1,721,005 research outputs found
Gas-phase kinetics of the self reactions of the radicals CH2F and CHF2
No full textFluorinated hydrocarbon radical-radical reactions in the gas phase have been studied at low pressure (0.5 less than or equal to p/mbar less than or equal to 2) and low temperature (253 less than or equal to T/K less than or equal to 333) using the discharge flow reactor molecular beam sampling mass spectrometry (MS) technique. Stable and labile species have been detected by MS applying low energy electron impact as well as multiphoton ionisation. For the combination reaction (1) CH2F + CH2F --> products the rate coefficient k(1) = (7.0 +/- 0.8) . 10(12) (T/298)((-3.9+/-1.0)) cm(3)/mol . s was determined. At low pressure the HF elimination pathway (1b) (1b) CH2F + CH2F --> C2H3F + HF is the main channel (k(1b)/k(1) = (0.82 +/- 0.015)). For the CHF, radical self reaction (2) CHF2 + CHF2 --> products the rate coefficient was measured as k(2) = (1.7 +/- 0.5) . 10(13) exp((-555 +/- 89)/T) cm(3)/ mol . s. The stabilisation of C2H2F4 is the main reaction pathway (2a) (k(2a)/k(2) greater than or equal to 0.5), the HF elimination is of minor importance (k(2b)/k(2) = 0.2). No pressure dependence of k(1) and k(2) was observed in the limited pressure range
Mechanisms and rates of the reactions C2H5+O and 1-C3H7+O
No full textThe mechanisms and rates of the reactions of the primary alkyl radicals ethyl and I-propyl with oxygen atoms at room temperature and low pressure (around 5 mbar) have been studied using two independent experimental arrangements. The reactants were generated by UV-laser flash photolysis with different precursors (C2H5COC2H5, C2H6 + CFCl3, C2H5I, C3H7COC3H7, SO2). Stable species concentrations were measured quantitatively by Fourier transform IR and OH radical concentrations of the ground (V = 0) and first vibrational (v = 1) state by time-resolved laser-induced fluorescence. For both reaction 1 and reaction 2, the mechanism is explained in terms of the formation and subsequent decomposition of a chemically activated alkoxy radical and a competing abstraction channel leading directly to OH and the alkene: C2H5 + O --> C2H5O (reaction 1a)/C2H5O --> HCHO + CH3 (reaction 1a(1))/CH3CHO + H (reaction 1a(2))//C2H5 + O --> C2H4 + OH (reaction 1b). The absolute branching ratio was determined preferentially using diethyl ketone as the C2H5 radical source leading to (1a(1))/(1a(2))/(1b), 32/44/24. Relative branching ratios for the C2H5 radical sources C2H6 + Cl and C2H5I were derived as (1a(1))/(1a(2)) = 1/1.5 and 1/1.55, respectively. The overall rate coefficient of the reaction C2H5 + O was measured as k(1) = (1.04 +/- 0.1) X 10(14) cm(3) mol(-1) s(-1) and in addition k(C2H5 + OH) = (7.0 +/- 1) X 10(13) cm(3) mol(-1) s(-1). The mechanism and the rate of reaction 2 were found as 1-C3H7 + O --> 1-C3H7O (reaction 2a)/I-C3H7O --> HCHO + C2H5 (reaction 2a(1))/C2H5CHO + H (reaction 2a(2))//1-C3H7 + O --> C3H6 + OH (reaction 2b) (branching ratio (2a(1))/(2a(2))/(2b), 44/ 32/ 24) and k(2) = (8.2 +/- 1) X 10(13) cm(3) mol(-1) s(-1). The results are discussed in terms of statistical rate theory
Gas-phase kinetics of the reactions of the CH2F radicals with the radicals CHF2, CH3, and C2H5
No full textThe mechanisms and the rates of the reactions of the fluorinated hydrocarbon radical CH2F with the fluorinated and pure hydrocarbon radicals CHF2 CH2, and C2H5, respectively, have been studied at low pressure (around 2 mbar) and room temperature using the discharge-flow reactor technique. Mass spectrometry either with electron impact or laser induced multiphoton ionization was applied for the detection of labile and stable species. The cross combination of the radicals is mainly followed by HF elimination from the chemically activated adduct. The overall rate coefficients at 298 K for CH2F + CHF2 --> C2H2F2 + HF (1) CH2F + CH3 --> C2H4 + HF (2) CH2F + C2H5 --> C3H6 + HF (3) were derived from the analysis of the radical-time profiles by fit procedures; the following values were obtained: k(1) = (3.0 + 1.5/ - 0.7) x 10(12) cm(3) mol(-1) s(-1) k(2) = (4.0 +/- 1.5) x 10(12) cm(3) mol(-1) s(-1). k(3) = (9.0 +/- 3) x 10(12) cm(3) mol(-1) s(-1). The results are discussed in terms of chemical activation mechanisms
Mechanism of the 1-C4H9+O reaction and the kinetics of the intermediate 1-C4H9O radical
No full textThe 1-C4H9 + O reaction has been investigated in two quasi-static reactors with different detection systems. From a time-resolved measurement of OH formation by laser induced fluorescence (T = 295 K, p = 21 mbar, bath gas: He) an inverted vibrational state distribution for OH X (2)Pi (v = 0, 1, 2) was observed. By using Fourier transform infrared spectroscopy, relative product yields of 0.55 +/- 0.08 for 1-C4H8, 0.397 +/- 0.05 for HCHO and 0.053 +/- 0.02 for C3H7CHO were determined (T = 298 K, p = 2 mbar, bath gas: He). The results are explained in terms of the formation and subsequent decomposition of an intermediate chemically activated 1-C4H9O radical and a competing abstraction channel leading directly to OH + 1-C4H8. A modeling by statistical rate theory based on ab initio results for the stationary points of the potential energy surface of C4H9O allows the quantitative description of the product branching ratios. From this modeling, threshold energies of E-06 = 55 +/- 6 and E-07 = 88 +/- 6 kJ mol(-1) for the beta -C-C and the beta -C-H bond dissociation, respectively, in 1-C4H9O are obtained. For the 1,5 H atom shift, a most probable value of E-05 = 40 +/- 5 kJ mol(-1) follows from a comparison of our quantum chemical results with data from the literature
Formation and decomposition of chemically activated cyclopentoxy radicals from the c-C5H9+O reaction
No full textThe formation and the decomposition of chemically activated cyclopentoxy radicals from the C-C5H9 + O reaction have been studied in the gas phase at room temperature. Two different experimental arrangements have been used. Arrangement A consisted of a laser-flash photolysis set up combined with quantitative Fourier transform infrared spectroscopy and allowed the determination of the stable products at 4 mbar. The c-C5H9 radicals were produced via the reaction c-C5H10 + Cl with chlorine atoms from the photolysis of CFCl3; the O atoms were generated by photolysis of SO2. Arrangement B, a conventional discharge flow-reactor with molecular beam sampling, was used to determine the rate coefficient. Here, the hydrocarbon radicals (c-C5H9, C2H5, CH2OCH3) were produced via the reaction of atomic fluorine with c-C5H10, C2H6, and CH3OCH3, respectively, and detected by mass spectrometry after laser photoionization. For the c-C5H9 + 0 reaction, the relative contributions of intermediate formation (c-C5H9O ) and direct abstraction (C-C5H8 + OH) were found to be 68 +/- 5 and 32 +/- 4%, respectively. The decomposition products of the chemically activated intermediate could be identified, and the following relative branching fractions were obtained: C-C5H8O + H (31 +/- 2%), CH2CH(CH2)(2)CHO + H (40 +/- 5%), 2 C2H4 + H + CO (17 +/- 5%), and C3H4O + C2H4 + H (12 +/- 5%). Additionally, the product formation of the C-C5H8 + 0 reaction was studied, and the following relative yields were obtained (mol %): C2H4, 24%; C3H4O, 18%; c-C5H8O, 30%; c-C5H8O, 23%; 4-pentenal, 5%. The rate coefficient of the c-C5H9 + O reaction was determined relative to the reactions C2H5 + 0 and CH3OCH2 + O leading to k = (1.73 +/- 0.05) x 10(14) cm(3) mol(-1) s(-1). The experimental branching fractions are analyzed in terms of statistical rate theory with molecular and transition-state data from quantum chemical calculations, and high-pressure limiting Arrhenius parameters for the unimolecular decomposition reactions of C5H9O species are derived
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
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
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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