1,721,005 research outputs found
The t-C4H9OCH2 radical in the gas phase: Detection by multiphoton ionization and the reactions with O, O-2, O-3, H, and NO
No full textThe detection of the t-C4H9OCH2 and the CH3OCH2 radical by the multiphoton ionization/mass spectrometry/discharge fast flow reactor technique has been investigated in the wavelength region lambda = 4200-530 nm. For both radicals, unstructured bands were observed. The rate of the five reactions of the t-C4H9OCH2 radical with O, O-2, O-3, H, and NO have been determined with reference to the corresponding CH3OCH2 radical reactions in the temperature range of 249-684 K at low pressure (around 2 mbar) t-C4H9OCH2 + O --> products k(1) = (1.80 +/- 0.5) . 10(14)(T/300 K)(0.46) (+/-) (0.15) cm(3) mol(-1) s(-1) (253 less than or equal to T/K less than or equal to 684) (1) t-C4H9OCH2 + O-2 --> products k(2) = (6.7 +/- 0.04) . 10(11)(T/300 K)(-1.1 +/- 0.2) cm(3) mol(-1) s(-1) (2) t-C4H9OCH2 + O-3 --> products k(3) = (6.7 +/- 0.1) . 10(12)(T/300 K)(-0.73 +/- 0.3) cm(3) mol(-1) s(-1) (249 less than or equal to T/K less than or equal to 363) (3) t-C4H9OCH2 + H products k(4) = (2.5 +/- 0.1) . 10(13)(T/300 K)(-0.50 +/-0.1) cm(3) mol(-1) s(-1) (4) t-C4H9OCH2 + NO --> products k(5) = (2.6 +/- 0.1) . 10(10)(T/300 K)(1.5 +/-0.6)cm(3) mol(-1) s(-1) The formation of the primary products is discussed on the basis of mass spectrometric detection using low-energy electron impact ionization
The elementary reaction of CHF((X)over-tilde(1)A ') with ozone
No full textThe reaction CHF((X) over tilde(1)A') + O-3 --> products (1) has been studied in an isothermal flow reactor in the temperature range from 235 K to 443 K and at a pressure of about p = 2 mbar under pseudo first order conditions, [O-3](0) >> [CHF](0). He was the main carrier gas. The CHF((X) over tilde) radical was produced in the reaction sequence CH3F + F --> CH2F + HF; CH2F + F --> CHF((X) over tilde) + HF and detected by laser induced fluorescence. For reaction (1) a value of the rate constant: k(1)(T) = (6.1+/-0.6) . 10(12)(T/298 K)(-0.6+/-0.4) [cm(3)/mol S] was obtained. No change in the rate constant was observed, substituting He by the quenchers N-2 or SF6. The reaction mechanism is discussed
Elementary reaction of CF2((X)over-tilde) with O(3): Primary products
No full textThe reaction of CF(2)((X) over tilde) with O(3) has been investigated in a discharge flow reactor at room temperature and low pressure (p = 2.5 mbar). CF(2)((X) over tilde) was produced by a microwave discharge of a mixture of CF(2)Br(2)/He. The reactants and products were detected by mass spectrometers (MS), which were connected to the flow systems via a continuous molecular beam sampling system. The following primary products were observed (CF(2)O(3) (m/z = 98); CF(2)O (m/z = 47, 66); O(2) (m/z = 32)). The reaction mechanism: CF(2)((X) over tilde) + O(3) reversible arrow CF(2)O(3)(#) --> CF(2)O(3) -> CF(2)O + O(2) explains the observed primary products. The molecule with the empirical formula CF(2)O(3) has been observed directly by MS; the structure is discussed
Mechanism and rate of the reaction CH3+O- revisited
No full textThe primary products and the rate of the reaction of methyl radicals with oxygen atoms in the gas phase at room temperature have been studied using three different experimental arrangements: ( A) laser. ash photolysis to produce CH3 and O from the precursors CH3I and SO2 (the educts and the products were detected by quantitative FTIR spectroscopy); ( B) the coupling of a conventional discharge flow reactor via a molecular sampling system to a mass spectrometer with electron impact ionization, which allowed the determination of labile and stable species; ( C) laser induced multiphoton ionization combined with a TOF mass spectrometer-molecular beam sampling-flow reactor, which was used for the specific and sensitive detection of the CH3, CD3, C2H5 and C2D5 radicals and the determination of rate coefficients. The branching ratio of the reaction channels was determined by the experimental arrangements (A) and (B) leading to CH3 + O -> HCHO + H (55 +/- 5)% -> CO + H-2 + H (45 +/- 5)%. The rate coefficients of the normal and deuterated methyl and ethyl radicals with atomic oxygen showed no isotope effect: k(CD3 + O)/k(CH3 + O) = 0.99 +/- 0.12, k(C2D5 + O)/k(C2H5 + O) = 1.01 +/- 0.07 (statistical error, 95% confidence level). The absolute rate coefficient of the reaction CH3 + O was derived with reference to the reaction C2H5 + O (k 1.04 x 10(14) cm(3) mol(-1) s(-1)) leading to k(CH3 + O) = (7.6 +/- 1.4) x 10(13) cm(3) mol(-1) s(-1)
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
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
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
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