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Rate Constants for OH with Selected Large Alkanes: Shock-Tube Measurements and an Improved Group Scheme

2009; American Chemical Society; Volume: 113; Issue: 17 Linguagem: Inglês

10.1021/jp810987u

1520-5215

Raghu Sivaramakrishnan, J. V. Michael,

Atmospheric chemistry and aerosols

High-temperature rate constant experiments on OH with the five large (C5−C8) 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 ∼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 ∼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 cm3 molecule−1 s−1 as follows: kOH+n-heptane=(2.48±0.17)×10−10exp[(−1927±69 K)/T](838−1287 K) kOH+2,2,3,3-TMB=(8.26±0.89)×10−11exp[(−1337±94 K)/T](789−1061 K) kOH+n-pentane=(1.60±0.25)×10−10exp[(−1903±146 K)/T](823−1308 K) kOH+n-hexane=(2.79±0.39)×10−10exp[(−2301±134 K)/T](798−1299 K) kOH+2,3-DMB=(1.27±0.16)×10−10exp[(−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 ∼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=2.059×10−15T1.401exp(33 K/T)cm3molecule−1s−1(241−1287 K) kOH+2,2,3,3-TMB=6.835×10−17T1.886exp(−365 K/T)cm3molecule−1s−1(290−1180 K) kOH+n-pentane=2.495×10−16T1.649exp(80 K/T)cm3molecule−1s−1(224−1308 K) kOH+n-hexane=3.959×10−18T2.218exp(443 K/T)cm3molecule−1s−1(292−1299 K) kOH+2,3-DMB=2.287×10−17T1.958exp(365 K/T)cm3molecule−1s−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 (C3−C5) 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.