Abstract
Reaction rate constants for seven large alkanes + hydroxyl (OH) radicals were measured behind reflected shock waves using OH laser absorption. The alkanes, n-hexane, 2-methyl-pentane, 3-methyl-pentane, 2,2-dimethyl-butane, 2,3-dimethyl-butane, 2-methyl-heptane, and 4-methyl-heptane, were selected to investigate the rates of site-specific H-abstraction by OH at secondary and tertiary carbons. Hydroxyl radicals were monitored using narrow-line-width ring-dye laser absorption of the R-1(5) transition of the OH spectrum near 306.7 nm. The high sensitivity of the diagnostic enabled the use of low reactant concentrations and pseudo-first-order kinetics. Rate constants were measured at temperatures ranging from 880 K to 1440 K and pressures near 1.5 atm. High-temperature measurements of the rate constants for OH + n-hexane and OH + 2,2-dimethyl-butane are in agreement with earlier studies, and the rate constants of the five other alkanes with OH, we believe, are the first direct measurements at combustion temperatures. Using these measurements and the site-specific H-abstraction measurements of Sivaramakrishnan and Michael (2009) [1,2], general expressions for three secondary and two tertiary abstraction rates were determined as follows (the subscripts indicate the number of carbon atoms bonded to the next-nearest-neighbor carbon):
S-20 = 1.58 x 10(-11)exp(-1550 K/T) cm(3) molecule(-1) s(-1) (887-1327 K)
S-30 = 2.37 x 10(-11)exp(1850 K/T) cm(3) molecule(-1) s(-1) (887-1327 K)
S-21 = 4.5 x 10(-12) exp(-793.7 K/T) cm(3) molecule(-1) s(-1) (833-1440 K)
T-100 = 2.85 x 10(-11) exp(-1138.3 K/T) cm(3) molecule(-1) s(-1) (878-1375 K)
T-101 = 7.16 x 10(-12) exp(-993 K/T) cm(3) molecule(-1) s(-1) (883-1362 K)
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