Photochemistry of CF₃(CH₂)₂CHO in air: UV absorption cross sections between 230 and 340nm and photolysis quantum yields at 308nm

This work constitutes the first study on the photochemical degradation process of CF₃(CH₂)₂CHO. Firstly, the wavelength and temperature dependence of the UV absorption cross sections, σλ, was determined. The n→π* electronic transition band of CO chromophore was characterized between 230 and 340nm in the 269–323K range. A hyperchromic effect was observed in the structured part of the band when the temperature decreases. Maximum σλ₌₂₈₃, ₂₉₁ₙₘ at 323K is ca. 22% larger than those at 269K. Secondly, the pulsed laser photolysis of a stationary mixture of CF₃(CH₂)₂CHO/cyclohexane (OH-scavenger)/air or N₂ was carried out at 308nm. On-line Fourier transform infrared (FTIR) spectroscopy was employed to monitor the decay of CF₃(CH₂)₂CHO and to obtain the photolysis quantum yield, Φλ₌₃₀₈ₙₘ, as a function of total pressure (20.5–760Torr). A slight curvature in the Stern–Volmer plot was observed at pressures lower than 75Torr. At high pressures, the pressure dependence of Φλ₌₃₀₈ₙₘ can be described by a Stern–Volmer relationship. Photodissociation of CF₃(CH₂)₂CHO at 308nm can produce HCO and CF₃(CH₂)₂ radicals (1a), CF₃CH₂CH₃ and CO (1b) and CF₃(CH₂)₂CO radicals and H atoms (1c). HCO radicals are rapidly converted into CO in the presence of O₂. Formation of CF₃CH₂CHO and CF₃CH₂CH₂OH evidences the importance of secondary chemistry involving CF₃(CH₂)₂ radicals formed in channel (1a). Further photodegradation of CF₃CH₂CHO yields mainly CF₃CHO. Small quantities of HC(O)OH were also detected. CF₃(CH₂)₂C(O)OH was only observed in the absence of OH-scavenger, implying that formation of CF₃(CH₂)₂CO radicals in channel (1c) is not an important photolysis pathway. Consequently, photodissociation of CF₃(CH₂)₂CHO in the actinic region is a source of shorter fluorinated oxygenated compounds, but it is not expected to be a source of fluorinated acids.