1,721,257 research outputs found
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
ChemInform Abstract: Towards a Quantitative Characterization of Photoisomerization Rates
No full text
Restoring detailed balance in the Landau-Teller probabilities for collision-induced vibrational transitions.
Full text link
Statistical theory for the reaction N + OH → NO + H: thermal low-temperature rate constants
Full text linkThe reaction N + OH → NO + H involves the intermediate formation of NOH adducts which in part rearrange to HNO conformers. A statistical treatment of the process is developed in which an initial adiabatic channel capture of the reactants is accompanied by partial primary redissociation of the N⋯OH collision pairs. A criterion for the extent of this primary redissociation in competition to the formation of randomized, long-lived, complex of NOH is proposed. The NOH adducts then may decompose to NO + H, rearrange in a unimolecular process to HNO, or undergo secondary redissociation back to the reactants N + OH, while HNO may also decompose to NO + H. As the reactants N(4S) + OH(2Π) have open electronic shells, non-Born–Oppenheimer effects have to be considered. Their influence on thermal rate constants of the reaction at low temperatures is illustrated and compared with such effects in other reactions such as C(3P) + OH(2Π)
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
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 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
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
- …
