Kinetics for the photo-chemical degradation of Methyl butyrate in presence of Cl atoms and OH radicals

Abstract

The Cl/OH initiated temperature dependent photo-oxidative reaction kinetics of methyl butyrate (MB) were examined using a relative rate (RR) technique. Gas chromatography with flame ionization and mass spectrometric detection were used to monitor the concentration of the reactants and to identify the products. The temperature dependent kinetics of MB with Cl atoms were measured with respect to the reaction of Cl with C₂H ₆ and C₂H₄. The temperature dependent kinetics for the reaction of MB with OH radicals were measured using n- propanol and iso -propanol as references. The obtained rate coefficients for the Cl and OH reactions with MB are, k Cl(Expt) (T) = [(7.76 ± 0.47) × 10 ⁻¹¹] exp [(10.31 ± 0.20)/T] cm³ molecule⁻¹ s⁻¹ and k OH(Expt) (T) = [(4.32 ± 0.21) × 10 ⁻¹²] exp [-(25.26 ± 0.39)/T] cm³ molecule⁻¹ s⁻¹ respectively. Dual level direct dynamics were used to perform the computational calculations to further elucidate the mechanisms over the studied temperature range. The rate coefficients for H-abstraction reactions were computed using Canonical Variational Transition State Theory with Small Curvature Tunneling (CVT/SCT) with Interpolated Single Point Energies (ISPE) method. The rate coefficients over the studied temperature range yielded the Arrhenius equations: k Cl(Theory) (200–400 K) = [(4.05 ± 0.54) × 10^–11] exp [-(2.80 ± 0.11)/T] cm³ molecule⁻¹ s⁻¹ and k OH(Theory) (200–400 K) = [(1.96 ± 0.68) × 10 -11] exp [-(384 ± 38)/T] cm³ molecule ⁻¹ s ⁻¹. Possible degradation mechanisms for the reactions are proposed based on the observed products. Thermo-chemical parameters, ozone formation potential, branching ratios, and the atmospheric lifetime of MB are calculated to understand the fate of MB in the atmosphere.