H-Abstraction by OH from Large Branched Alkanes: Overall Rate Measurements and Site-Specific Tertiary Rate Calculations

Abstract

Reaction rate coefficients for the reaction of hydroxyl (OH) radicals with nine large branched alkanes (i.e., 2-methyl-3-ethyl-pentane, 2,3-dimethyl-pentane, 2,2,3-trimethylbutane, 2,2,3-trimethyl-pentane, 2,3,4-trimethyl-pentane, 3-ethyl-pentane, 2,2,3,4-tetramethyl-pentane, 2,2-dimethyl-3-ethyl-pentane, and 2,4-dimethyl-3-ethyl-pentane) are measured at high temperatures (900-1300 K) using a shock tube and narrow-line-width OH absorption diagnostic in the UV region. In addition, room-temperature measurements of six out of these nine rate coefficients are performed in a photolysis cell using high repetition laser-induced fluorescence of OH radicals. Our experimental results are combined with previous literature measurements to obtain three-parameter Arrhenius expressions valid over a wide temperature range (300-1300 K). The rate coefficients are analyzed using the next-nearest-neighbor (N-N-N) methodology to derive nine tertiary (T, T, T, T, T, T, T, T, and T) site-specific rate coefficients for the abstraction of H atoms by OH radicals from branched alkanes. Derived Arrhenius expressions, valid over 950-1300 K, are given as (the subscripts denote the number of carbon atoms connected to the next-nearest-neighbor carbon): T = 1.80 × 10 exp(-2971 K/T) cm molecule s; T = 9.36 × 10 exp(-3024 K/T) cm molecule s; T = 4.40 × 10 exp(-4162 K/T) cm molecule s; T = 1.47 × 10 exp(-3587 K/T) cm molecule s; T = 6.06 × 10 exp(-3010 K/T) cm molecule s; T = 3.98 × 10 exp(-1617 K/T) cm molecule s; T = 9.08 × 10 exp(-3661 K/T) cm molecule s; T = 6.74 × 10 exp(-7547 K/T) cm molecule s; T = 3.47 × 10 exp(-1802 K/T) cm molecule s.

Collision-Energy Dependence of the Uptake of Hydroxyl Radicals at Atmospherically Relevant Liquid Surfaces

“…There is overwhelming evidence that this is a positively activated process for alkanes in the gas phase. , However, the quantitative variation in reactivity that might be expected here obviously depends on the relationship between the collision energies spanned and the barrier heights. This is complicated by the presence of primary, secondary, and tertiary sites in the squalane molecule, with significantly different bond strengths and corresponding activation energies. , More fundamentally, for all three C–H bond types the barriers are relatively low, below ∼10 kJ mol –1 ; Arrhenius plots are found to be strongly curved over the accessible range of temperatures for which they have been measured (typically 300–1200 K) for related alkane molecules in the gas phase. , As expressed in the Tolman interpretation, it is not sufficient to therefore simply equate the Arrhenius activation energy, E a , measured in any particular temperature range to the threshold energy, E 0 . To proceed to a reasonable estimate of E 0 for the different C–H bond types in squalane, we have extrapolated from the relationship between E a and E 0 for the well-studied parent molecule, ethane.…”

Developing detailed chemical kinetic mechanisms for fuel combustion

“…Given their apparent sensitivity, rate constants for H-atom abstraction from alkanes by hydroxyl radicals have been measured experimentally, earlier studies were performed by Tully et al [250]- [254] and Bott and Cohen [255]- [258] with more recent measurements by Farooq (b) (a) et al [259]- [261] and correlations based on a group-additivity transition state-theory model were developed by Cohen [262] and updated by Sivaramakrishnan and Michael [263]. There is evidence that group-additivity correlations work well based on the comparisons of the measurements of rate constants for H-atom abstraction from propane and n-butane by ȮH radicals measured by Badra et al [259] compared to the measurements and group-additivity correlations provided by Sivaramakrishnan and Michael [263].…”

Ignition delay measurements of a low-octane gasoline blend, designed for gasoline compression ignition (GCI) engines

“…Interestingly, although cyclopentane has shorter ignition delay times and lower concentration than toluene in MCS, the overall reactivity (or OH concentration) is more sensitive to cyclopentane than toluene. This is probably due to the presence of 10 secondary hydrogen atoms in cyclopentane which makes the H-abstraction by OH radical faster [39][40][41][42] and thereby strongly affects the radical pool and overall reactivity at low temperatures. We showed in our previous work [28] that replacing toluene by cyclopentane in a TPRF blend leads to a noticeable increase in IDTs at low and intermediate temperatures.…”