Rate constants for OH with selected large alkanes: shock-tube measurements and an improved group scheme

High-temperature rate constant experiments on OH with the five large (C(5)-C(8)) saturated hydrocarbons n-heptane, 2,2,3,3-tetramethylbutane (2,2,3,3-TMB), n-pentane, n-hexane, and 2,3-dimethylbutane (2,3-DMB) were performed with the reflected-shock-tube technique using multipass absorption spectrometric detection of OH radicals at 308 nm. Single-point determinations at approximately 1200 K on n-heptane, 2,2,3,3-TMB, n-hexane, and 2,3-DMB were previously reported by Cohen and co-workers; however, the present work substantially extends the database to both lower and higher temperature. The present experiments span a wide temperature range, 789-1308 K, and represent the first direct measurements of rate constants at T > 800 K for n-pentane. The present work utilized 48 optical passes corresponding to a total path length of approximately 4.2 m. As a result of this increased path length, the high OH concentration detection sensitivity permitted pseudo-first-order analyses for unambiguously measuring rate constants. The experimental results can be expressed in Arrhenius form in units of cm(3) molecule(-1) s(-1) as follows: kOH+n-heptane)(2.48(0.17) x 10(-10) exp[(-1927(69 K) / T] (838-1287 K)kOH+2,2,3,3-TMB)(8.26(0.89)x10(-11) exp[(-1337(94 K)/ T] (789-1061 K)kOH+n-pentane)(1.60(0.25) x 10(-10) exp[(-1903(146 K) / T] (823-1308 K)kOH+n-hexane)(2.79(0.39) x 10(-10) exp[(-2301(134 K) /T] (798-1299 K)kOH+2,3-DMB)(1.27(0.16) x 10(-10) exp[(-1617(118 K)/ T] (843-1292 K) The available experimental data, along with lower-T determinations, were used to obtain evaluations of the experimental rate constants over the temperature range from approximately 230 to 1300 K for most of the title reactions. These extended-temperature-range evaluations, given as three-parameter fits, are as follows: kOH+n-heptane)2.059 x 10(-15)T1.401 exp(33 K/ T) cm3 molecule-1 s-1 (241-1287 K)kOH+2,2,3,3-TMB)6.835 x 10(-17)T1.886 exp(-365 K/ T) cm3 molecule-1 s-1 (290-1180 K)kOH+n-pentane)2.495 x 10(-16)T1.649 exp(80 K/T) cm3 molecule-1 s-1 (224-1308 K)kOH+n-hexane)3.959 x 10(-18)T2.218 exp(443 K/T) cm3 molecule-1 s-1 (292-1299 K)kOH+2,3-DMB)2.287 x 10(-17)T1.958 exp(365 K/ T) cm3 molecule-1 s-1 (220-1292 K). The experimental data and the evaluations obtained for these five larger alkanes in the present work were used along with prior data/evaluations obtained in this laboratory for H abstractions by OH from a series of smaller alkanes (C(3)-C(5)) to devise rate rules for abstractions from various types of primary, secondary, and tertiary H atoms. Specifically, the current scheme was applied with good success to H abstractions by OH from a series of n-alkanes (n-octane through n-hexadecane). The total rate constants using this group scheme for reactions of OH with selected large alkanes are given as three-parameter fits in this article. The rate constants for the various abstraction channels in any large n-alkane can also be obtained using the groups listed in this article. The present group scheme serves to reduce the uncertainties in rate constants for OH + alkane reactions.