Kinetics of the Reactions of n-Alkyl (C₂H₅, n-C₃H₇, and n-C₄H₉) Radicals with CH₃

The ethyl-methyl, n -propyl-methyl, and n -butyl-methyl cross-radical reactions were studied by laser photolysis/photoionization mass spectroscopy. Overall rate constants were obtained in direct real-time experiments in the temperature region 297−800 K and bath gas (helium) density (3−36) × 10 16 atom cm -3 . The observed overall C 2 H 5 + CH 3 (1), n -C 3 H 7 + CH 3 (2), and n -C 4 H 9 + CH 3 (3) rate constants demonstrate negative temperature dependences. Master equation modeling of collisional effects indicates that the n -C 3 H 7 + CH 3 and the n -C 4 H 9 + CH 3 reactions are at the high-pressure limit under all experimental conditions used. The C 2 H 5 + CH 3 reaction is not at the high-pressure limit and falloff in reaction 1 cannot be neglected at 800 K. Falloff corrections applied to reaction 1, on average, reach 45% at 800 K and introduce noticeable uncertainties in the extrapolated high-pressure-limit rate constant values. The following expressions for the high-pressure-limit rate constants of reaction 1−3 were obtained: k 1 ∞ = 2.36 × 10 -11 exp(433 K/ T ), k 2 ∞ = 3.06 × 10 -11 exp(387 K/ T ), and k 3 ∞ = 2.28 × 10 -11 exp(473 K/ T ) cm 3 molecule -1 s -1 . C 3 H 8 was detected as a product of reaction 1 and C 5 H 12 and C 4 H 8 were detected as products of reaction 3.