Temperature dependence of reactions of the nitrate radical with alkanes

Abstract

Rate constants have been determined as a function of temperature for the reaction of NO₃ with ethane, n-butane, isobutane (2-methylpropane) and isopentane (2-methylbutane), at temperatures up to 553 K. Allowance was was made in analysing experimental data for probable secondary reactions. Abstractions of H atoms at primary, secondary, and tertiary sites possess different temperature coefficients. Arrhenius plots for the alkanes n-butane, isobutane and isopentane are linear at the lower temperatures employed, but show curvature at the higher temperatures, probably because of the participation of a sequence of secondary reactions. The rate constants at 298 K were found to be 0.45 ± 0.06, 1.1 ± 0.2 and 1.6 ± 0.2 (in units of 10^–16 cm³ molecule^–1 s^–1) for n-butane, isobutane and isopentane, respectively. Rate constants for ethane were determined only at elevated temperatures (453 K and above). We propose the following expressions for abstraction per H atom at primary, secondary and tertiary sites (in cm³ molecule^–1 s^–1); (9.5 ± 6.8)× 10^–13 exp –(4400 ± 350/T), (6.3 ± 1.5)× 10^–13 exp –(3250 ± 90/T) and (2.3 ± 0.6)× 10^–12 exp –(2960 ± 90/T).