A new approach for studying aqueous phase OH kinetics: application of Teflon waveguides

Abstract

Bimolecular rate coefficients for the reactions of the hydroxyl radical, OH, with methanol, ethanol, tetrahydrofuran, dimethylmalonate [CH₃OC(O)CH₂C(O)OCH₃], dimethylsuccinate [CH₃OC(O)CH₂CH₂C(O)OCH₃], dimethylcarbonate [CH₃OC(O)OCH₃] and diethylcarbonate [CH₃CH₂OC(O)OCH₂CH₃] in aqueous solutions have been measured using a novel experimental approach. The centrepiece of the new experimental technique reported in this work is a Teflon AF 2400 liquid core waveguide. The physical properties of the Teflon AF 2400 liquid core waveguide allow for the construction of a micro-flowtube reaction photolysis cell with an extremely low volume and a very long optical pathway. Such a reaction system allows for a very sensitive detection of chemical transients in the aqueous phase. The micro-flowtube experiments involved competition kinetics of the OH radical with the organic reactant of interest and an SCN⁻ anion, (k_OH+SCN⁻ = 1.29 × 10¹⁰ M⁻¹ s⁻¹). The (SCN)⁻₂ anion was detected using UV-visible spectroscopy following a medium pressure mercury lamp photolysis (λ ≥ 366 nm) of H₂O/H₂O₂/reactant/KSCN mixtures. All experiments were carried out at room temperature. Measured rate coefficients for the reaction of the OH radical with methanol, ethanol, tetrahydrofuran, dimethylmalonate, dimethylsuccinate, dimethylcarbonate and diethylcarbonate are (units are 10⁸ M⁻¹ s⁻¹): k_OH+methanol = 13 ± 4, k_OH+ethanol = 19 ± 5, k_OH+THF = 38 ± 10, k_{OH+dimethylmalonate} = 2.7 ± 0.9, k_{OH+dimethylsuccinate} = 5.3 ± 2.9, k_{OH+dimethylcarbonate} = 0.51 ± 0.22, k_{OH+diethylcarbonate} = 7.9 ± 3.2. Uncertainties in the above expressions are ±2σ and represent precision only. The reported rate coefficients for the reactions of OH with ethanol, methanol and THF agree very well with the currently recommended values. To date, there is no kinetic data reported in the literature for the OH radical reaction with dimethylmalonate, dimethylsuccinate, dimethylcarbonate and diethylcarbonate. The reaction mechanism is briefly discussed as a function of bond energies.