Experimental and Theoretical Study of the Kinetics of the OH + Methyl Ethyl Ketone Reaction as a Function of Temperature

The kinetics of the reaction of OH radicals with methyl ethyl ketone (MEK) (2-butanone), methyl ethyl ketone-d₃, and methyl ethyl ketone-d₅ were measured over the temperature range of 273-373 K between 4 and 5 Torr using discharge-flow techniques coupled with laser-induced fluorescence or resonance fluorescence (RF) detection of OH. The average room temperature (298 K) rate constant for the OH + methyl ethyl ketone reaction was found to be (11.53 ± 0.35) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹, in reasonable agreement with previous studies at higher pressures. In contrast to some previous measurements that displayed non-Arrhenius behavior at low temperatures, measurements of the temperature dependence of the reaction between 273 and 373 K followed Arrhenius behavior characterized by k = (6.53 ± 1.24) × 10⁻¹² exp(-(523 ± 63)/T) cm³ molecule⁻¹ s⁻¹. Ab initio calculations of the potential energy surface for this reaction were performed at the CCSDT//M06-2X/6-311+G(2dp,2f) level of theory, indicating a reaction mechanism that is dominated by hydrogen abstraction of the secondary α-carbon and the primary β-carbon, consistent with the measured kinetic isotope effect (KIE). Hydrogen-bonded complexes were found for multiple pathways, resulting in a multistep reaction mechanism. High-pressure limiting (HPL) kinetic calculations were performed using transition state (TS) theory along with low-pressure limiting (LPL) calculations using RRKM theory to evaluate the impact of the prereactive complexes (PRC) on the reaction kinetics. The experimental and theoretical results suggest that this reaction may exhibit a pressure dependence below room temperature.