Kinetic and Spectroscopic Studies of the Reaction of CF with H in Shock Waves.

The reaction of CF with H was studied in shock waves by monitoring UV absorption signals. CF was prepared by thermal dissociation of CF (or of c-CF). The rate constant of the reaction CF + H → CHF + HF near 2000 K was found to be close to 10 cm mol s, consistent with earlier information on the reverse reaction CHF + HF → CF + H and a modeled equilibrium constant. The kinetic studies were accompanied by spectroscopic investigations. Absorption cross sections of CF between 190 and 220 nm were measured near 1000 K and compared with room-temperature values from the literature. Likewise, absorption cross sections of CF near 2000 K were measured between 210 and 300 nm and compared with room-temperature data. Additional superimposed absorption signals were recorded during the reaction and identified by their time dependence and by quantum-chemical calculations employing time-dependent density functional theory. A previously unknown absorption spectrum of CHF radicals near 200 nm was identified and its wavelength dependence determined. Further strong absorptions between 190 and 300 nm were attributed to CHF and CF radicals. Absorptions at longer wavelengths, reaching up to 510 nm, were postulated to arise from C radicals formed at later stages of the reaction.