Arrhenius Pre-exponential Factor Research Articles

A study on the thermal degradation kinetics of syndiotactic polystyrene by thermogravimetric analysis

The thermal degradation kinetics of syndiotactic polystyrene (sPS) has been studied by thermogravimetric analysis (TGA). Applying both integral and differential methods, the values of the activation energy ( E a) of the degradation process during its early stages and, when possible, the Arrhenius pre-exponential factor have been determined. All the methods appear to be suitable for the data treatment and give an E a value of about 50 kcal mol −1, compared with 60 kcal mol −1 for atactic PS.

Ring opening of a formal cyclopentylmethyl radical in the thermolysis of di(tert-alkyl)(1-norbornyl)methanols

In the thermolysis of di(1-adamantyl)(1-norbonyl)methanol, Ad2NorCOH, 1a, in toluene at 220–265 °C, C–C bond cleavage within the norbornyl group of the first-formed (1-adamantyl)(1-norbornyl)ketyl radical (by loss of Ad˙) leads to ring-opened ketones and several ketonic cross-products. These are isomeric with the secondary alcohol, AdNorCHOH, and with the regular cross-product, AdNorSCOH (S = benzyl), respectively, also present in the product mixture. Formation of the ring-opened thermolysis products is particularly favoured by high temperature and the use of deuteriated solvent, which slows hydrogen transfer from the solvent to the intermediate ketyl radical. The new products are cyclopentane derivatives, formed by cleavage of the norbornyl C(1)–C(2) bond, in agreement with MM2 calculations on the transition states. Self-consistent values for the cage effect have been determined by measuring the extent of 1-[2H] labelling of the adamantane formed in [2H8] toluene and by scavenging the ketyl radical with benzenethiol in [1H8] toluene. The product composition of the scavenger-free reaction in [1H8] or [2H8] toluene has been interpreted by kinetic simulation based on the steady state approximation, a Simplex procedure being used to optimise several rate constants, in particular those for hydrogen transfer from toluene to the ketyl radical and ring opening of the latter. The Arrhenius pre-exponential factor and activation energy are both much greater for ring opening than for hydrogen transfer.

Temperature dependence of the rate constants for oxidation of organic compounds by peroxyl radicals in aqueous alcohol solutions

Rate constants for reactions of chlorinated methylperoxyl radicals with chlorpromazine (2-chloro-10-[3-(dimethylamino)-propyl]phenothiazine), trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid), and ascorbate in aqueous alcohol solutions have been measured by pulse radiolysis as a function of temperature, generally between 5 and 75{degrees}C. The rate constants varied between 10{sup 6} and 10{sup 9} M{sup {minus}1} s{sup {minus}1}, the calculated Arrhenius activation energies ranged from 1 to 30 kJ mol{sup {minus}1}, and the preexponential factors also varied considerably, with log A ranging from 7 to 14. In general, room temperature rate constants increase with an increase in the number of chlorine atoms on the radical (increasing its electron affinity and thus the driving force for the reaction) and with an increase in the solvent polarity. The Arrhenius preexponential factor and the activation energy both increased as the proportion of water in the solvent mixture increased; i.e., the increase in rate constant with solvent polarity is a result of two compensating effects. Electron transfer from the organic reductants to the chlorinated methylperoxyl radicals is suggested to take place via an inner-sphere mechanism involving a transient adduct of the peroxyl radical to the reductant. 15 refs., 2 figs., 3 tabs.

Studies on the kinetics of photo‐initiated radical polymerisation of modified epoxy resin

AbstractThe diglycidyl ether of bisphenol‐A (DGEBPA) was modified by acrylic acid. The reaction between the active epoxy groups of DGEBPA and the carboxylic group of acrylic acid was catalysed by triphenyl phosphine to prepare di(vinyl‐2‐hydroxypropanoate) ether of bisphenol‐A (DEBA). The DEBA so formed was co‐polymerised with 2‐ethylhexylacrylate using benzophenone as the photo‐initiator to yield a flexible polymer. The kinetics of the photo‐initiated radical polymerisation was studied by a new technique called differential photo‐calorimetery and the results thus obtained were analysed to determine the Arrhenius activation energy and pre‐exponential factor.

High secondary α-deuterium kinetic isotope effect in the acetolysis and formolysis of dideuterioferrocenylmethyl benzoate

Acetolysis and formolysis of dideuterioferrocenylmethyl benzoate exhibit large secondary α-deuterium kinetic isotope effects and an abnormal temperature dependence. In the presence of LiClO4(ca. 0.1 mol dm–3), which prevents the reversion from solvent-separated to contact ion-pairs, kH/kD at 25 °C amount to 1.53 ± 0.02 (acetolysis) and 1.48 ± 0.03 (formolysis). In the presence of LiClO4 the ratios of Arrhenius pre-exponential factors, AH/AD, are significantly less than unity and amount to 0.49 ± 0.01 (acetolysis) and 0.38 ± 0.04 (formolysis). In the absence of LiClO4 the AH/AD ratios are much smaller (0.02 both in acetolysis and formolysis). We suggest that these surprisingly low values result from a change in rate-determining step over the temperature range, from formation of the solvent-separated ion-pair at low temperatures to reaction of the dissociated carbocation with solvent at the highest temperatures. Whether tunnelling plays any role in these solvolyses is discussed.

Tungsten(V) binuclear thiocomplexes with diethyldithiocarbamate. Synthesis, spectral, thermal and kinetic studies

We report the synthesis, using reactions in different solvents, and the study of di-μ-sulfido-bis(sulfido N,N-diethyldithiocarbamate) dy-tungsten(V) adducts with pyridine or substituted pyridines, with formula assigned as W 2S 4(diethyldtc) 2B 2, where dtc=dithiocarbamate; B=pyridine (Py), 3-methylpyridine (3-MP), 4-methylpyridine (4-MP), 3,5-dimethylpyridine (3,4-DMP), 3-aminopyridine (3-AP) and 4-aminopyridine (4-AP). The complexes obtained have been identified by IR and electronic spectra, magnetic susceptibility measurements and analytical data. We have also studied the thermal behaviour and kinetic parameters, by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), of the thermal decomposition of these adducts in the solid state. From the DSC curves, the activation energies and pre-exponential Arrhenius factors for the endothermic process corresponding to the loss of two moles of coordinated base were calculated. We have also deduced the mechanism of the reaction using a new non-isothermal kinetic method. The effects of steric hindrance and the inductive effects exercised by the functional groups, amino and methyl of the pyridine, in the formation of the adducts are discussed. The relationship between the basicity of the pyridines, IR and electronic spectral data and activation energies has also been explored.

Thermal and kinetic parameters of phyllosilicate dehydration

This paper describes the application of differential scanning calorimetry (DSC), thermogravimetry and differential thermogravimetry to the analysis of the thermal dehydration of a natural phyllosilicate (Sierra del Segura, Spain). The thermal and kinetic parameters of the thermal dehydration, under nitrogen atmosphere, have also been determined by differential scanning calorimetry (DSC). From the DSC curve, activation energies and pre-exponential Arrhenius factors for the endothermic processes of the loss of water, have been calculated using a new method, and the mechanism of these reactions has been determined. The samples have been identified by IR spectroscopy and X-ray powder diffraction.

Thermal and kinetic studies on the adducts with pyridines of bis(8-quinolinol) oxovanadium(IV)

The kinetic study by differential scanning calorimetry (DSC) of the thermal decomposition under nitrogen atmosphere is described for several adducts with pyridine or substituted pyridines of bis(8-quinolinol) oxovanadium(IV), whose assigned formula is VO(ox) 2·B, where ox ≡ 8-quinolinol and B ≡ pyridine (Py), 3-methylpyridine (3-MP), 4-methylpyridine (4-MP), 3,5-dimethylpyridine (3,5-DMP), 3-aminopyridine (3-AP) or 4-aminopyridine (4-AP). From the DSC curves, the activation energies and pre-exponential Arrhenius factors, for the endothermic loss of one mole of coordinated base were calculated using a new method, and the mechanism of this reaction was determined. A relationship between the activation energies and the properties of the ligands has also been established.

Catalytic Control of SiO2 Sol-Gel Kinetics - a Mechanistic Study of Bases

ABSTRACTThe aim of this research was to determine the relative effects of three different base catalysts (NaOH, NH4OH, and RbOH) on the rate of gelation of SiO2 from reactive silicon ethoxide solutions in alcohol (the sol/gel method). The time to gelation was determined using a reaction protocol which assured that condensation was the rate controlling step. NMR spectroscopy was used to verify that the nature of the reacting polymers remained similar as the base catalyst was varied. It was determined that under these conditions the condensation reactions avoid diffusional limitations. Furthermore, the base catalyst serves to modulate both the activation energy and also the Arrhenius preexponential factor. This leads to the proposal of a new condensation mechanism.

Modelling of urethane formation

AbstractThe formation of polyurethanes has been followed by in‐situ 1H NMR. The experimental data have been used in a mathematical model. The model was found to be representative for a major part of the reaction. The Arrhenius preexponential factors and activation energies for the formation reactions were calculated based on the model.

Determination of gross kinetic parameters for petroleum formation from Jurassic source rocks of different maturity levels by means of laboratory experiments

Solvent-extracted core samples of Lower Toarcian carbonate-rich shales (“Posidonia Shale”, Lias ϵ) of the Hils syncline area, Lower Saxony Basin, northern Germany, covering a wide maturity range (vitrinite reflectance, R r = 0.48, 0.88 and 1.45%) and a Kimmeridge mudstone ( R r = 0.45%) are pyrolyzed at three different heating rates (0.1, 0.7 and 5.0 K/min). By application of non-isothermal first-order reaction kinetics to the resulting hydrocarbon evolution curves activation energy distributions between 46 and 70 kcal/mol and pre-exponential Arrhenius factors are obtained by an iteration method. Hypothetical hydrocarbon generation curves are calculated for the Lias ϵ assuming three geological heating rates (0.5, 5 and 50 K/10 6a). Peak hydrocarbon generation temperatures range from 135 to 165°C.

Evidence that both protium and deuterium undergo significant tunneling in the reaction catalyzed by bovine serum amine oxidase.

The magnitudes of primary and secondary H/T and D/T kinetic isotope effects have been measured in the bovine serum amine oxidase catalyzed oxidation of benzylamine from 0 to 45 degrees C. Secondary H/T and D/T kinetic effects are small and in the range anticipated from equilibrium isotope effects; Arrhenius preexponential factors (AH/AT and AD/AT) determined from the temperature dependence of isotope effects also indicate semiclassical behavior. By contrast, primary H/T and D/T isotope effects, 35.2 +/- 0.8 and 3.07 +/- 0.07, respectively, at 25 degrees C, are larger than semiclassical values and give anomalously low preexponential factor ratios, AH/AT = 0.12 +/- 0.04 and AD/AT = 0.51 +/- 0.10. Stopped-flow studies indicate similar isotope effects on cofactor reduction as seen in the steady state, consistent with a single rate-limiting C-H bond cleavage step for Vmax/Km. The comparison of primary and secondary isotope effects allows us to rule out appreciable coupling between the primary and secondary hydrogens at C-1 of the substrate. From the properties of primary isotope effects, we conclude that both protium and deuterium undergo significant tunneling in the course of substrate oxidation. These findings represent the first example of quantum mechanical effects in an enzyme-catalyzed proton abstraction reaction.

Catalytic Activity of Sulphonated Styrene—Divinylbenzene Resins

The evolution of the catalytic activity of sulphonated styrene—divinylbenzene resins was studied in the vapour-phase esterification of acetic acid with ethanol. The activity of fresh resins decreases during the initial period when the reactants are first introduced into the reactor. Also, a variation is observed in the global activation energy and in the Arrhenius pre-exponential factor during this period. After that period (ca. 2 h), the activity reaches a stable value, which remains constant during the experiment and in successive runs. Once the resins have been used, and after a long stoppage, the activity decreases. After a 2-month stoppage, the activity decreases by 30%. This variation affects only the pre-exponential factor and not the activation energy. The different activity levels observed and the evolution can be related to a change in the nature of catalytic sites. Before the addition of reactants to a fresh resin, sulphonic groups inside the resin are associated, whereas after the passage of reactants these sulphonic groups dissociate. A mathematical model has been developed to explain this evolution.

Application of a new method to the kinetic study of the thermal decarbonylation of sodium oxalate

The application of a new method for studying the kinetic parameters of the thermal decarbonylation of sodium oxalate in dinitrogen using differential scanning calorimetry is described. The activation energy and the pre-exponential Arrhenius factor for the loss of carbon monoxide are calculated from the DSC curves and the reaction order and mechanism are also determined. A comparison is made between the results obtained using the proposed method and those reported in the literature.

Thermal and kinetic studies on the adducts with pyridines of bis(acetylacetonate)oxovanadium(IV)

The kinetics of the thermal decomposition, under nitrogen, of several adducts with pyridine or substituted pyridines of bis(acetylacetonate)oxovanadium(IV), {[VO(acac) 2,B] where acac = acetylacetonate and B = pyridine (Py), 3-methyl-pyridine (3-MP), 4-methylpyridine (4-MP), 3,5-dimethylpyridine (3,5-DMP), 3-aminopyridine (3-AP) and 4-aminopyridine (4-AP)}, have been studied by differential scanning calorimetry (DSC). Activation energies and pre-exponential Arrhenius factors, for the endothermic process of the loss of one mole of coordinated base, have been calculated using a new method, and the mechanism of this reaction has been determined. A relationship between activation energies and the properties of the ligands has also been determined.

Reaction branching as a mechanistic alternative to tunneling in explaining anomalous temperature‐dependencies of kinetic isotope effects

AbstractThe temperature‐dependence of the kinetic isotope effects for branched reactions proceeding via a common intermediate has been simulated by calculations. It is shown that, under certain conditions, anomalously small isotope effects on the Arrhenius preexponential factors, as well as unusually large observed isotope effects, may originate from the branching.

Steady-state ablation of an Arrhenius material by external heating

This paper calculates the recession speed and surface temperature of materials that thermally decompose over a range of temperatures according to a first-order Arrhenius rate law. Due to in-depth reactions, the surface is mushy and the surface temperature is not only unknown in advance, but also requires a careful definition. An application to Teflon includes improved calculations of the in-depth distribution of density and temperature. The results strongly depend on the heat exchange processes in the evolving gas and adjacent boundary layer. Nomenclature A = Arrhenius pre-exponential factor c (T) = specific heat of polymer cg(T) = specific heat of gas 40)(a) =seeEq.(22) 4°(a) =seeEq. (22) F+ = total heat flux at cutoff density

Influence of steric hindrance in molecules of substituted 2-phenyl-1,1,3,3-tetramethylguanidines on the kinetics and isotope effects of their proton transfer reactions with 4-nitrophenylnitromethane in acetonitrile solvent

Rate constants have been determined for the proton and deuteron transfer reactions of 4-nitrophenylnitromethane, NO2C6H4CH2NO2, with three phenyl substituted derivatives of 2-phenyl-1,1,3,3-tetramethylguanidine, X—C6H4—N=C[N(CH3)2]2, where X = 4-Cl, 2-Cl, and 2-Br, in the temperature range 278–308 K in acetonitrile solvent. In the concentration ranges used, the reaction to form an ion-paired product is first order in acid and first order in base. The reaction was followed spectrophotometrically using the stopped-flow technique. The reaction is fastest for the 4-Cl substituted base and slowest for the 2-Br analogue. The observed deuterium kinetic isotope effects show a clear dependence on the position and the size of the substituent in the base molecule. Kinetic isotope effects at 25 °C, kH/kD = 9.7 ± 0.3, 12.5 ± 0.3, and 15.6 ± 0.3, were observed for 4-Cl, 2-Cl, and 2-Br substituted bases respectively. The appropriate ratios of Arrhenius preexponential factors, AH/AD = 0.98 ± 0.10, 0.87 ± 0.10, and 0.77 ± 0.10, may indicate increasing importance of tunnelling of the proton when the substituent is moved from the para to the ortho position or when the size of ortho substituent increases. Values of tunnelling correction factors were calculated using Bell's equation by means of the Caldor and Saunders method.

Temperature dependent ac impedance studies of glass to metal contacts in solid state glass ph sensors

Two major types of electrode contact were studied: vacuum evaporated Au, Ag and Cu and thick film-deposited contacts which are fused at high temperature (1000 °C) and involve Pt, Cu, Fe and their oxides. The frequency dependent ac spectra of the real and imaginary contributions to conduction were monitored over a frequency range 10 −4 Hz to 10 6 Hz. Temperatures were varied between ambient and 431 K. The spectral information was studied in both complex impedance and complex admittance forms; these two representations of the same information are used to determine simple equivalent circuits and relevant energies of activation for each relaxation. Arrhenius pre-exponential factors were also obtained for the thermally activated processes.

Barrier permeabilities for a symmetric Eckart potential as studied by the kinetic isotope effects for hydrogen/deuterium abstraction from neopentane by hydrogen atoms in the gas phase

Kinetic isotope effects kH/kD have been determined in the gas phase over the temperature range 363–473 K for the abstraction reactions [graphic omitted] where RH is an alkane and R′D is the corresponding perdeuterated alkane. The alkanes studied include some n-alkanes, cycloalkanes and branched-chain alkanes. The ratio of the Arrhenius pre-exponential factors AH/AD varies from 0.32 to 0.70 and the difference in Arrhenius activation energy ED–EH ranges from 7.1 to 11.0 kJ mol,–1 depending on the strength of the C—H bond being broken. The small A factor ratios, coupled with the large activation energy differences, are taken to indicate the importance of the tunnelling effect in these reactions. A theoretical interpretation of the kinetic isotope effects (KIEs) was made for abstraction of a hydrogen atom from neopentane within the framework of transition-state theory using a London–Eyring–Polanyi–Sato (LEPS) potential-energy surface. Approximating the LEPS profile along the reaction path by a symmetric Eckart barrier, some characteristics of tunnelling phenomena are discussed in great detail, although in an elementary way. Good agreement between experiment and theory was obtained when the barrier permeabilities were taken into consideration.