A novel long path photolysis cell—application to the reactivity of selected organic compounds toward the nitrate radical (NO3)

Abstract

Bimolecular rate coefficients for the reactions of the nitrate radical, NO₃, with methanol, ethanol, acetaldehyde tert-butyl methyl ether, propionic acid, dimethylmalonate, dimethylsuccinate, dimethyl carbonate and diethylcarbonate in aqueous solutions have been measured using a novel experimental approach. This study was performed using laser flash photolysis (LFP) with a capillary made of Teflon AF2400 as a long path capillary photolysis cell. Taking benefit of this new material allowing a long optical path length for a very limited irradiated solution volume, new rate constants were established. All experiments were carried out at room temperature. Measured rate coefficients for the reaction of the NO₃ radical with methanol, ethanol, acetaldehyde, dimethylcarbonate, diethylcarbonate, dimethylmalonate, dimethyl succinate, propionic acid and tert-butyl-methyl ether are (units are 10⁵ M⁻¹ s⁻¹): k_{NO3 + ethanol} = 4.8 ± 0.5, k_{NO3 + ethanol} = 19 ± 3, k_{NO3 + acetaldehyde} = 20 ± 3, k_{NO3 + dimethylcarbonate} = 0.15 ± 0.04, k_{NO3 + diethylcarbonate} = 0.84 ± 0.12, k_{NO3 + dimethylmalonate} = 0.26 ± 0.07, k_{NO3 + dimethylsuccinate} = 0.34 ± 0.02, k_{NO3 + propionic acid} = 0.77 ± 0.02, k_{NO3 + tert-butyl-methyl ether} = 3.9 ± 1.3. The uncertainties in the above expressions are ±2σ and represent precision only. The reported rate coefficients for the reactions of NO₃ with methanol, ethanol and acetaldehyde agree well with currently recommended values. To date, there is no kinetic data reported in the literature for the NO₃ radical reaction with dimethylcarbonate, diethylcarbonate, dimethylmalonate, dimethylsuccinate, propionic acid and tert-butyl-methyl ether. The reaction mechanism is briefly discussed as a function of bond energies