Autocatalytic Rate Constant Research Articles

Spontaneous Emergence of Transient Chirality in Closed, Reversible Frank-like Deterministic Models.

To explore abiotic theories related to the origin of biomolecular homochirality, we analyze two entirely reversible kinetic models composed of an enantioselective autocatalysis with limited stereoselectivity that is coupled to an enantiomeric mutual inhibition (Frank-like models). The two models differ in their autocatalytic steps in respect to the formation of monomer species in one model and of dimer species in the other. While fully reversible and running in a closed system, spontaneous mirror symmetry breaking (SMSB) gives rise to transient chiral excursions, even when starting from a strictly achiral situation. Before the SMSB, the two models differ in the main dissipative processes. At the SMSB, the entropy production rate reaches its maximum in both models. Here it is the enantioselective autocatalysis with retention of the winner enantiomer that dominates. During the terminal phase, the enantioselective autocatalysis with inversion prevails, while the entropy production rate vanishes, thus fulfilling the conditions of microscopic reversibility. SMSB does not occur if the autocatalytic rate constant is too strong or too weak. However, when the autocatalysis is relatively weak, the temporary chiral excursions last for long periods of time and could be the starting point of a cascade of asymmetric reactions. The realism of such Frank-like models is discussed from the viewpoint of their relevance to prebiotic chemistry.

Noncatalytic and Autocatalytic Rate Constants of the Reaction of Phenyl Isocyanate with Butan-1-ol

The kinetics of the reaction of phenyl isocyanate with butan-1-ol have been studied in the temperature range from 30 to 60°C. The rate constants of the noncatalytic and autocatalytic reaction paths have been determined from the concentration dependences of the effective second-order rate constants. The autocatalytic reaction is characterized by lower enthalpies and entropies of activation than those of the noncatalytic reaction. The autocatalytic reaction is the major formation path of butyl phenylcarbamate.

Curing Kinetics of Biobased Epoxies for Tailored Applications

The curing kinetics of a family of biobased epoxies derived from n-alkyl diphenolate esters differing in ester side chain length were compared with diglycidyl ether of bisphenol A (DGEBA). Isothermal isoconversional analysis and Kamal–Sourour model fitting by differential scanning calorimetry (DSC) were utilized to obtain reaction constants. The biobased epoxides and DGEBA have reaction orders that are comparable while the autocatalytic rate constant of DGEBA was larger than those of the biobased epoxies. As the n-alkyl side chain length of diphenolate esters increased, the autocatalytic rate constant decreased. Furthermore, the non-autocatalytic rate constant for DGEBA is smaller than that of the biobased epoxies. The cause for the difference in rate constants is discussed, and applications are assigned to the epoxies based on curing kinetics.

Cure kinetics of epoxy–amine resins used in the restoration of works of art from glass or ceramic

The cure kinetics of two epoxy/amine resins, Araldite 2020 and AY103-HY956 widely used as adhesives in the restoration of works of art from glass or ceramic was investigated using FTIR spectroscopy. These resins are two-part adhesives, consisting of a resin – A, based on a diglycidyl ether of bisphenol A, and a hardener – B which is either a cycloaliphatic amine (isophorone diamine) for Araldite 2020, or a mixture of three aliphatic amines in HY956. The study was based on the collection of IR spectra, in the middle range (4000–600 cm −1), of mixtures of resin and hardener at different proportions and isothermal temperatures (22–70 °C) as a function of curing time. A kinetic model was employed to simulate the experimental data using two kinetic rate constants. Diffusion control was incorporated to describe the cure behaviour at high degrees of conversion. From fitting to experimental data the kinetic and diffusional parameters were estimated, together with the activation energies of the kinetic and autocatalytic rate constants. It was found that higher degrees of curing are obtained at higher temperatures and increased amounts of hardener. Differences in the performance of the two adhesives are explained based on the type of the amines used as hardener.

Cure Kinetics of an Epoxy-Anhydride-Imidazole Resin System by Isothermal DSC

The cure kinetics of a dimethyl imidazole catalyzed diglycidyl ether of bisphenol A - methyltetrahydrophthalic anhydride resin system was examined. Degree of conversion versus cure time was obtained using isothermal DSC over the temperature range from 80 °C to 140 °C. The kinetic data were analysed using both nth order and autocatalytic kinetic models. The results indicated that the epoxy curing process could be described satisfactorily by Kamal's four-parameter generalised autocatalytic model modified with an exponential function of conversion defined as the diffusion factor. The curing process was predominantly autocatalytic in nature. The autocatalytic rate constant of the resin system was 10 to 60 times larger than the nth order rate constant. The corresponding activation energies obtained from Arrhenius equation were 78.1 kJ/mol and 67.6 kJ/mol, respectively. The approximate order of reaction was 0.39 for the autocatalytic reactions and 1.0 for the nth order reactions.

A self-replicating ligase ribozyme.

A self-replicating molecule directs the covalent assembly of component molecules to form a product that is of identical composition to the parent. When the newly formed product also is able to direct the assembly of product molecules, the self-replicating system can be termed autocatalytic. A self-replicating system was developed based on a ribozyme that catalyzes the assembly of additional copies of itself through an RNA-catalyzed RNA ligation reaction. The R3C ligase ribozyme was redesigned so that it would ligate two substrates to generate an exact copy of itself, which then would behave in a similar manner. This self-replicating system depends on the catalytic nature of the RNA for the generation of copies. A linear dependence was observed between the initial rate of formation of new copies and the starting concentration of ribozyme, consistent with exponential growth. The autocatalytic rate constant was 0.011 min(-1), whereas the initial rate of reaction in the absence of pre-existing ribozyme was only 3.3 x 10(-11) M.min(-1). Exponential growth was limited, however, because newly formed ribozyme molecules had greater difficulty forming a productive complex with the two substrates. Further optimization of the system may lead to the sustained exponential growth of ribozymes that undergo self-replication.

Comparative DSC kinetics of the reaction of DGEBA with aromatic diamines. II. Isothermal kinetic study of the reaction of DGEBA with m-phenylene diamine

In the first part of the present series the non-isothermal kinetics of the reaction of an epoxy resin based on diglycidyl ether of bis-phenol A (DGEBA) with m-phenylene diamine (mPDA) was studied. A four step kinetic analysis was applied using differential scanning calorimetry (DSC) data. It allowed us to confirm the validity of the three molecular autocatalytic model of this reaction, as well as to obtain reliable kinetic data in programmed temperature mode.The isothermal study of the DGEBA–mPDA reaction was performed applying a similar kinetic approach: (i) analysis at the peak maximum of the DSC curves; (ii) apparent activation energy analysis of the isothermal DSC data; (iii) integral and differential curve fitting methods; and (iv) modeling of the reaction and comparison of the model with the experiment.It was established that the overall kinetic parameters measured under programmed temperature conditions sufficiently well described the isothermal shift of the DSC curves along the logarithmic time scale, especially the initial stage of the reaction. A more precise analysis of the data showed that the isothermal DSC kinetics obeyed a formal model whose power exponent was approximately 2.5, or it was not well represented by the mechanistic-like three molecular autocatalytic velocity equation. Nevertheless, the activation energy of the autocatalytic rate constant determined at constant temperature mode, i.e. Ea,i=50.67kJmol−1, was found out in close agreement with the one obtained previously in programmed temperature mode, Ea,n=50.50kJmol−1. On the contrary, the ratio of the impurity catalytic to autocatalytic rate constant was slightly temperature dependent.

Reaction kinetics and mechanism of formation of [MnMo9O32]6? by hypochlorous acid oxidation of Mn2+(aq) in the presence of molybdate

Oxidation of Mn2+(aq) by HOCl in weakly acidic solution, in the presence of molybdate, results in the formation of the soluble heteropolymolybdate [MnMo9O32]6–. The speciation of Mn2+ in solution prior to oxidation has been examined using ESR spectroscopy and it has been established that the principal species is [Mn(OH2)6]2+, there being evidence for weak interactions only with molybdate and isopolymolybdate anions. In keeping with this conclusion, the secondary manganese(II) polymolybdate species Na4[MnMo8O27]·20H2O was crystallized from an acidic (pH 4.5) solution containing Mn2+(aq) and isopolymolybdate ions by slow vapour diffusion of ethanol into the aqueous phase. The compound crystallizes in the triclinic space group P, a= 9.568(2), b= 9.868(2), c= 11.703(3)A, α= 103.60(1), β= 100.86(1) and γ= 96.05(1)°, and the structure was determined by X-ray diffraction methods to an R of 0.026 (R′= 0.040) for 3182 independent observed reflections. It consists of polymerized octamolybdate units linked through the sharing of common oxygen atoms and hydrated Mn2+ groups resulting in a sheet structure, with hydrated Na+ ions also attached to the octamolybdate units. Uncomplexed water is also present. The kinetics of oxidation of MnII by HOCl was studied at 5–20 °C over the range pH 4.0–5.4 and found to exhibit solution autocatalytic behaviour. The oxidation kinetics followed the expanded rate expression +d[MnMo9O326–]/dt=kAC[Mn2+][MnMo9O326–][HOCl][MoO42–][HMoO4–] based on an examination of [HOCl], pH and total [MoO42–] dependences, and arguments based on molybdate speciation under the reaction conditions. An approximate value of kAC at 25 °C, 1.36(10)× 108 dm12 mol–4 s–1, was estimated from an extrapolated value of the second-order autocatalytic rate constant using an Arrhenius plot and the reported formation constants describing the speciation of molybdate, isopolymolybdate and their protonated analogues in solution. A mechanism involving sequential one-electron oxidation steps of Mn2+ through to Mn4+via Mn3+, together with the equilibrium Mn2++ Mn4+⇌ 2Mn3+, which favours Mn2+ and Mn4+, is able to account for the observed autocatalysis.

Differences in effect of Mn(II), Fe(III), Co(II), and Zn(II) ions on trypsinogen activation.

The influence of Mn2+, Fe3+, Co2+, and Zn2+ ions on the extent of trypsinogen activation has been determined for several ion concentrations at pH 7.4 and 36.4 degrees C. For the Mn2+ ion also the autocatalytic rate constants have been detected. The effect of Ca2+ has been reinvestigated for comparison purposes. The apparent dissociation constants of KMn2+ = 0.01 (M) and KCa2+ = 0.02 (M) have been found for the given metal ion-trypsinogen complexes. For Co2+ ion, however, only a slight effect and for Fe3+ and Zn2+ ions no significant effect could be detected on trypsinogen activation. The investigated ions are of empty, open, and completed d subshells of electrons and they are different also in their ionic size. The differences in effects of the ions are discussed on the basis of these factors.

Initial stage of the chlorination of natural rubber

Kinetics of the initial stage of the chlorination of natural rubber(NR), up to chlorine content ⩽1 Cl/C 5, have been investigated using a stop-flow method in CCl 4 at room temperature. The chlorination is very fast and is accelerated by the product HCl. The first orders of the reaction rate relative to NR, Cl 2, and HCl were established. A proposed mechanism includes formation of molecular complexes between polymer units, Cl 2 and HCl; the corresponding kinetic equation described satisfactorily the experimental data. Kinetic characteristics of the chlorination of the model compound 2-methylpentene-2(MP) are the same as for NR. The noncatalytic and autocatalytic rate constants for MP are close to those for NR. Compositional heterogeneity of chlorinated NR(CNR) containing <1 Cl/C 5 has been studied by cross-fractionation. The sample chlorinated by the routine procedure is markedly heterogeneous whereas the sample prepared in the stop-flow unit (where extremely fast mixing is realized) is quite homogeneous. These results lead to the conclusions: (1) high reaction rate is caused by high reactivity of “kinetically independent” NR units, not by a polymeric effect; (2) great compositional heterogeneity of CNR samples prepared by the routine procedure is caused by macrokinetic factors.