Kinetic and Spectroscopic Studies of the Reaction of CF2 with H2 in Shock Waves.

The reaction of CF2 with H2 was studied in shock waves by monitoring UV absorption signals. CF2 was prepared by thermal dissociation of C2F4 (or of c-C3F6). The rate constant of the reaction CF2 + H2 → CHF + HF near 2000 K was found to be close to 1011 cm3 mol-1 s-1, consistent with earlier information on the reverse reaction CHF + HF → CF2 + H2 and a modeled equilibrium constant. The kinetic studies were accompanied by spectroscopic investigations. Absorption cross sections of C2F4 between 190 and 220 nm were measured near 1000 K and compared with room-temperature values from the literature. Likewise, absorption cross sections of CF2 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 CH2F and CF radicals. Absorptions at longer wavelengths, reaching up to 510 nm, were postulated to arise from C2 radicals formed at later stages of the reaction.