in an inert atmosphere, with a catalyst and ensuring removal of product water. All of the studied reactions are chemically identical, and their rates may differ only in so far as reactivity affected by molecular weight. Our analysis of the kinetic data reveals that all reactions obey third‐order kinetics, and the degree of polymerization increases even upto 20 h, if same stoichiometry for both the reactants is maintained. The velocity constants for the reactions were found to be in the order for the acids as succinic > oxalic > sebacic > adipic. The rates are also comparable to similar reactions of diols and diacids. The analysis further reveals that the chain length as well the structural characteristics of the reactant molecules govern the speed of the reaction. It seems that the proper conformations and structural geometry do play an important role in the collisional process of formation of products in a desired time. The probable implications of the results and analysis of these observations are discussed.     

Kinetic study of the oxidation of ethanol by 3,4-lutidine chromium(VI) peroxide in dichloromethane solution

The oxidation kinetics of ethanol with 3,4-lutindine chromium(VI) peroxide (LCP) were investigated by monitoring the absorbance change at 565 nm in dichloromethane solution. The reaction had a first-order dependence on oxidant and a fractional (one half) dependence on reactant. The stoichiometric ratio between LCP and ethanol was 1 : 2. The activation parameters were determined from temperature dependence of the reaction rate. It was found that the cleavage of the peroxide groups of LCP is primarily responsible for the oxidant of ethanol to acetaldehyde. Based on the kinetic results obtained (including deuterium isotope effect) a plausible mechanism is proposed. © 1994 John Wiley & Sons, Inc.     

A generalized kinetic model for heterogeneous gas-solid reactions

We present a generalized kinetic model for gas-solid heterogeneous reactions taking place at the interface between two phases. The model studies the reaction kinetics by taking into account the reactions at the interface, as well as the transport process within the product layer. The standard unreacted shrinking core model relies on the assumption of quasi-static diffusion that results in a steady-state concentration profile of gas reactant in the product layer. By relaxing this assumption and resolving the entire problem, general solutions can be obtained for reaction kinetics, including the reaction front velocity and the conversion (volume fraction of reacted solid). The unreacted shrinking core model is shown to be accurate and in agreement with the generalized model for slow reaction (or fast diffusion), low concentration of gas reactant, and small solid size. Otherwise, a generalized kinetic model should be used.     

A dual-path sequence of first-order reactions the protiodeacylation of acetylpropionylmesitylene

An example of a sequence of competing first-order reactions, leading to a common product, has been found for the sulfodeacylation of acetylpropionylmesitylene. The basic reaction scheme is Scheme l and a kinetic analysis of the component rate constants allows an estimate to be made of the concentrations of the reactant and products as a function of time. In the present system the total concentration of the intermediates B, C, and D never exceeds 0.26% at 25°C, and therefore the kinetics, which were followed spectrophotometrically, were essentially of the conversion of substrate A to the final product E. The kinetics of protiodeacylation have been measured, over a range of temperatures, of propionyl-, dipropionyl-, and acetylpropionylmesitylenes in 89.8% (w/w) sulfuric acid.     

One- and three-parameter density functional theory and high level ab initio theory study of the CH+CH disproportionation reaction

Complete basis set (CBS) ab initio computational studies were performed with the target being to explore the CH+CH potential energy surface. Several closed and open shell intermediates were located on the potential energy surface. Computed enthalpies for the branching reactions, as well as heats of formation are in excellent agreement. Although CBS computed energies are of high quality, this computational study is not capable of predicting the branching product ratio due to fact that neither the MP2 nor the 6-311G(2d,2p) basis set are sufficient to locate the reactant complexes and the transition state structures for the hydrogen and carbon transfer reactions in the reaction complexes. To properly explore the CH+CH potential energy surface a much higher ab initio theory level is required.     

Determination of overall kinetic constants for mediated electrochemical oxidation of phenol from CO<Subscript>2</Subscript> measurements

Mediated electrochemical oxidation is the latest achievement in environmental electrochemistry for the complete oxidation of organic pollutants. Transition or inner transition elements in an acid medium are usually employed as the mediator-electrolyte combination. The organic pollutants upon oxidation are completely converted to carbon dioxide and water. Since the oxidizing ability of the medium is so vigorous, the changes in the reactant concentrations or intermediates formed are usually difficult to analyze, but the product formed (CO₂) can be measured and quantified in most of the cases. Therefore, in MEO reactions the kinetics can be followed either by monitoring the oxidant concentration changes or by measuring the product concentrations. In real applications the oxidant is regenerated continuously in situ and, hence, the oxidant concentration is maintained throughout the system. Therefore, in continuous organic feeding reactions, the product CO₂ could be monitored and kinetics could be followed. We report in this paper a simple procedure for the calculation of the overall kinetic constants for the destruction of phenol from CO₂ measurements. The procedure is based on the summation of the difference between the total amounts of organic added to the system and reacted to obtain CO₂ evolution patterns. The CO₂ patterns were then fitted with the experimental results to obtain the overall kinetic constants. Thermodynamic parameters have been obtained for phenol destruction from the overall kinetic constants.     

Kinetics of the single-electron chemical oxidation of rhenium(V) meso-phenyl-β-octaethylporphyrinate

The states and reactions of rhenium(V) complexes with meso-monophenyl-β-octaethylporphines containing Cl^? and OPh^? as axial ligands O=Re(Cl)MPOEP and O=Re(OPh)MPOEP in concentrated sulfuric acid at 298–348 K are studied via spectral and kinetic methods. While stable along M-N bonds, O=Re(Cl)MPOEP is found to undergo slow oxidation after transforming into axial hydrosulfate complex O=Re(HSO₄)MPOEP. It is shown that the sole electron oxidizing agent is atmospheric oxygen (with the assistance of highly concentrated protons) and the sites of reduction are aromatic ligands. The reaction product was identified as π-radical cation O=Re(HSO₄)MPOEP^·+. Forward and inverse chemical kinetics solutions are used to obtain a full kinetic equation and the reaction rate parameters of elementary steps, and to establish the stoichiometric mechanism of the composite oxidation of the complex. Complex O=Re(OPh)MPOEP in the form O=Re(OPh)(O₂)MPOEP with coordinated oxygen is shown experimentally to be stable with respect to oxidation. The obtained results are important for identifying intermediates in processes catalyzed by stable metal porphyrins.     

Laser flash photolysis generation and kinetic studies of corrole-manganese(V)-oxo intermediates

Corrole-manganese(V)-oxo intermediates were produced by laser flash photolysis of the corresponding corrole-manganese(IV) chlorate complexes, and the kinetics of their decay reactions in CH2Cl2 and their reactions with organic reductants were studied. The corrole ligands studied were 5,10,15-tris(pentafluorophenyl)corrole (H3TPFC), 5,10,15-triphenylcorrole (H3TPC), and 5,15-bis(pentafluorophenyl)-10-(p-methoxyphenyl)corrole (H3BPFMC). In self-decay reactions and in reactions with substrates, the order of reactivity of (Cor)Mn(V)(O) was TPC > BPFMC > TPFC, which is inverted from that expected based on the electron-demand of the ligands. The rates of reactions of (Cor)Mn(V)(O) were dependent on the concentration of the oxidant and other manganese species, with increasing concentrations of various manganese species resulting in decreasing rates of reactions, and the apparent rate constant for reaction of (TPFC)Mn(V)(O) with triphenylamine was found to display fractional order with respect to the manganese-oxo species. The kinetic results are consistent in part with a reaction model involving disproportionation of (Cor)Mn(V)(O) to give (Cor)Mn(IV) and (Cor)Mn(VI)(O) species, the latter of which is the active oxidant. Alternatively, the results are consistent with oxidation by (Cor)Mn(V)(O) which is reversibly sequestered in non-reactive complexes by various manganese species.     

Specific Features of a Two-Phase Bimolecular Chemical Reaction in the Case of the Association of Both Reactants

The effect of the association of both reactants on the kinetics of their bimolecular reaction in the liquid phase is studied. The mathematical modeling of chemical reactions that are described by nonlinear differential equations is performed. The steady states, the conditions for the emergences of intermediates, and the nature of their concentration oscillations in the reaction system are described. It is found that the concentration of the intermediates has two types of oscillations (harmonic and relaxation oscillations) characterized by significantly different times. The relationship between the observed rate constant of the process, the rate constants for the elementary stages, and the reactant concentrations is found.     

Catalytic reduction of 4-nitrophenol by means of nanostructured polymeric Schiff base complexes

Polymeric Schiff base ligands were synthesized using 2-hydroxybenzaldehyde (L²), 4-hydroxy-3-methoxybenzaldehyde (L⁴), and 5-aminoisophthalic acid. The nanostructured complexes were then synthesized using Ni²⁺, Cu²⁺, and Mn³⁺. The ligands and complexes thus synthesized were characterized using Fourier-transform infrared spectroscopy, X-ray diffraction, thermogravimetric analysis (TGA), and field-emission scanning electron microscopy. The thermal stability of the complexes was confirmed using TGA. The synthesized complexes were used as catalysts in the reduction of 4-nitrophenol (4-NP) to 4-aminophenol in an aqueous phase in the presence of sodium borohydride. In this work, the catalytic reactivity of nanostructured complexes was compared using the rate constant (k) of the reaction. The reaction time for the reduction of 4-NP was 5–14 min for different complexes. The catalytic system based on Ni²⁺/2-hydroxybenzaldehyde was the most active and displayed reusability in the reduction of 4-NP.