Evaluation of activation energy conformity derived from model-free non-isothermal predictions and Arrhenius isothermal results

Organically modified montmorillonite (OMMT) was used as synergist to enhance the flame-retardant and mechanical properties of poly(butylene succinate)/intumescent flame retardant (PBS/IFR) composites. The flame-retardant, thermal degradation and combustion properties of PBS and its flame-retardant composites were characterized by limiting oxygen index (LOI) test, vertical burning (UL-94) test, thermogravimetric analysis, cone calorimeter and scanning electron microscopy, respectively. The results indicate that PBS/IFR composites exhibit excellent flame retardance when OMMT is at an appropriate content. PBS/IFR composite with 20 wt% IFR and 1.5 wt% OMMT has an LOI of 40.1% and can pass the UL-94 V0 rating. The synergistic effect between OMMT and IFR on the flame-retardant properties of PBS depends on the content of OMMT, and excessive OMMT diminish this synergistic effect. The possible flame-retardant mechanism of OMMT on PBS/IFR composite is proposed. The results of mechanical test also indicate that OMMT can effectively increase the notched impact strength of PBS/IFR composites.     

Thermal hydrocracking of indan. Effects of the hydrogen pressure on thekinetics and Arrhenius parameters

The kinetics of the thermal hydrocracking of indan were investigatedin a high-pressure flow reactor at temperatures from 470 to 530°C, total pressures of up to 300 atm, and molar ratios from 3 to 40. The effect of the hydrogen pressure was reflected especially in a change of the experimental rate equations for the formation of toluene from r_T=k [indan]^0.5 [hydrogen] to r_T=k′ [indan] ^0.75[hydrogen]^0.75 with hydrogen partial pressureincreasing from 73 to 230 atm. The rate equation of n-propylbenzene remained constant at r_Pr=k″ [indan] [hydrogen]^1.5. Simultaneously the Arrheniusparameters of toluene changed significantly, while those of n-propylbenzene remained unchanged. The observed effect of the hydrogen pressure is explained as a change inthe rates of the intermediate reactions; it provides an excellent agreementbetween the theoretical and experimental data. It was found that the steady-state concentration of the hydrogen atoms, which act as chain carriers in the thermal hydrocracking, was much smaller than the thermodynamic equilibrium concentrations     

Numerical investigation of the uncertainty of Arrhenius parameters

The temperature dependence of rate coefficient k is usually described by the Arrhenius expression ln k = ln A − (E/R)T ⁻¹. Chemical kinetics databases contain the recommended values of Arrhenius parameters A and E, the uncertainty parameter f (T) of the rate coefficient and temperature range of validity of this information. Taking ln k as a random variable with known normal distribution at two temperatures, the corresponding uncertainty of ln k at other temperatures was calculated. An algorithm is provided for the generation of the histogram of the transformed Arrhenius parameters ln A and E/R, which is in accordance with their 2D normal probability density function (pdf). The upper and the lower edges of the 1D normal distribution of ln k correspond to the two opposite edge regions of the 2D pdf of the transformed Arrhenius parameters. Changing the temperature, these edge regions move around the 2D cone. The rate parameters and uncertainty data belonging to reactions H + H₂O₂ = HO₂ + H₂ and O + HO₂ = OH + O₂ were used as examples.     

Arrhenius activation parameters for the loss of neutral nucleobases from deprotonated oligonucleotide anions in the gas phase

Arrhenius activation parameters (E(a) and A) for the loss of neutral nucleobase from a series of doubly deprotonated oligodexoynucleotide 10-mers of the type XT(9), T(9)X, and T(5)XT(4), where X = A, C, and G, have been determined using the blackbody infrared radiative dissociation technique. At temperatures of 120 to 190 degrees C, the anions dissociate exclusively by the loss of a neutral nucleobase (XH), followed by cleavage of the sugar 3' C-O bond leading to (a-XH) and w type ions or, in the case of the T(9)X(2-) ions, the loss of H(2)O. The dissociation kinetics and energetics are sensitive to the nature and position of X. Over the temperature range investigated, the kinetics for the loss of AH and GH were similar, but approximately 100 times faster than for the loss of CH. For the loss of AH and GH, the values of E(a) are sensitive to the position of the base. The order of the E(a)s for the loss of XH from the 5' and 3' termini is: C > G > A; while for T(5)XT(4) the order is: C > A > G. The trends in the values of E(a) do not parallel the trend in deprotonation enthalpies or proton affinities of the nucleobases in the gas phase, indicating that the energetic differences do not simply reflect differences in their gas phase acidity or basicity. The pre-exponential factors (A) vary from 10(10) to 10(15) s(-1), depending on the nature and position of X. These results suggest that the reactivity of individual nucleobases is influenced by stabilizing intramolecular interactions.     

On the errors of Arrhenius parameters and estimated rate constant values

A detailed statistical study is presented, based on simulated experimental data, on the estimation of activation parameters using the Arrhenius equation: k = A exp(B/T). The close correlation of the two parameters is shown, which requires the computation of the covariance matrix for the representation of uncertainties. This matrix facilitates the correct estimation of the confidence interval for interpolated (or extrapolated) values of rate coefficients. It is proposed that the full correlation matrix should be published in any article dealing with the determination of Arrhenius parameters. The importance of correct weighting is emphasized. Nonlinear fitting to the Arrhenius equation can be carried out without weighting only in case the (absolute) error of rate coefficient is independent of the temperature. Simulated experiments show that noncorrect weighting shifts the average values of fitted parameters and increases the variance of the parameters as well. With respect to the modified Arrhenius equation: k = A · Tⁿ exp(B/T), statistical analysis shows that the physically meaningful estimation of all three parameters is impossible. Nonlinear fitting of three parameters is suggested for interpolation (and extrapolation) of rate coefficients, whereas in case of activation parameter estimation, the fixing of “n” on the basis of theoretical considerations is advised followed by the estimation of the remaining two parameters.     

On the influence of spatial correlations on the rate of chemical reactions in dense systems. II. Numerical results

The explicit expressions for the rate constant k_f and k_b of a dense system obtained in the preceding paper are investigated numerically. The pair correlation functions representing the spatial correlations are calculated from an associate nonreactive hard sphere system by means of the multicomponent Percus—Yevick equation. The rate constants are found to differ in their dependence on density and time up to an order of magnitude from the corresponding dilute gas value. The time behaviour of k_f and k_b was found to depend sensitively on a relation between the total volumes of the reactant and product molecules.     

Shrinking droplets in electrospray ionization and their influence on chemical equilibria

We investigated how chemical equilibria are affected by the electrospray process, using simultaneous in situ measurements by laser-induced fluorescence (LIF) and phase Doppler anemometry (PDA). The motivation for this study was the increasing number of publications in which electrospray ionization mass spectrometry is used for binding constant determination. The PDA was used to monitor droplet size and velocity, whereas LIF was used to monitor fluorescent analytes within the electrospray droplets. Using acetonitrile as solvent, we found an average initial droplet diameter of 10 microm in the electrospray. The PDA allowed us to follow the evolution of these droplets down to a size of 1 microm. Rhodamine B-sulfonylchloride was used as a fluorescent analyte within the electrospray. By spatially resolved LIF it was possible to probe the dimerization equilibrium of this dye. Measurements at different spray positions showed no influence of the decreasing droplet size on the monomer-dimer equilibrium. However, with the fluorescent dye pair DCM and oxazine 1 it was shown that a concentration increase does occur within electrosprayed droplets, using fluorescence resonance energy transfer as a probe for the average pair distance.     

Arrhenius parameters for reactions of oxygen atoms with the fluorinated ethylenes

Competitive studies of the reactions of ground-state oxygen atoms, generated by mercury-photosensitized decomposition of nitrous oxide, have been carried out with ethylene and all the fluoroethylenes using 2-(trifluoromethyl)-propene as reference compound. From measurements at 25°C and 150°C relative rate constants have been determined and used to calculate the Arrhenius parameters shown in the following table:

Hydrogen bonds involving C-H groups of 1,1,2,2-tetrabromoethane and their effect on the conformation equilibrium

The solvent and polymer matrix effects on the conformation equilibrium in 1,1,2,2-tetrabromoethane (TBE) are investigated by IR spectroscopy. The free energy differences (ΔG₀) of TBE conformations correlate with the dielectric permittivity function of the medium. A pronounced solvent effect on the absolute integrated intensities of the IR bands arising from the C-H stretching vibrations indicates H-bonding between the C-H groups of TBE and the solvent. The enthalpies of specific interactions due to hydrogen bonding were estimated using the intensity rule (1–3 kcal/mole). The absence of correlations between ΔG₀ and the solvent basicity parameter leads us to conclude that the trans- and gosh-conformers of TBE form nearly equally strong hydrogen bonds. Kazan State University. Kazan State Technological University. Translated fromZhurnal Strukturnoi Khimii, Vol. 39, No. 3, pp. 478–483, May–June, 1998. This work was supported by RFFR grant No. 96-03-32171a.     

Study of Arrhenius activation energy on the thermo-bioconvection nanofluid flow over a Riga plate

Journal of Thermal Analysis and Calorimetry - This article deals with a study of Arrhenius activation energy on thermo-bioconvection nanofluid propagates through a Riga plate. The Riga plate is...     

Evaluation of the influence of experimental parameters on the photogalvanic potential of some photosensitive chemical systems

Summary The paper deals with the mathematical evaluation of three important experimental parameters (pH, concentration and temperature) influencing the photogalvanic potential created in solutions of aliphatic alcohols. Factorial design was used to obtain reliable information for the effects of each parameter or their combined influence. The statistical models obtained make it possible not only to evaluate the weight of any experimental value but also to have an idea for possible analytical application of the photochemical systems studied.     

The influence of chemical structure of sulfonamides on the course of their thermal decomposition

The thermal decomposition of several sulfonamides and potassium salts of sulfonamides was investigated. The analyses were performed using a derivatograph in an air atmosphere, sample sizes were from 50 to 200 mg and heating rate from 2.5 to 20 K min^-1. It has been established, that the thermal destruction of studied compounds occurs via three stages with formation of potassium carbonate as a final product of the complete combustion of potassium salts of sulfonamides. The temperature ranges, in which the analyzed compounds undergo thermal transformations were established. For evaluation of the results the principal component analysis (PCA) was applied. By this method the influence of the specific functional groups on the thermal decomposition of sulfonamides and potassium salts of sulfonamides was determined. It has also been recognized, that better discrimination among the analyzed compounds is obtained for the data set of the DTA. This revised version was published online in July 2006 with corrections to the Cover Date.     

Comparison of methods of determining Arrhenius equation parameters by the least squares method

The parameters of the Arrhenius equation determined by the linear, weighted linear and non-linear least squares methods and by the simplex method are compared. Since the non-linear least squares method permits the consideration of statistical weights of both the dependent (k) and independent (T) variables and does not involve logarithmic transformation, it is advisable to calculate the parameters of the Arrhenius equation by means of the non-linear least squares method.

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Zusammenfassung Arrhenius-Parameter bestimmt nach der linearen, mit Gewichten operierenden linearen und nicht-linearen Methode der kleinsten Fehlerquadrate oder nach der Simplex-Methode werden miteinander verglichen. Da bei der nicht-linearen Methode der kleinsten Fehlerquadrate sowohl die abhÄngige (K) als auch die unabhÄngige (T) Variable mit statistischen Gewichten versehen werden kann und keine logarithmische Transformation vorgenommen wird, ist es zweckmÄ\ig, die Berechnung der Parameter der Arrhenius-Gleichung nach der nicht-linearen Methode der kleinsten Fehlerquadrate auszuführen.

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