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

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 (T₀₀₃, T₀₁₂, T₀₁₃, T₀₂₂, T₀₂₃, T₁₁₁, T₁₁₂, T₁₁₃, and T₁₂₂) 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): T₀₀₃ = 1.80 × 10^-10 exp(-2971 K/T) cm³ molecule^-1 s^-1; T₀₁₂ = 9.36 × 10^-11 exp(-3024 K/T) cm³ molecule^-1 s^-1; T₀₁₃ = 4.40 × 10^-10 exp(-4162 K/T) cm³ molecule^-1 s^-1; T₀₂₂ = 1.47 × 10^-10 exp(-3587 K/T) cm³ molecule^-1 s^-1; T₀₂₃ = 6.06 × 10^-11 exp(-3010 K/T) cm³ molecule^-1 s^-1; T₁₁₁ = 3.98 × 10^-11 exp(-1617 K/T) cm³ molecule^-1 s^-1; T₁₁₂ = 9.08 × 10^-12 exp(-3661 K/T) cm³ molecule^-1 s^-1; T₁₁₃ = 6.74 × 10^-9 exp(-7547 K/T) cm³ molecule^-1 s^-1; T₁₂₂ = 3.47 × 10^-11 exp(-1802 K/T) cm³ molecule^-1 s^-1.