RESUMEN
The thermal dissociation reactions of C2F4 and C2F6 were studied in shock waves over the temperature range 1000-4000 K using UV absorption spectroscopy. Absorption cross sections of C2F4, CF2, CF, and C2 were derived and related to quantum-chemically modeled oscillator strengths. After confirming earlier results for the dissociation rates of C2F4, CF3, and CF2, the kinetics of secondary reactions were investigated. For example, the reaction CF2 + CF2 â CF + CF3 was identified. Its rate constant of 1010 cm3 mol-1 s-1 near 2400 K is markedly larger than the limiting high-pressure rate constant of the dimerization CF2 + CF2 â C2F4, suggesting that the reaction follows a different path. When the measurements of the thermal dissociation CF2 (+Ar) â CF + F (+Ar) are extended to temperatures above 2500 K, the formation of C2 radicals was shown to involve the reaction CF + CF â C2F + F (modeled rate constant 8.0 × 1012 (T/3500 K)1.0 exp(-4400 K/T) cm3 mol-1 s-1) and the subsequent dissociation C2F (+Ar) â C2 + F + (Ar) (modeled limiting low-pressure rate constant 3.0 × 1016 (T/3500 K)-4.0 exp(-56880 K/T) cm3 mol-1 s-1). This mechanism was validated by monitoring the dissociation of C2 at temperatures close to 4000 K. Temperature- and pressure-dependences of rate constants of reactions involved in the system were modeled by quantum-chemistry based rate theory.
RESUMEN
The thermal decomposition of perfluorotriethylamine, (C2F5)3N, was investigated in shock waves by monitoring the formation of CF2. Experiments were performed over the temperature range of 1120-1450 K with reactant concentrations between 100 and 1000 ppm of (C2F5)3N in the bath gas Ar and with [Ar] in the range of (0.7-5.5) × 10-5 mol cm-3. The experiments were accompanied by quantum-chemical calculations of the energies of various dissociation paths and by rate calculations, in particular for the dissociation of C2F5via C2F5 â CF3 + CF2. The overall reaction can proceed in different ways, either by a sequence of successive C-N bond ruptures followed by fast C2F5 decompositions, or by a sequence of alternating C-C and C-N bond ruptures. A cross-over between the two pathways can also take place. At temperatures below about 1300 K, yields of less than one CF2 per (C2F5)3N decomposed were observed. On the other hand, at temperatures around 2000 K, when besides the parent molecule, CF3 also dissociates, yields of six CF2 per (C2F5)3N decomposed were measured. The rate-delaying steps of the dissociation mechanism at intermediate temperatures were suggested to be the processes (C2F5)NCF2 â (C2F5)N + CF2 and (CF2)N â N + CF2. The reduction of the CF2 yields at low temperatures was tentatively attributed to a branching of the mechanism at the level of (C2F5)2NCF2, from where the cyclic final product perfluoro-N-methylpyrrolidine, (C4F8)NCF3, is formed which was identified in earlier work from the literature.
RESUMEN
The unimolecular dissociation of CH2F2 leading to CF2 + H2, CHF + HF, or CHF2 + H is investigated by quantum-chemical calculations and unimolecular rate theory. Modeling of the rate constants is accompanied by shock wave experiments over the range of 1400-1800 K, monitoring the formation of CF2. It is shown that the energetically most favorable dissociation channel leading to CF2 + H2 has a higher threshold energy than the energetically less favorable one leading to CHF + HF. Falloff curves of the dissociations are modeled. Under the conditions of the described experiments, the primary dissociation CH2F2 â CHF + HF is followed by the reaction CHF + HF â CF2 + H2. The experimental value of the rate constant for the latter reaction indicates that it does not proceed by an addition-elimination process involving CH2F2* intermediates, as assumed before.
RESUMEN
The 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 × 1014 exp(-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 â CF2 + COF2) = 7.1 × 1014 exp(-320 kJ mol-1/RT) s-1 and k6,∞(CF3COF â CF3 + FCO) = 3.9 × 1015 exp(-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.
RESUMEN
The 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.
RESUMEN
The 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 = 10(15.97) 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.
RESUMEN
In June and September 1988, the USDA Food Safety and Inspection Service sampled raw chicken carcasses at a federally inspected slaughter establishment in Puerto Rico to determine the effects of changing the scalding equipment on bacterial contents of raw poultry products. The scalding equipment was changed to a countercurrent configuration, with a postscald hot-water rinse cabinet that sprayed carcasses as they exited the scalder. Analysis of 250 carcass-rinse samples collected at preevisceration, prechill, and postchill sites over 7 days indicated that carcasses had mean aerobe plate counts of log(10)3.73 before evisceration, 3.18 before chilling, and 2.87 after chilling; Enterobacteriaceae counts of log(10)2.70 before evisceration, 2.25 before chilling, and 1.56 after chilling; and Escherichia coli counts of log(10)2.09 before evisceration, 1.61 before chilling, and 0.89 after chilling. Salmonellae were found on 24% of the carcasses before evisceration, on 28% before chilling, and on 49% after chilling. Although bacterial count reductions were significant at all 3 sites, the proportion of carcasses contaminated with salmonellae in this study was higher at the postchill than prechill site (49 vs 28%). This no doubt was caused by cross-contamination in the chiller. These percentages indicated that although simple scalder changes contributed substantially to the improvement of the bacterial quality of chicken carcasses, additional interventions in the chilling process (such as chlorination of chill water) are important to control cross-contamination and to preserve the positive effects obtained by the scalder changes.