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.