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 (S-20, S-21, S-22, S-35, S-31, S-32, and S-33) and two tertiary (T-100 and T-101) 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 (2501450 K). 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 (822-1367 K) using a shock tube and OH absorption diagnostic. Hydroxyl radicals were detected using the narrow-line-width ring-dye laser absorption of the R-1(5) transition of OH spectrum near 306.69 nm.
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 (250-1450 K). 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:
S-20 = 8.49 x 10(-17)T(1.52) exp(73.4 K/T) cm(3) molecule-1 s(-1) (250-1450 K)
S-21 = 1.07 x 10(-13)T(1.07) exp(208.3 K/T) cm(3) molecule(-1) s(-1) (296-1440 K)
S-22 = 2.88 x 10(-13)T(0.41)exp(-291.5 K/T) cm(3) molecule(-1) s(-1) (272-1311 K)
S-30 = 3.35 x 10(-18)TL(1.97)exp(323.1 K/T) cm(3) molecule(-1) s(-1) (250-1366 K)
S-31 = 1.60 x 10(-18)T(2.0) exp(500.0 K/T) cm(3) molecule(-1) s(-1) (250-1351 K)
S-32 = 9.65 x 10(-12)T(1.45) exp(180.0 K/T) cm(3) molecule(-1) s(-1) (297-1367 K)
S-33 = 2.83 x 10(-19)T(2.25) exp(501.3 K/T) cm(3) molecule(-1) s(-1) (250-1365 K)
T-100 = 5.69 x 10(-16)T(1.32)exp(76.8K/T) cm(3) molecule(-1)s(-1) (297-1375 K)
T-101 = 1.05 x 10(-16)T(1.38)exp(577.9 K/T) cm(3) molecule(-1) s(-1) (297-1362 K)
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