1,720,966 research outputs found
The Thermal DissociationRecombination Reactions of SiF4, SiF3, and SiF2: A Shock Wave and Theoretical Modeling Study
No full textMonitoring UV absorption signals of SiF2and SiF, the thermal dissociation reactions of SiF4and SiF2were studied in shock waves. Rationalizing the experimental observations by standard unimolecular rate theory in combination with quantum-chemical calculations of the reaction potentials, rate constants for the thermal dissociation reactions of SiF4, SiF3, and SiF2and their reverse recombination reactions were determined over broad temperature and pressure ranges. A comparison of fluorosilicon and fluorocarbon chemistry was finally made.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, Lars. Universität Göttingen; Alemania. Max-Planck-Institut für Multidisziplinäre Naturwissenschaften; AlemaniaFil: Tellbach, Elsa. Universität Göttingen; Alemania. Max-Planck-Institut für Multidisziplinäre Naturwissenschaften; AlemaniaFil: Troe, Jürgen. Universität Göttingen; Alemania. Max-Planck-Institut für Multidisziplinäre Naturwissenschaften; Alemani
Shock wave and modelling study of the UV spectra of perfluorocarbon iodides and perfluorocarbon radicals
No full textThe UV absorption spectrum of C2F5I was studied in shock waves over the temperature range 580–1200 K and, together with room temperature data, was represented in extended Sulzer–Wieland form. As a case study, the work illustrates the properties of high-temperature UV absorption continua for use in kinetics experiments in shock waves. A comparison with experimental data for CF3I and their representation is made. Quantum-chemical model calculations of positions and oscillator strengths are tested and, besides CF3I and C2F5I, applied also to spectra of C2F4I, C2F4, CF2I, C2F2, C2F, CF2, CFI, CF, and IF. Likewise, modelling and experimental results for the high-temperature UV absorption spectra of CF3 and C2F5 radicals are considered.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: Hintzer, Klaus. Dyneon GmbH; AlemaniaFil: Sölter, Lars. Universität Göttingen; AlemaniaFil: Tellbach, Elsa. Universität Göttingen; AlemaniaFil: Thaler, Arne. Dyneon GmbH; AlemaniaFil: Troe, Jürgen. Universität Göttingen; Alemania. Max-Planck-Institut für biophysikalische Chemie; Alemani
Role of Water Complexes in the Reaction of Propionaldehyde with OH Radicals
No full textThere has been considerable debate and speculation about the role of weakly bound complexes in radical molecule reactions in the gas phase, especially in atmospheric chemistry. Among the significant number of potentially important molecular aggregates in chemical reactions, water complexes are of particular interest. Beyond the well-known energy transfer role of water in complex-forming reactions, it has been shown that water may also have a catalytic effect on the kinetics of radical-molecule reactions because of reduced reaction barrier heights for the complexes. Here we report studies of the reaction of OH radicals and propionaldehyde in the presence and absence of water vapor between 300 and 60 K in Laval nozzle expansions. Water accelerates the overall reaction at low temperatures but much less pronounced than for the reaction of OH with acetaldehyde reported recently. Quantum chemical calculations help us to understand this behavior, which can be rationalized in terms of the stability of intermediate reaction complexes and the effect of water aggregation on the barrier separating prereactive complexes and products
Shock wave and modeling study of the reaction CF4 (+ M) <--> CF3 + F (+ M).
No full textThe thermal decomposition of CF4 (+Ar) → CF3 + F (+Ar) was studied in shock waves over the temperature range 2000–3000 K varying the bath gas concentration [Ar] between 4 × 10−6 and 9 × 10−5 mol cm−3. It is shown that the reaction corresponds to the intermediate range of the falloff curve. By combination with room temperature data for the reverse reaction CF3 + F (+He) → CF4 (+He) and applying unimolecular rate theory, falloff curves over the temperature range 300–6000 K are modeled. A comparison with the reaction system CH4 (+M) ⇔ CH3 + H (+M) is made
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
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
Practical Aspects of Thermal Dissociation and Recombination Reactions: the Reaction Systems CF3X(+M)<-> CF3+X (+M) with X=F, Cl, Br, and I
Full text linkThe thermal dissociation/recombination reactions of the perfluoromethyl halides CF4↔CF3+F, CF3Cl↔CF3+Cl, CF3Br↔CF3+Br, and CF3I↔CF3+I are analyzed with respect to their transition (for increasing pressures) from second-order to first-order dissociation (or from third-order to second-order recombination). The dependence of this transition on the temperature is documented. Practical aspects of the modelling of falloff curves are discussed
Shock wave studies of the pyrolysis of fluorocarbon oxygenates. I. The thermal dissociation of C3F6O and CF3COF
Full text linkThe thermal decomposition of hexafluoropropylene oxide, C3F6O, to perfluoroacetyl fluoride, CF3COF, and CF2 has been studied in shock waves highly diluted in Ar between 630 and 1000 K. The measured rate constant k1 = 1.1 × 1014exp(-162(±4) kJ mol-1/RT) s-1 agrees well with literature data and modelling results. Using the reaction as a precursor, equimolar mixtures of CF3COF and CF2 were further heated. Combining experimental observations with theoretical modelling (on the CBS-QB3 and G4MP2 ab initio composite levels), CF3COF is shown to dissociate on two channels, either leading to CF2 + COF2 or to CF3 + FCO. By monitoring the CF2 signals, the branching ratio was determined between 1400 and 1900 K. The high pressure rate constants for the two channels were obtained from theoretical modelling as k5,∞(CF3COF → CF2 + COF2) = 7.1 × 1014exp(-320 kJ mol-1/RT) s-1 and k6,∞(CF3COF → CF3 + FCO) = 3.9 × 1015exp(-355 kJ mol-1/RT) s-1. The experimental results obtained at [Ar] ≈ 5 × 10-6 mol cm-3 were consistent with modelling results, showing that the reaction is in the falloff range of the unimolecular dissociation. The mechanism of secondary reactions following CF3COF dissociation has been analysed as well.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: Hintzer, K.. Dyneon GmbH; AlemaniaFil: Sölter, L.. Universität Göttingen; AlemaniaFil: Tellbach, E.. Universität Göttingen; AlemaniaFil: Thaler, A.. Dyneon GmbH; AlemaniaFil: Troe, J.. Universität Göttingen; Alemania. Max-Planck-Institut für Biophysikalische Chemie; Alemani
Shock wave studies of the pyrolysis of fluorocarbon oxygenates. II. the thermal dissociation of C4F8O
Full text linkThe thermal decomposition of octafluorooxalane, C4F8O, to C2F4 + CF2 + COF2 has been studied in shock waves highly diluted in Ar between 1300 and 2200 K. The primary dissociation was shown to be followed by secondary dissociation of C2F4 and dimerization of CF2. The primary dissociation was found to be in its falloff range and falloff curves were constructed. The limiting low and high pressure rate constants were estimated and compared with modelling results. Quantum-chemical calculations identified possible reaction pathways, either leading directly to the final products of the reaction or passing through an open-chain CF2CF2CF2 intermediate which dissociates in a second step.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: Hintzer, K.. Dyneon GmbH; AlemaniaFil: Sölter, L.. Universität Göttingen; AlemaniaFil: Tellbach, E.. Universität Göttingen; AlemaniaFil: Thaler, A.. Dyneon GmbH; AlemaniaFil: Troe, J.. Universität Göttingen; Alemania. Max-Planck-Institut für Biophysikalische Chemie; Alemani
Shock wave study and theoretical modeling of the thermal decomposition of c-C4F8.
Full text linkThe thermal dissociation of octafluorocyclobutane, c-C4F8, was studied in shock waves over the range 1150–2300 K by recording UV absorption signals of CF2. It was found that the primary reaction nearly exclusively produces 2 C2F4 which afterwards decomposes to 4 CF2. A primary reaction leading to CF2 + C3F6 is not detected (an upper limit to the yield of the latter channel was found to be about 10 percent). The temperature range of earlier single pulse shock wave experiments was extended. The reaction was shown to be close to its high pressure limit. Combining high and low temperature results leads to a rate constant for the primary dissociation of k1 = 1015.97 exp(−310.5 kJ mol−1/RT) s−1 in the range 630–1330 K, over which k1 varies over nearly 14 orders of magnitude. Calculations of the energetics of the reaction pathway and the rate constants support the conclusions from the experiments. Also they shed light on the role of the 1,4-biradical CF2CF2CF2CF2 as an intermediate of the reaction
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