Formation kinetics of heteroligand Ni(II) complex with alkyl-substituted 2,2′-dipyrrolylmethenes

The formation kinetics of heteroligand Ni(II) complexes with alkylated dipyrrolylmethenes of symmetric and asymmetric structure was studied. The kinetic and activation parameters of the reactions were determined. The spectral and kinetic data obtained, as well as available data on the solvation of Ni(II) acetate in electron-donor solvents, allowed us to propose an associatively activated concerted mechanism. The Ni(II) complex formation is mainly determined by the rate of substitution of the entering ligand for solvent molecules in the initial Ni(II) complex, that is, by creation of conditions for efficient donor-acceptor interaction.     

Excimer kinetics in di(naphthyl)propane

The kinetics of excimer formation in 1,3-di(1-naphthyl)propane (DPN) in isooctane were investigated. Time-resolved fluorescence measurements carried out as a function of temperature and emission wavelength were successfully analysed in terms of a simple kinetic scheme, involving two ground state DNP conformations, an excimer and a contribution from photodimer formation.     

Fluorescence quenching by reversible excimer formation: Kinetics and yield predictions for a classical potential association-dissociation model

Fluorescence quenching by reversible excimer formation is studied on the assumption that excimer formation and dissociation can be modelled as entering and leaving the attractive region of an monomer excited-monomer interaction potential by diffusion. To get some general insight in the kinetic consequences of such a type of modelling, the simple case of an attractive square-well potential is investigated. It is shown that three different kinetic regimes have to be distinguished: Two "reversible" ones in case of slow excimer radiative decay, in which the quenching kinetics can be formulated by Markovian or non-Markovian rate equations with both excimer formation and excimer dissociation terms, and an effectively "irreversible" regime if the excimer radiative decay is too rapid to allow the excimer equilibration. In the latter case a dissociation coefficient can no longer be defined and the quenching kinetics can only be predicted on the basis of generalized rate equations of a net-excimer-formation type. It is shown how the quenching constant formula must be generalized to be applicable in all kinetic situations.     

In Situ Mass Spectrometric and Kinetic Investigations of Soai's Asymmetric Autocatalysis

Chemical reactions that lead to a spontaneous symmetry breaking or amplification of the enantiomeric excess are of fundamental interest in explaining the formation of a homochiral world. An outstanding example is Soai's asymmetric autocatalysis, in which small enantiomeric excesses of the added product alcohol are amplified in the reaction of diisopropylzinc and pyrimidine-5-carbaldehydes. The exact mechanism is still in dispute due to complex reaction equilibria and elusive intermediates. In situ high-resolution mass spectrometric measurements, detailed kinetic analyses and doping with in situ reacting reaction mixtures show the transient formation of hemiacetal complexes, which can establish an autocatalytic cycle. We propose a mechanism that explains the autocatalytic amplification involving these hemiacetal complexes. Comprehensive kinetic experiments and modelling of the hemiacetal formation and the Soai reaction allow the precise prediction of the reaction progress, the enantiomeric excess as well as the enantiomeric excess dependent time shift in the induction period. Experimental structural data give insights into the privileged properties of the pyrimidyl units and the formation of diastereomeric structures leading to an efficient amplification of even minimal enantiomeric excesses, respectively.     

The use of highly ordered vesicle gels as template for the formation of silica gels

A spontaneously forming gel of unilamellar vesicles based on sodium oleate (Na oleate) and 1-octanol as amphiphiles has been employed as a template in the formation of a silica gel formed by the hydrolysis of the inorganic precursor tetraethyl orthosilicate (TEOS). Up to about 10 wt % TEOS can be incorporated into this vesicle gel without phase separation and in a fully homogeneous formation process by simple mixing of the components. The process itself relies solely upon the self-organizing properties of this amphiphilic template system. The formation process was followed by means of time-resolved turbidity, rheology, and small-angle neutron scattering (SANS) experiments. It can be concluded that the presence of the precursor TEOS affects the kinetics of the process but the original vesicle gel structure is retained even up to highest TEOS content. The kinetic studies confirm that under the chosen conditions the vesicle formation proceeds much faster than the hydrolysis of TEOS and the subsequent formation of the silica gel. SANS displays in the low q-range an additional scattering due to the silica gel network, i.e., a hybrid material of an amphiphilic vesicle gel and an inorganic oxide gel is formed. Thus, this method is a very facile novel route of forming a highly ordered silica/vesicle gel by employing a self-organizing amphiphilic system as template and the formation of the silica network proceeds in a fully homogeneous fashion under kinetic control.     

A kinetic study of the interaction of DNA with gold nanoparticles: mechanistic aspects of the interaction

A kinetic study of the interaction of gold nanoparticles capped with N-(2-mercaptopropionyl)glycine with double stranded DNA was carried out in water and in salt (NaCl) solutions. The kinetic curves are biexponential and reveal the presence of three kinetic steps. The dependence of the reciprocal fast and slow relaxation time, on the DNA concentration, is a curve and tends to a plateau at high DNA concentrations. The simplest mechanism consistent with the kinetic results involves a simple three-step series mechanism reaction scheme. The first step corresponds to a very fast step that is related to a diffusion controlled formation of an external precursor complex (DNA, AuNPs); the second step involves the formation of a (DNA/AuNPs)(I) complex, as a result of the binding affinity between hydrophilic groups of the tiopronin and the DNA grooves. Finally, the third step has been interpreted as a consequence of a conformational change of the (DNA/AuNPs)(I) complex formed in the second step, to a more compacted form (DNA/AuNPs)(II). The values of the rate constants of each step decrease as NaCl concentration increases. The results have been discussed in terms of solvation of the species and changes in the viscosity of the solution.     

Intensification of the performance of kinetic inhibitors in the presence of polyethylene oxide and polypropylene oxide for simple gas hydrate formation in a flow mini-loop apparatus

The main objective of the present work is enhancement of the performance of gas hydrate kinetic inhibitors in the presence of polyethylene oxide (PEO) and polypropylene oxide (PPO) for simple gas hydrate formation in a flow mini-loop apparatus. PEO and PPO are high molecular weight polymers that are not kinetic inhibitors by their self. For this investigation, a laboratory flow mini-loop apparatus was set up to measure the induction time and rate of gas hydrate formation when a hydrate-forming substance (such as C1, C3, CO₂ and i-C4) is contacted with water containing dissolved inhibitor in presence or absence of PEO or PPO under suitable temperature and pressure conditions. In each experiment, water containing inhibitors blend saturated with pure gas is circulated up to a required pressure. Pressure is maintained at a constant value during experimental runs by means of required gas make-up. The effect of PEO and PPO on induction time and gas consumption during hydrate formation is investigated in the presence or absence of PVP (polyvinylpyrrolidone) and l-tyrosine as kinetic inhibitors. Results were shown that the induction time is prolonged in the presence of PEO or PPO compared to the inhibitor only. Inclusion of PPO into a kinetic hydrate inhibitor solution shows a higher enhancement in its inhibiting performance compare to PEO. Thus, the induction time for simple gas hydrate formation in presence of kinetic hydrate inhibitor with PPO is higher, compare to kinetic hydrate inhibitor with PEO.     

Experimental and modeling study on hydrate formation in wet activated carbon

The formation of methane hydrate in wet activated carbon was studied. The experimental results demonstrated that the formation of methane hydrate could be enhanced by immersing activated carbon in water. A maximum actual storage capacity of 212 standard volumes of gas per volume of water was achieved. The apparent storage capacity of the activated carbon + hydrate bed increased with the increasing of mass ratio of water to carbon until reaching a maximum, then decreased drastically as the bulk water phase emerged above the wet carbon bed. The highest apparent storage capacity achieved was 140 v/v. A hydrate formation mechanism in the wet activated carbon was proposed and a mathematical model was developed. It has been shown that the proposed model is adequate for describing the hydrate formation kinetics in wet activated carbon. The kinetic model and the measured kinetic data were used to determine the formation conditions of methane hydrate in wet carbon, which are in good agreement with literature values and demonstrate that hydrate formation in wet carbon requires lower temperature or higher pressure than in the free water system.     

Kinetic Features of Aerosol Formation in the Branched Chain Process of Dichlorosilane Oxidation under Static Conditions: I. Initiated Ignition

It is found experimentally that the initial pressure of aerosol formation progressively decreases with an increase in the initial concentration of dichlorosilane in the initiated ignition of dichlorosilane mixtures with oxygen at 293 K. The dependence of the maximal aerosol concentration on the total pressure is S-shaped. A generalized kinetic scheme is proposed that qualitatively describes the regularities observed in the experiments. The most important calculated parameters are the heat evolved in the chain process and the dependence of the pressure of the saturated vapor of a new phase on temperature. It is shown that the specific features of branched-chain processes under nonisothermic conditions determine the kinetic regularities of new phase formation. The optimal range of pressures is recommended for obtaining particles with as low dispersivity as possible.     

Kinetic Resolution of Acylsilane Cyanohydrins via Organocatalytic Cycloetherification

An asymmetric cyanation of acylsilanes involving the in‐situ formation of chiral acylsilane cyanohydrins followed by their kinetic resolution via organocatalytic cycloetherification is described. The highly enantio‐ and diastereoselective cycloetherification was crucial for achieving a high efficiency in the kinetic resolution. Consequently, acylsilane cyanohydrins containing a tetrasubstituted chiral carbon atom bearing silyl, cyano, and hydroxy groups were obtained in an enantioenriched form. This protocol therefore offers an efficient catalytic approach to optically active acylsilane cyanohydrins, which exhibit potential as chiral building blocks for the synthesis of pharmaceutically relevant chiral organosilanes.     

Interfacial reaction in the synergistic extraction rate of Ni(II) with dithizone and 1,10-phenanthroline

The kinetic synergistic effect of 1,10-phenanthroline (phen) on the extraction rate of Ni(II) with dithizone (HDz) into chloroform was studied by means of a high-speed stirring method combined with photodiode-array spectrophotometry. The initial extraction rate of the adduct complex NiDz(2)phen depended upon the concentrations of both HDz and phen, suggesting the formation of NiDzphen(+) as the rate-controlling step. When [HDz] < [phen], the initial extraction of NiDz(2)phen competed with the formation of an intermediate complex, which was adsorbed at the interface and assigned most probably to NiDzphen(+)(2). The intermediate complex was gradually converted to NiDz(2)phen at a later stage of the extraction. The rate constants for the formation and consumption of the intermediate were determined, and the kinetic mechanism in the synergistic extraction was discussed.     

Total cross sections for K + Br2 at relative energies between 0 and 8000 eV

Trajectory Surface Hopping (TSH) calculations have been applied to the non-elastic scattering in the K + Br₂ collision system over a wide range of relative kinetic energies from 0 to 8000 eV. Absolute total cross sections have been computed for the formation of various collision products with an accuracy of 5% with respect to statistical errors. The following non-elastic processes have been studied: chemical reaction, inelastic neutral scattering, neutral dissociation and ion pair formation, yielding atomic as well as molecular negative bromine ions together with PC ions. The absolute values of the respective total cross sections, obtained from the TSH calculations, are in close agreement with the available experimental data, both for chemical reaction and for ion pair formation, over the whole energy range considered. The three particle character of the collision system is important in describing the experimental results quantitatively at relative kinetic energies below 100 eV.     

Kinetic justification of the solubility product: application of a general kinetic dissolution model

Using a simple, feldspar-like model and the crystal-based reaction mechanism for water-rock kinetics being developed before, we show directly how the dissolution of euhedral faces of crystals are governed by the nonlinear quantity represented by the solubility product. The kinetic approach requires recognition of the essential role played by the correlation of the dynamics of neighboring sites in a crystal, the statistical dynamics of steps, the coupling of the various kink sites on the surface by the crystal structure, and the inclusion of bond formation as well as bond rupture into the kinetic reaction mechanism. The same kinetic approach, which accounts for the role of the solubility product (or DeltaG) in the overall rate, is then shown also to explain the observed inhibition behavior in feldspars as well as the often-written phenomenological rate law, involving a product of a pH term, an activation energy term, and a DeltaG term.     

Influence of the Electron Transfer Rate on Electrochemically Controlled Hydrogen Bonding

Hydrogen bonding is an essential interaction in natural and artificial systems. Its strength can be modulated by employing process known as Electrochemically Controlled Hydrogen bonding (ECHB). Although these processes are assumed to operate under thermodynamic control no experimental evidence for kinetic control exists. In this work, ECHB processes where studied using electrogenerated radical anions from 5-nitroimidazole derivatives as receptor molecules and 1,3-diethylurea as hydrogen bond donor species. Results revealed that kinetic control occurs due to an increase in the internal reorganization energy of the receptor molecule, which cause a decrease in electron transfer rate. Electronic structure calculations and experimental K_b values suggested that kinetic limitations were the product of a competition between intra and intermolecular hydrogen bonding formation during the global process.     

Kinetic model for formation of DMPO-OH in water under ultrasonic irradiation using EPR spin trapping method

The rate of the formation of the 2-hydroxy-5,5-dimethyl-1-pyrrolidinyloxy (DMPO-OH) radical in water during ultrasonic irradiation was evaluated both experimentally and theoretically. The hydroxyl radical (OH radical) was indirectly detected using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as the spin trapping compound, and the generated DMPO-OH by the reaction between the OH radical and DMPO was measured by an electron paramagnetic resonance. The rate of change in the concentration of the DMPO-OH decreased with time, suggesting that not only the formation reaction of DMPO-OH but also the degradation reaction would take place by ultrasonic irradiation. The formation rate of the DMPO-OH was higher with ultrasonic power intensity and lower with reaction temperature. Based on the experimental results, a kinetic model for the formation of the DMPO-OH was proposed by considering the formation reaction, the ultrasonic degradation, and spontaneous degradation of DMPO-OH. The model well described the effect of the ultrasonic power intensity and the reaction temperature on the formation rate of DMPO-OH. The rate of the formation of the DMPO-OH was evaluated with the aid of the kinetic model.     

Direct kinetic formation of nonanomeric [6.5]-spiroketals in aqueous media

The direct kinetic formation of spiroketals from mixed ketal-alcohol precursors under acid catalysis was studied using four differently substituted systems. In all cases, the exclusive formation of the anomeric isomer was observed under equilibrating conditions. However, the formation of the nonanomeric spiroketal isomer was observed if the reaction was performed under kinetic conditions using an appropriately tuned acid. Water had a dramatic accelerating effect on the spiroketalization reactions that were performed in THF, and the highest yields of the nonanomeric products were obtained in aqueous THF. The nonanomeric/anomeric product ratio was also strongly affected by the substituents and the stereochemistry of the starting alcohol.     

Analyses of three-way data from equilibrium and kinetic investigations

In kinetic or equilibrium investigations it is common to measure two-way multiwavelength data, e.g. absorption spectra as a function of time or reagent addition. Often it is advantageous to acquire experimental data at various initial conditions or even on different instruments. A collection of these measurements can be arranged in three-dimensional arrays, which can be analysed as a whole under the assumption of a superimposed function, e.g. a kinetic model, and/or common properties of the subsets, such as molar absorptivity. As we show on selected formation equilibria (Zn²⁺/phen) and kinetic studies (Cu²⁺/cyclam) from our own research, an appropriate combination of multivariate data can lead to an improved analysis of the investigated systems.     

Dynamic parallel kinetic resolution of α-ferrocenyl cation initiated by chiral Brønsted acid catalyst

The dynamic parallel kinetic resolution (DPKR) of an α-ferrocenyl cation intermediate under the influence of a chiral conjugate base of a chiral phosphoric acid catalyst has been demonstrated in an S_N1 type substitution reaction of a racemic ferrocenyl derivative with a nitrogen nucleophile. The present method provides efficient access to a ferrocenylethylamine derivative in a highly enantioselective manner, which is potentially useful as a key precursor of chiral ligands for metal catalysis. The mechanism of the present intriguing resolution system was elucidated by control experiments using the enantio-pure precursor of relevant α-ferrocenyl cation intermediates and the hydroamination of vinylferrocene. Further theoretical studies enabled the elucidation of the origin of the stereochemical outcome as well as the efficient DPKR. The present DPKR, which opens a new frontier for kinetic resolution, involves the racemization process through the formation of vinylferrocene and the chemo-divergent parallel kinetic resolution of the enantiomeric α-ferrocenyl cations generated by the protonation/deprotonation sequence of vinylferrocene.

The dynamic parallel kinetic resolution (DPKR) of an enantiomeric α-ferrocenyl cation using a chiral phosphate anion of an acid catalyst was accomplished by the combination of the PKR and the racemization through the formation of vinylferrocene.     

Molecular associations between indolizines and tetracyanoethylene

Spectroscopic, thermodynamic and kinetic properties of molecular interactions between indolizines and tetracyanoethylene are reported. The interactions involved in the complexes are weak as evidenced by the low formation constants. The kinetic behaviour indicates the initial formation of a pI-complex that is rapidly transformed into a σ-complex and thereafter into the final adduct(s). Apart from the results obtained by other techniques, infrared data clearly evidence vibrational contributes characteristic of the complexes, the tricyanovinyl derivatives and the tetracyanoethylene radical anion.     

Differentiation of Epoxide Enantiomers in the Confined Spaces of an Homochiral Cu(II) Metal-Organic Framework by Kinetic Resolution

TAMOF-1 , a homochiral metal-organic framework (MOF) constructed from an amino acid derivative and Cu(II), was investigated as a heterogeneous catalyst in kinetic resolutions involving the ring opening of styrene oxide with a set of anilines. The branched products generated from the ring opening of styrene oxide with anilines and the unreacted epoxide were obtained with moderately high enantiomeric excesses. The linear product arising from the attack on the non-benzylic position of styrene oxide underwent a second kinetic resolution by reacting with the epoxide, resulting in an amplification of its final enantiomeric excess and a concomitant formation of an array of isomeric aminodiols. Computational studies confirmed the experimental results, providing a deep understanding of the whole process involving the two successive kinetic resolutions. Furthermore, TAMOF-1 activity was conserved after several catalytic cycles. The ring opening of a meso-epoxide with aniline catalyzed by TAMOF-1 was also studied and moderate enantioselectivities were obtained.