Chloracylierung und Bromacylierung von Carbonylverbindungen. III. Lage des Gleichgewichts

Chloroacylation and Bromoacylation of Carbonyl Compounds III. Reactant/Product Equilibria Aliphatic, α, β-unsaturated and aromatic aldehydes 1 as well as aliphatic ketones react with acyl halides 2 to α-haloalkyl esters 3 . In the presence of Lewis acids there is an equilibrium between reactants and product. The position of the equilibrium depends on the nature of the carbonyl compound as well as that of the acyl halide: The products 3 are favoured in the case of aldehydes, cyclobutanone and cyclohexanone, and the equilibrium constant increases in the series F < Cl < Br < I. Low reaction temperature, nonpolar solvents as well as high reactant concentrations favour the product 3 .     

Activity-Based Models to Predict Kinetics of Levulinic Acid Esterification

The solvent is of prime importance in biomass conversion as it influences dissolution, reaction kinetics, catalyst activity and thermodynamic equilibrium of the reaction system. So far, activity-based models were developed to predict kinetics and equilibria, but the influence of the catalyst on kinetics has not been succesfully predicted by thermodynamic models. In this work, the thermodynamic model ePC-SAFT advanced was used to predict the activities of the reactants and of the catalyst at various conditions (temperature, reactant concentrations, γ-valerolactone GVL cosolvent addition, catalyst concentration) for the homogeneously acid-catalyzed esterification of levulinic acid (LA) with ethanol. Different kinetic models were applied, and it was found that the catalyst influence on kinetics could be predicted correctly by simultaneously solving the dissociation equilibrium of H₂SO₄ catalyst along the reaction coordinate and by relating reaction kinetics to proton activity. ePC-SAFT advanced model parameters were only fitted to reaction-independent phase equilibrium data. The key reaction properties were determined by applying ePC-SAFT advanced to one experimental kinetic curve for a set of temperatures, yielding the reaction enthalpy at standard state , activation energy and the intrinsic reaction rate constant k=0.011 s⁻¹ at 323 K, which is independent of catalyst concentration. The new procedure allowed an a-priori identification of the effects of catalyst, solvent and reactant concentration on LA esterification.     

Kinetics of diffusion-controlled reaction between chemically asymmetric molecules. II. Approximate steady-state solution

The previously published equation for the rate of a diffusion-limited bimolecular reaction between chemically asymmetric molecules is studied numerically for the case that one of the reactant molecules is uniform. The results are reproduced quite well by a simple approximate chemical-kinetic steady-state scheme and, in principle, allow estimates of the size of the reactive region and of the activation-controlled rate to be made from the observed dependence of rate on solvent viscosity. The simple scheme is easily generalized to the case of two nonuniform reactants. In general, restriction of reactivity to some fraction of the molecular surface (i.e., a steric factor) must reduce the observable reaction rate, but to an extent which is moderated by the rotational diffusion of the reactant molecules.     

Manifestation of macroscopic correlations in elementary reaction kinetics. II. Irreversible reaction A+B→C

The applicability of the Encounter Theory (ET) (the prototype of the Collision Theory) concepts for widely occurring diffusion assisted irreversible bulk reaction A+B→C (for example, radical reaction) in dilute solutions with arbitrary ratio of initial concentrations of reactants has been treated theoretically with modern many-particle method for the derivation of non-Markovian binary kinetic equations. The method shows that, just as in the reaction A+A→C considered earlier, the agreement with the Encounter Theory is observed when the familiar Integral Encounter Theory is used which is just a step in the derivation of kinetic equations in the framework of the method employed. It allows for two-particle correlations only, and fails to consider the correlation of reactant simultaneously with a partner and with a reactant in the bulk. However, the next step leading to the Modified Encounter Theory under reduction of equations to a regular form both extends the time applicability interval of ET homogeneous rate equation (as for reactions proceeding in excess of one of the reactants), and yields the inhomogeneous equation of the Generalized Encounter Theory (GET) that reveals macroscopic correlations induced by the encounters in a reservoir of free walks in full agreement with physical considerations. This means that the encounters of reactants in solution are correlated at rather large time interval of the reaction course. However, unlike the reaction A+A→C of identical reactants, the reaction A+B→C accumulation of the above macroscopic correlations (even with the initial concentrations of reactants being equal) proceeds much slower. Another distinction is that for the reaction A+A→C the long-term behavior of ET and GET kinetics is the same, while in the reaction A+B→C these kinetics behave differently. It is of interest that just taking account of the above macroscopic correlations in the reaction A+B→C (in GET) results in the universal character of the long-term behavior of the kinetics for the case of equal initial concentrations of reactants and that where one of the reactants is in excess. This is more natural from the point of view of the reaction course on the encounters of reactants in solutions.     

An investigation into the preparation of poly(1-methylpyrol-2-ylsquaraine) particles

Poly(1-methylpyrol-2-ylsquaraine) particles can be prepared in a one-pot reaction by refluxing equimolar amounts of squaric acid (3,4-dihydroxycyclobut-3-ene-1,2-dione) and 1-methylpyrrole in butan-1-ol and precipitate from the reaction as porous micrometer-sized spheres that have potential use as molecular adsorbents in chromatography and sensors, as well as ion batteries. Previous papers that have examined the electrical, photophysical, and adsorption properties of these particles have reported their size and shape, with appropriate proof in the form of electron microscope images but have paid little attention as to the actual limits of the particle size and spherical formation. Thus, a range of experiments have been undertaken to examine the robustness, in terms of reaction variation, of the construction of these spheres. Reaction characteristics such as solvent, sequence in which reactants are added, reaction time, reactant concentration, and mode of reaction agitation have all been examined to determine the optimal conditions for the production of these spheres. Fortuitously, butan-1-ol remains the most suitable solvent, but reactant concentration is important to produce uniformly sized spherical particles with minimal interparticle growth.     

Chemical applications of group theory and topology

The following procedure is described for investigating the qualitative dynamics of simple chemical systems: 1) A so-called influence diagram is generated representing the relationships between the reference reactants (phase-determining intermediates); 2) This influence diagram is used to generate a truth table indicating possible transitions between state vectors representing the signs of the time derivatives of of the reference reactant concentrations; 3) The truth table is used to determine a state transition diagram representing the flow topology around unstable equilibrium points; 4) The characteristic equation of the adjacency matrix of the influence diagram is solved in order to determine the presence of such unstable equilibrium points. The two types of qualitative dynamics possible for chemical systems containing two reference reactants and one feedback circuit are bifurcation between two attracting regions (bistability) and limit cycle oscillation. However, in two reference reactant systems oscillation requires an additional self-activating loop to generate the unstable equilibrium point required for its realization. Bistability and limit cycle oscillation are also two of the possible types of qualitative dynamics for chemical systems containing three reference reactants. However, chemical systems with three reference reactants and two or more feedback circuits can also contain interlocking limit cycles, which can lead to toroidal oscillations or chaos. The influence diagrams are given for the systems exhibiting these various types of dynamic behavior along with a summary of the important properties of all 729 possible influences for simple chemical systems containing three reference reactants.     

Kinetics of liquid phase photocatalyzed reactions: An illuminating approach

Analysis of photocatalyst kinetics to date have relied largely on Langmuir-Hinshelwood rate forms, which assume equilibrated adsorption of reactants and, correspondingly, a slow, rate-controlling surface step. Alternatively, and more generally, a pseudo-steady state analysis based upon the stationary state hypothesis for reaction intermediates may be applied. We show here that only this second approach is consistent with the reported intensity dependence of apparent adsorption (and desorption) binding constants, as well as the catalytic rate constant. In consequence, we show that for at least some photocatalyzed reactions, adsorption/desorption reaction equilibria are not established during reaction, because the substantial reactivity of an adsorbed active species (e.g., hole (h+), radical (*OH), etc.) causes a continued displacement from equilibrium of the adsorbed reactant concentration.     

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.     

A three-state mechanism for DNA hairpin folding characterized by multiparameter fluorescence fluctuation spectroscopy

The folding of a dye-quencher labeled DNA hairpin molecule was investigated using fluorescence autocorrelation and cross-correlation spectroscopy (FCS) and photon counting histogram analysis (PCH). The autocorrelation and cross-correlation measurements revealed the flow and diffusion times of the DNA molecules through two spatially offset detection volumes, the relaxation time of the folding reaction, and the total concentration of DNA molecules participating in the reaction. The PCH measurements revealed the equilibrium distribution of DNA molecules in folded and unfolded conformations and the specific brightnesses of the fluorophore in each conformational state. These measurements were carried out over a range of NaCl concentrations, from those that favored the open form of the DNA hairpin to those that favored the closed form. DNA melting curves obtained from each sample were also analyzed for comparison. It was found that the reactant concentrations were depleted as the reaction progressed and that the equilibrium distributions measured by FCS and PCH deviated from those obtained from the melting curve analyses. These observations suggest a three-state mechanism for the DNA hairpin folding reaction that involves a stable intermediate form of the DNA hairpin. The reaction being probed by FCS and PCH is suggested to be a rapid equilibrium between open and intermediate conformations. Formation of the fully closed DNA hairpin is suggested to occur on a much longer time scale than the FCS and PCH measurement time. The closed form of the hairpin thus serves as a sink into which the reactants are depleted as the reaction progresses.     

Solvent Dependence on Cooperative Vibrational Strong Coupling and Cavity Catalysis**

Strong light-matter coupling offers a unique way to control chemical reactions at the molecular level. Here, we compare the solvent effect on an ester solvolysis process under cooperative vibrational strong coupling (VSC). Three reactants, para-nitrophenylacetate, 3-methyl-para-nitrophenylbenzoate, and bis-(2, 4-dinitrophenyl) oxalate are chosen to study the effect of VSC on the solvolysis reaction rates. Two solvents, ethyl acetate and cyclopentanone, are also considered to compare the cavity catalysis by coupling the C=O stretching band of the reactant and the solvent molecules to a Fabry-Perot cavity mode. Interestingly, both solvents enhance the chemical reaction rate of para-nitrophenylacetate and 3-methyl-para-nitrophenylbenzoate under cooperative VSC conditions. However, the resonance effect is observed at different temperatures for different solvents, which is further confirmed by thermodynamic studies. Bis-(2, 4-dinitrophenyl) oxalate doesn′t respond to VSC in either of the solvent systems due to poor overlap of reactant and solvent C=O vibrational bands. Cavity detuning and other control experiments suggest that cooperative VSC of the solvent plays a crucial role in modifying the activation free-energy of the reaction. These findings, along with other observations, cement the concept of polaritonic chemistry.     

An efficient molecular dynamics simulation method for calculating the diffusion-influenced reaction rates

We present a molecular dynamics (MD) simulation method for calculating the diffusion-influenced reaction rates in the limit of low reactant concentrations. To calculate the reaction rate coefficient, we use MD trajectories of a nonreactive equilibrium system that are initiated with a pair of reactant molecules in reactive configuration. Hence reaction systems involving complicated reactant molecules with geometrically restricted reactivities can be treated with comparable efficiency as the simple hard-sphere reaction system. Compared to the similar MD method proposed by Van Beijeren, Dong, and Bocquet [J. Chem. Phys. 114, 6265 (2001)], the present method has a couple of advantages. First, reactions involving more general sink functions can be treated. Second, more accurate results can be obtained when the reaction probability upon collision is less than unity. As an application, we investigate the effects of nondiffusive dynamics and hydrodynamic interaction of reactants on the reaction rate.     

Calculation of equilibrium compositions of systems of enzyme-catalyzed reactions

When a system of enzyme-catalyzed reactions does not involve H(2)O as a reactant, the equilibrium composition at specified temperature, pH, and ionic strength can be calculated using the Mathematica programs equcalcc, which uses the conservation matrix, or equcalcrx, which uses the stoichiometric number matrix. When H(2)O is involved as a reactant, equcalcrx must be used because H(2)O is not in the stoichiometric number matrix. It is shown here that the use of equcalcrx is equivalent to using the further transformed Gibbs energy G" that eliminates oxygen from the conservation matrix. Calculations presented here show that the calculation of further transformed Gibbs energies of formation of reactants other than coenzymes can be avoided by using equcalcrx to calculate equilibrium concentrations.     

Thermodynamic vs. kinetic control of excited-state proton transfer reactions

The “Excited-State Intramolecular Proton Transfer” (ESIPT) reactions in a number of organic fluorophores are among the fastest basic chemical reactions known so far and their rates can be observed even on femtosecond time scale. Accordingly, the reactant concentration, as monitored by its emission, should be negligibly small. In sharp contrast to this conventional wisdom, however, the coexistence of the reactant and the product of this reaction is so frequently observed in condensed media. We then discuss two possible origins of these effects: when the ESIPT reaction is perturbed and hence is slow on the time scale of emission (kinetic control) or when the reverse reaction repopulating the reactant state is fast and leads to the excited-state equilibrium (thermodynamic control). Upon reviewing a great number of ESIPT prototypical systems, we summarize and discuss different criteria for distinguishing these cases based on the steady-state and time-resolved spectroscopic studies and derive correlations between reversibility of these reactions and the solvent-dependent effects observed in fluorescence spectra.     

Rate kernel theory for pseudo-first-order kinetics of diffusion-influenced reactions and application to fluorescence quenching kinetics

Theoretical foundation of rate kernel equation approaches for diffusion-influenced chemical reactions is presented and applied to explain the kinetics of fluorescence quenching reactions. A many-body master equation is constructed by introducing stochastic terms, which characterize the rates of chemical reactions, into the many-body Smoluchowski equation. A Langevin-type of memory equation for the density fields of reactants evolving under the influence of time-independent perturbation is derived. This equation should be useful in predicting the time evolution of reactant concentrations approaching the steady state attained by the perturbation as well as the steady-state concentrations. The dynamics of fluctuation occurring in equilibrium state can be predicted by the memory equation by turning the perturbation off and consequently may be useful in obtaining the linear response to a time-dependent perturbation. It is found that unimolecular decay processes including the time-independent perturbation can be incorporated into bimolecular reaction kinetics as a Laplace transform variable. As a result, a theory for bimolecular reactions along with the unimolecular process turned off is sufficient to predict overall reaction kinetics including the effects of unimolecular reactions and perturbation. As the present formulation is applied to steady-state kinetics of fluorescence quenching reactions, the exact relation between fluorophore concentrations and the intensity of excitation light is derived.     

Phase-transfer catalytic reaction of 2,4,6-tribromophenol and dibromomethane: effect of potassium hydroxide

The reaction of 2,4,6-tribromophenol with dibromomethane in an alkaline solution of KOH/dibromomethane two-phase medium, catalyzed by tetrabutylammonium bromide (TBAB or QBr), was carried out. Both mono-substituted as well as bi-substituted products were found to have formed during or after the reaction, when dibromomethane was used both as organic solvent as well as organic-phase reactant. The active catalyst tetrabutylammonium 2,4,6-tribromophenoxide (ArOQ) was identified during the reaction, from which the organic-phase reaction was inferred to be the rate controlling step. The mass transfer of both the catalysts viz. QBr and ArOQ between the two phases was found to be fast. A peculiar phenomenon was observed while investigating the effect of KOH on the reaction rate, viz. the reaction rate does not monotonously increase or decrease with increase in the amount of KOH. This phenomenon is attributed to the activity of ArOQ, the distribution of active catalyst (ArOQ) between the two phases and the hydration of active catalyst in the organic phase, both of which are affected by the amount of KOH. An effective method is proposed to determine the two intrinsic rate constants of the organic-phase reaction, based on the reaction carried out at high KOH concentration.     

Calculation of thermodynamic properties of species of biochemical reactants using the inverse Legendre transform

The determination of apparent equilibrium constants and heats of enzyme-catalyzed reactions provides a way to determine Delta(f)G degrees and Delta(f)H degrees of species of biochemical reactants. These calculations are more difficult than the calculation of transformed thermodynamic properties from species properties, and they are an application of the inverse Legendre transform. The Delta(f)G degrees values of species of a reactant can be calculated from an apparent equilibrium constant if the Delta(f)G degrees values are known for all the species of all the other reactants and the pKs of the reactant of interest are known. The Delta(f)H degrees of species of a reactant can be calculated from the heat of reaction if the Delta(f)H degrees values are known for all species of the other reactants and Delta(f)G degrees values are known for all species in the reaction. The standard enthalpies of acid dissociation of the reactant of interest are also needed. The inverse Legendre transformation is accomplished by using computer programs to set up the simultaneous equations that involve the Delta(f)H degrees of the species and solving them. Thirty two new species matrixes providing Delta(f)G degrees values and eight new species matrixes providing Delta(f)H degrees values are calculated. It is the specificity and speed of enzyme-catalyzed reactions that make it possible to determine standard thermodynamic properties of complicated species in aqueous solution that could never have been obtained classically.     

Novel quantum mechanical/molecular mechanical method combined with the theory of energy representation: free energy calculation for the Beckmann rearrangement promoted by proton transfers in the supercritical water

The Beckmann rearrangement of acetone oxime promoted by proton transfers in the supercritical water has been investigated by means of the hybrid quantum mechanical/molecular mechanical approach combined with the theory of energy representation (QM/MM-ER) recently developed. The transition state (TS) structures have been explored by ab initio calculations for the reaction of hydrated acetone oxime on the assumption that the reaction is catalyzed by proton transfers along the hydrogen bonds connecting the solute and the solvent water molecules. Up to two water molecules have been considered as reactants that take part in the proton transfers. As a result of the density functional theory calculations with B3LYP functional and aug-cc-pVDZ basis set, it has been found that participation of two water molecules in the reaction reduces the activation free energy by -12.3 kcal/mol. Furthermore, the QM/MM-ER simulations have revealed that the TS is more stabilized than the reactant state in the supercritical water by 2.7 kcal/mol when two water molecules are involved in the reaction. Solvation free energies of the reactant and the TS have been decomposed into terms due to the electronic polarization of the solute, electron density fluctuation, and others to elucidate the origin of the stabilization of the TS as compared with the reactant. It has been revealed that the promotion of the chemical reaction due to the hydration mainly originates from the interaction between the nonpolarized solute and the solvent water molecules at the supercritical state.     

络合法制备纳米硫酸钡及其影响因素的分析

以Na2SO4和BaCl2为原料,EDTA为络合剂,水为反应介质,采用络合法制备纳米BaSO4。并研究了反应物浓度、体系pH、反应温度、滴加速度和干燥方式等因素对产物粒径大小和粒径分布的影响。通过考察和分析,初步得出纳米BaSO4最佳制备条件为:反应温度为35℃,pH为6,3种反应物EDTA、BaCl2、Na2SO4的反应浓度均为0.5 mol/L。