Experimentelle und theoretische Untersuchungen der Dissoziationen von Tetrafluorethen, Hexafluorpropen und Hexafluorcyclopropan

Abstract

The thermal dissociation reactions of C2F4, C3F6 and c-C3F6 were studied in shock waves monitoring CF2 radicals by their UV absorption. C2F4: The absorption coefficients as functions of wavelength and temperature were redetermined and are represented in analytical form. Dissociation rate constants as functions of bath gas concentration [M] and temperature, from previous and the present work, are presented analytically employing falloff expressions from unimolecular rate theory. Equilibrium constants are determined between 1200 and 1500 K. The data are shown to be consistent with a C-C bond energy of 67.5 (± 0.5) kcal mol^-1. High pressure limiting rate constants for dissociation and recombination are found to be unusually small. This phenomenon can be attributed to an unusually pronounced anisotropy of the potential energy surface such as demonstrated by quantum-chemical calculations of the potential energy surface. C3F6: The thermal dissociation of C3F6 was studied between 1330 and 2210K. 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. c-C3F6: The thermal dissociation of c-C3F6 has been studied over the range 620 – 1030 K. The reaction was studied close to its high pressure limit, but some high temperature falloff was accounted for. Quantum-chemical and kinetic modeling rationalize the experimental data. The reaction is suggested to involve the 1,3 biradical CF2CF2CF2 intermediate. CF2 formed by the dissociation of c-C3F6 dimerizes to C2F4. The measured rate of this reaction is also found to correspond to the falloff range. Rate constants for 2 CF2 -> C2F4 as a function of temperature and bath gas concentration [Ar] are given and shown to be consistent with literature values for the high pressure rate constants from experiments at lower temperatures and dissociation rate constants obtained in the falloff range at higher temperatures. The onset of falloff at intermediate temperatures is analyzed.