Site-specific reaction rate constant measurements for various secondary and tertiary H-abstraction by OH radicals

Abstract

Reaction rate constants for nine site-specific hydrogen atom (H) abstraction by hydroxyl radicals (OH) have been determined using experimental measurements of the rate constants of Alkane+OH→Products reactions. Seven secondary (S20, S21, S22, S30, S31, S32, and S33) and two tertiary (T100 and T101) site-specific rate constants, where the subscripts refer to the number of carbon atoms (C) connected to the next–nearest–neighbor (N–N–N) C atom, were obtained for a wide temperature range (250–1450K). This was done by measuring the reaction rate constants for H abstraction by OH from a series of carefully selected large branched alkanes. The rate constant of OH with four different alkanes, namely 2,2-dimethyl-pentane, 2,4-dimethyl-pentane, 2,2,4-trimethyl-pentane (iso-octane), and 2,2,4,4-tetramethyl-pentane were measured at high temperatures (822–1367K) using a shock tube and OH absorption diagnostic. Hydroxyl radicals were detected using the narrow-line-width ring-dye laser absorption of the R1(5) transition of OH spectrum near 306.69nm.Previous low-temperature rate constant measurements are added to the current data to generate three-parameter rate expressions that successfully represent the available direct measurements over a wide temperature range (250–1450K). Similarly, literature values of the low-temperature rate constants for the reaction of OH with seven normal and branched alkanes are combined with the recently measured high-temperature rate constants from our group [1]. Subsequent to that, site-specific rate constants for abstractions from various types of secondary and tertiary H atoms by OH radicals are derived and have the following modified Arrhenius expressions:S20=8.49×10-17T1.52exp(73.4K/T)cm3molecule-1s-1(250–1450K)S21=1.07×10-15T1.07exp(208.3K/T)cm3molecule-1s-1(296–1440K)S22=2.88×10-13T0.41exp(-291.5K/T)cm3molecule-1s-1(272–1311K)S30=3.35×10-18T1.97exp(323.1K/T)cm3molecule-1s-1(250–1366K)S31=1.60×10-18T2.0exp(500.0K/T)cm3molecule-1s-1(250–1351K)S32=9.65×10-17T1.45exp(180.0K/T)cm3molecule-1s-1(297–1367K)S33=2.83×10-19T2.25exp(501.3K/T)cm3molecule-1s-1(250–1365K)T100=5.69×10-16T1.32exp(76.8K/T)cm3molecule-1s-1(297–1375K)T101=1.05×10-16T1.38exp(577.9K/T)cm3molecule-1s-1(297–1362K)