Synthesis and characterization of pentafluoroethyl peroxynitrate, C2F5OONO2. Photooxidation of C2F5C(O)Cl in the presence of NO2

ADRIANA G. BOSSOLASCO; FABIO E. MALANCA *; GUSTAVO A. ARGÜELLO

Mendoza

Congreso; 21st IAPS Winter Conference; 2011

Peroxynitrates (ROONO2) are important species in the atmosphere because they act as reservoirs of NO2 and ROO. radicals. Their atmospheric thermal lifetimes (from hours to several days) are sufficiently long to be transported from their sources to remote places or to the stratosphere. Industrial compounds such as chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), and hydrofluoroethers (HFEs) lead to the formation of fluorinated peroxynitrates, for example, trifluoromethyl (CF3OONO2), trifluoroacetyl (CF3C(O)OONO2), and trifluoromethoxycarbonyl (CF3OC(O)OONO2) peroxynitrates.In this work we present the photochemical synthesis and characterization of pentafluoroethyl peroxynitrate, C2F5OONO2. The synthesis was carried out by photolysing C2F5C(O)Cl at 254nm (that provides the C2F5 fragment) in presence of NO2 and O2. To establish the correct experimental conditions for the synthesis, the mechanism of photooxidation of C2F5C(O)Cl in the presence of NO2 and O2 was previously studied. Photolysis was run in a standard infrared glass cell, located in the optical path of a FTIR equipment to follow the temporal variation of reactant and products. The photolysis leads to the formation of C2F5OONO2 (54%), CF2O (63%), CF3ONO2 (12%) and CO as the main carbonated products, at short photolysis time. As time evolves, the CO concentrations increase in the system and NO2 decrease, leading to the formation of CF2O, CO2 and CF3OC(O)OONO2. It can be seen a slow disappearance of C2F5OONO2, which act as a reservoir of the NO2 needed for the production of  CF3OC(O)OONO2. A complete reaction mechanism explaining the formation of the products is presented in the Figure.The synthesis of C2F5OONO2 was carried out in a 5 L glass flask maintained at -20 ºC. The progress of its formation was followed every 30 min. through infrared spectroscopy and was stopped when NO2 concentration decrease to one third of its initial concentration. After photolysis, the mixture was collected by slowly passing it through three traps kept at -186ºC, to remove the excess of O2. C2F5OONO2 was further purified by distillation.The infrared spectrum and thermal stability of C2F5OONO2 was determined. Absorption infrared cross-sections at the main peaks (2.54, 1.63, 4.8, 1.57, 2.2, 1.11 x 10-18 cm2 molecule-1 at 1764, 1304, 1244, 1188, 1085 and 790 cm-1 respectively) were measured. Kinetics parameters for its thermal decomposition between 279 and 290 K at total pressure of 9.0 and 200 mbar show that its thermal stability depends on the temperature as well as the total pressure, like in other peroxynitrates: (Ea = 84,2 and 96,5 kJ/mol, at 9 and 200 mbar respectively). Comparison with another peroxynitrates and atmospheric implications are also presented.