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 à Products reactions. Seven secondary (S 20 , S 21 , S 22 , S 30 , 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 (250 -1450 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 to 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 threeparameter 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:

Rate Constants for the Reaction of OH Radicals with Hydrocarbons in a Smog Chamber at Low Atmospheric Temperatures

“…For 2,2-dimethylpentane and 2,2-dimethylhexane, calculated values (based on a structure-activity relationship (SAR) [51]) at room temperature are given for these two compounds to evaluate the temperature dependence. The results of 2,2-dimethylpentane fit well to the non-linear fit provided by Badra and Farooq [52]. A SAR estimation is given for 2,2-dimethylhexane, and our data fall very close to the estimation line.…”

A comprehensive iso-octane combustion model with improved thermochemistry and chemical kinetics

“…In this work, the rate coefficients of all H-atom abstraction reactions were updated based on the rate rules suggested by Sarathy et al [63], except for H-atom abstraction by ȮH radicals, for which experimentally-derived rate rules were recently made available [64,65].…”

“…For example, P 3 is a primary carbon bonded to a carbon connected to three other carbons (i.e., the neighbor is a quarternary site); and T 002 is a tertiary carbon bonded to two primary carbons and one tertiary carbon. Badra et al [65] conducted a similar study wherein they measured the total rate of H-atom abstraction by ȮH radicals from different molecules, including iso-octane. Based on their measurements, and the rate rules reported by [64], Badra et al [65] derived rate rules for H-atom abstraction by ȮH radicals from S 20 , S 21 , S 22 , S 30 , S 31 , S 32 , S 33 , T 100 and T 101 sites.…”

Additional chain-branching pathways in the low-temperature oxidation of branched alkanes

“…The initial distribution of primary, secondary and tertiary fuel radicals at 510 K is approximately 26%, 41%, and 33% based on the sitespecific OH radical H-atom abstraction rate coefficients proposed by Badra and Farooq [56] and Sivaramakrishnan and Michael [57] (Fig. S2).…”