Luc Vereecken

Products and mechanism of the OH-initiated photo-oxidation of perfluoro ethyl vinyl ether, C2F5OCFCF2

The OH-initiated photo-oxidation of perfluoro ethyl vinyl ether (C2F5OCF[double bond, length as m-dash]CF2, PEVE) in air (298 K, 50 and 750 Torr total pressure) was studied in a photochemical reactor using in situ detection of PEVE and its products by Fourier transform IR absorption spectroscopy. The relative rate technique was used to derive the rate coefficient, k1, for the reaction of PEVE with OH as k1 = (2.8 ± 0.3) × 10-12 cm3 molecule-1 s-1. The photo-oxidation of PEVE in the presence of NOx at 1 bar results in formation of C2F5OCFO, FC(O)C(O)F and CF2O in molar yields of 0.50 ± 0.07, 0.46 ± 0.07 and 1.50 ± 0.22, respectively. FC(O)C(O)F and CF2O are formed partially in secondary, most likely heterogeneous processes. At a reduced pressure of 50 Torr, the product distribution is shifted towards formation of FC(O)C(O)F, indicating the important role of collisional quenching of initially formed association complexes, and enabling details of the reaction mechanism to be elucidated. An atmospheric photo-oxidation mechanism for PEVE is presented and the environmental implications of PEVE release and degradation are discussed.

Temperature dependence of stable carbon kinetic isotope effect for the oxidation reaction of ethane by OH radicals: Experimental and theoretical studies

The stable carbon kinetic isotope effect (KIE) of ethane photo-oxidation by OH radicals was deduced by employing both laboratory measurements and theoretical calculations. The investigations were designed to elucidate the temperature dependence of KIE within atmospherically relevant temperature range. The experimental KIE were derived from laboratory compound specific isotope analyses of ethane with natural isotopic abundance exposed to OH at constant temperature, showing e values of 7.16 ± 0.54 ‰ (303 K), 7.45 ± 0.48 ‰ (288 K), 7.36 ± 0.28 ‰ (273 K), 7.61 ± 0.28 ‰ (263 K), 8.89 ± 0.90‰ (253 K), and 9.42 ± 2.19% (243 K). Compared to previous studies, a significant improvement of the measurement precision was reached at the high end of the investigated temperature range. The KIE was theoretically determined as well, in the temperature range of 150 K to 400 K, by calculating the reaction rate coefficients of 12C and singly 13C substituted ethane isotopologues applying chemical quantum mechanics together with transition state theory. Tunneling effect and internal rotations were also considered. The agreement between experimental and theoretical results for rate coefficients and KIE in an atmospherically relevant temperature range is discussed. However, both laboratory observations and computational predictions show no significant temperature dependence of the KIE for the ethane oxidation by OH radicals.