1,720,976 research outputs found
Prediction of reduced falloff curves for recombination reactions at low temperatures
No full textStrong collision falloff curves for barrierless recombination reactions at low temperatures are calculated taking into account transitional modes only. Specific rate constants k(E, J) from statistical adiabatic channel/classical trajectory (SACM/CT) calculations are used. Broadening factors of the falloff curves are found to depend only weakly on the temperature. A systematic analysis of the influence of the centrifugal barriers E-0(J), which are governed by the potential energy surface of the bond energies E-0, and of the number of transitional modes is made. Guidelines for estimating center broadening factors and shape functions for the broadening factors are given
Shock wave and modelling study of the dissociation kinetics of C₂F₅I
Full text linkThe thermal dissociation of C₂F₅I was studied in shock waves monitoring UV absorption signals from the reactant C₂F₅I and later formed reaction products such as CF, CF₂, and C₂F₄. Temperatures of 950–1500 K, bath gas concentrations of [Ar] = 3 × 10⁻⁵ –2 × 10⁻⁴ mol cm⁻³, and reactant concentrations of 100–500 ppm C₂F₅I in Ar were employed. Absorption-time profiles were recorded at selected wavelengths in the range 200–280 nm. It was found that the dissociation of C₂F₅I → C₂F₅ + I was followed by the dissociation C₂F₅ → CF₂ + CF₃, before the dimerization reactions 2CF₂ → C₂F₄ and 2CF₃ → C₂F₆ and a reaction CF₂ + CF₃ → CF + CF₄ set in. The combination of iodine atoms with C₂F₅ and CF₃ had also to be considered. The rate constant of the primary dissociation of C₂F₅I was analyzed in the framework of statistical unimolecular rate theory accompanied by a quantum-chemical characterization of molecular parameters. Rates of secondary reactions were modelled as well. Experimental rate constants for the dissociations of C₂F₅I and C₂F₅ agreed well with the modelling results. The comparably slow dimerization 2CF₂ → C₂F₄ could be followed both by monitoring reactant CF₂ and product C₂F₄ absorption signals, while CF₃ dimerization was too fast to be detected. A competition between the dimerization reactions of CF₂ and CF₃, the recombination of CF₂ and CF₃ forming C₂F₅, and CF-forming processes like CF₂ + CF₃ → CF + CF₄ finally was discussed.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada
Falloff curves and mechanism of thermal decomposition of CF3I in shock waves
Full text linkThe falloff curves of the unimolecular dissociation CF3I (+Ar) - CF3 + I (+Ar) are modelled by combining quantum-chemical characterizations of the potential energy surface for the reaction, standard unimolecular rate theory, and experimental information on the average energy transferred per collision between excited CF3I and Ar. The (essentially) parameter-free theoretical modelling gives results in satisfactory agreement with data deduced from earlier shock wave experiments employing a variety of reactant concentrations (between a few ppm and a few percent in the bath gas Ar). New experiments recording absorption–time signals of CF3I, I2, CF2 and (possibly) IF at 450–500 and 200–300 nm are reported. By analysing the decomposition mechanism, besides the unimolecular dissociation of CF3I, these provide insight into the influence of secondary reactions on the experimental observations.Fil: Cobos, Carlos Jorge. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; ArgentinaFil: Sölter, L.. Universität Göttingen; AlemaniaFil: Tellbach, E.. Universität Göttingen; AlemaniaFil: Troe, J.. Universität Göttingen; Alemania. Institut Max Planck fuer Bioanorganische Chemie; Alemani
Shock wave and modelling study of the unimolecular dissociation of Si(CH₃)₂F₂: an access to spectroscopic and kinetic properties of SiF₂
Full text linkThe thermal dissociation of Si(CH₃)₂F₂ was studied in shock waves between 1400 and 1900 K. UV absorption-time profiles of its dissociation products SiF₂ and CH₃ were monitored. The reaction proceeds as a unimolecular process not far from the high-pressure limit. Comparing modelled and experimental results, an asymmetric representation of the falloff curves was shown to be most realistic. Modelled limiting high-pressure rate constants agreed well with the experimental data. The UV absorption spectrum of SiF₂ was shown to be quasi-continuous, with a maximum near 222 nm and a wavelength-integrated absorption cross section of 4.3 (±1) × 10⁻²³ cm³ (between 195 and 255 nm, base e), the latter being consistent with radiative lifetimes from the literature. Experiments over the range 1900-3200 K showed that SiF₂ was not consumed by a simple bond fission SiF₂ →SiF + F, but by a bimolecular reaction SiF₂ + SiF₂ → SiF + SiF₃ (rate constant in the range 10¹¹-10¹² cm³ mol⁻¹ s⁻¹), followed by the unimolecular dissociation SiF₃ → SiF₂ + F such that the reaction becomes catalyzed by the reactant SiF₂. The analogy to a pathway CF₂ + CF₂ → CF + CF₃, followed by CF₃ → CF₂ + F, in high-temperature fluorocarbon chemistry is stressed. Besides the high-temperature absorption cross sections of SiF₂, analogous data for SiF are also reported.Facultad de Ciencias ExactasInstituto de Investigaciones Fisicoquímicas Teóricas y Aplicada
Shock wave and modeling study of the thermal decomposition reactions of Pentafluoroethane and 2-H Heptafluoropropane
Full text linkThe thermal decomposition reactions of CF3CF2H and CF3CFHCF3 have been studied in shock waves by monitoring the appearance of CF2 radicals. Temperatures in the range 1400-2000 K and Ar bath gas concentrations in the range (2-10) × 10(-5) mol cm(-3) were employed. It is shown that the reactions are initiated by C-C bond fission and not by HF elimination. Differing conclusions in the literature about the primary decomposition products, such as deduced from experiments at very low pressures, are attributed to unimolecular falloff effects. By increasing the initial reactant concentrations in Ar from 60 to 1000 ppm, a retardation of CF2 formation was observed while the final CF2 yields remained close to two CF2 per C2F5H or three CF2 per C3F7H decomposed. This is explained by secondary bimolecular reactions which lead to comparably stable transient species like CF3H, releasing CF2 at a slower rate. Quantum-chemical calculations and kinetic modeling help to identify the reaction pathways and provide estimates of rate constants for a series of primary and secondary reactions in the decomposition mechanism.Fil: Cobos, Carlos Jorge. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina. Universidad Nacional de La Plata; ArgentinaFil: Sölter, L.. Universitat of Gottingen; AlemaniaFil: Tellbach, E.. Universitat of Gottingen; AlemaniaFil: Troe, J.. Max-Planck-Institut für Biophysikalische Chemie; Alemani
Temperature and pressure dependence of the reaction 2CF(3) (+ M) double left right arrow C2F6 (+ M).
No full textLimiting low- and high-pressure rate coefficients as well as full falloff curves have been modeled by unimolecular rate theory for the recombination reaction 2CF(3) (+ M) -> C2F6 (+ M) and the reverse dissociation of C2F6. The results are compared with experimental data from the literature. Although there are considerable discrepancies (up to a factor of 5) between various experimental data near 300 K and the database for high temperatures is still limited, we try to conclude on the temperature dependence of the high-pressure rate coefficient. We suggest that there is only a small and probably positive temperature coefficient of the latter quantity. The present theoretical modeling seems to be in agreement with this experimental result, but it is in disagreement with conclusions from earlier theoretical work. The difference is attributed to different empirical assumptions about the anisotropy of the potential. It is shown that nearly all previous experiments (except high-temperature shock wave and very low pressure pyrolysis/photolysis experiments) correspond to nearly limiting high-pressure conditions
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Shock wave study of the thermal decomposition of CF3 and CF2 radicals.
No full textThe thermal dissociation reactions CF3 + M -> CF2 + F + M (reaction 1) and CF2 + M -> CF + F + M (reaction 3) were studied behind shock waves. CF.) radicals were monitored through their UV absorption. By working at very low reactant concentrations, the rate coefficients of the unimolecular processes could be derived. Reaction 1 was investigated between 1600 and 2300 K in the intermediate range of the falloff curves, at similar to 10 times larger bath gas pressures than employed in earlier work (Srinivasan, N. K.; Su, M.-C.; Michael, J. V.; Jasper, A. W.; Klippenstein, S. J.; Harding, L. B. J. Phys. Chem. A 2008, 112, 31). The combination of the two sets of data, together with theoretical modeling, allows one to construct falloff curves and to provide complete representations of the temperature and pressure dependences of the rate coefficients. Reaction 3 was studied in the limiting low-pressure range and, over the range 2900-3800 K, a rate coefficient k(3) = [Ar] 1.6 x 10(15) exp(-48 040 K/T) cm(3) molecule(-1) s(-1) was obtained. Representations of the rate coefficients over the full falloff curves were again derived by theoretical modeling
Shock Wave Study of the Thermal Dissociations of C3F6 and c-C3F6. I. Dissociation of Hexafluoropropene
Full text linkThe thermal dissociation of C3F6 was studied between 1330 and 2210 K in shock waves monitoring the UV absorption of CF2. CF2 yields of about 2.6 per parent C3F6 were obtained at reactant concentrations of 500-1000 ppm in the bath gas Ar. These yields dropped to about 1.8 when reactant concentrations were lowered to 60 ppm. The increase of the CF2 yield with increasing concentration was attributed to bimolecular reactions between primary and secondary dissociation products. Quantum-chemical and kinetic modeling calculations helped to estimate the contributions from the various primary dissociation steps. It was shown that the measurements correspond to unimolecular reactions in their falloff range. Falloff representations of the rate constants are given, leading to an overall high pressure rate constant k∞ = 2.0 × 10(17)(-104 kcal mol(-1)/RT) s(-1) and a relative rate of about 2/3:1/3 for the reactions C3F6 → CF3CF + CF2 versus C3F6 → C2F3 + CF3.Fil: Cobos, Carlos Jorge. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina. Universidad Nacional de La Plata; ArgentinaFil: Sölter, L.. Universitat of Gottingen; AlemaniaFil: Tellbach, E.. Universitat of Gottingen; AlemaniaFil: Troe, J.. Max-Planck-Institut für Biophysikalische Chemie; Alemania. Universitat of Gottingen; Alemani
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