Rate Constants for the Reactions of CH₃O with Cyclohexane, Cyclohexene, and 1,4-Cyclohexadiene: Variable Temperature Experiments and Theoretical Comparison of Addition and H-Abstraction Channels

Abstract

First kinetic measurements for CH3O reactions have been obtained for three cyclohydrocarbons using the discharge flow reactor combined with the laser induced fluorescence technique to detect CH3O radicals over the pressure and temperature ranges 1−7 Torr of helium and 300−513 K. Measurements have been performed for the cyclohydrocarbons c-C6H12 (k1), c-C6H10 (k2), and 1,4-c-C6H8 (k3). In addition to the experimental work, we have performed ab initio molecular orbital computations to get an insight into the mechanism of the three reactions, using PMP2, HF-DFT (B3LYP), and CASPT2 methods. The rate constant k1 has been calculated using the Transition State Theory. Measured rate constants are pressure independent in our experimental range. Arrhenius expressions are (ki in cm3 s-1) k1 = 8.8 × 10-12 exp[−(24.5 ± 3.0) kJ mol-1/RT] (403−513 K), k2 = (3.1 ± 0.8) × 10-12 exp[−(15.3 ± 0.8) kJ mol-1/RT] (300−503 K), and k3 = 1.9 × 10-12 exp[−(7.6 ± 1.9) kJ mol-1/RT] (300−513 K). A good agreement between the experimental and theoretical rate constants k1 has been found. The comparison between the computed and the experimentally determined barrier heights serves as an endorsement of the increasing reactivity in the series from cyclohexane system to the 1,4-cyclohexadiene system.