Temperature-dependent rate constants for hydroxyl radical reactions with organic compounds in aqueous solutions

Abstract

The OH radical is the most important oxidant in both the tropospheric gas and aqueous phase. Its main sink processes in clouds appear to be reactions with organics but due to the lack of appropriate kinetic data current cloud chemistry models consider only reactions with C₁ and C₂ compounds. Therefore, in this study temperature dependent rate constants for the reactions of the OH radical with organic compounds (≥C₂) were determined. These investigations were performed by competition kinetics (reference substance: SCN⁻). Initially the experimental system was checked reinvestigating kinetic data for OH reactions with formate (R-1) and tert-butanol (R-2) available from literature. For the reactions (R-1) and (R-2) the following results were obtained: k₁(298 K) = (2.4 ± 0.4) × 10⁹ M⁻¹ s⁻¹; A₁ = (7.9 ± 0.7) × 10¹⁰ M⁻¹ s⁻¹; E_A,1 = (9 ± 5) kJ mol⁻¹ and k₂(298 K) = (5.0 ± 0.6) × 10⁸ M⁻¹ s⁻¹; A₂ = (3.3 ± 0.1) × 10¹⁰ M⁻¹ s⁻¹; E_A,2 = (10 ± 3) kJ mol⁻¹ for formate and tert-butanol, respectively. Temperature dependent rate constants for the reactions of OH with ethanol (k₃(298 K) = (2.1 ± 0.1) × 10⁹ M⁻¹ s⁻¹; A₃ = (1.0 ± 0.1) × 10¹¹ M⁻¹ s⁻¹; E_A,3 = (10 ± 5) kJ mol⁻¹), 1-propanol (k₄(298 K) = (3.2 ± 0.2) × 10⁹ M⁻¹ s⁻¹; A₄ = (5.6 ± 0.6) × 10¹⁰ M⁻¹ s⁻¹; E_A,4 = (8 ± 6) kJ mol⁻¹), acetone (k₅(298 K) = (2.1 ± 0.6) × 10⁸ M⁻¹ s⁻¹; A₅ = (3.4 ± 0.4) × 10¹¹ M⁻¹ s⁻¹; E_A,5 = (18 ± 11) kJ mol⁻¹) and methylglyoxal (k₆(298 K) = (1.1 ± 0.1) × 10⁹ M⁻¹ s⁻¹; A₆ = (2.9 ± 0.3) × 10¹¹ M⁻¹ s⁻¹; E_A,6 = (13 ± 6) kJ mol⁻¹), Propionic acid: k₇(298 K) = (3.2 ± 0.5) × 10⁸ M⁻¹ s⁻¹; A₇ = (7.6 ± 0.9) × 10¹¹ M⁻¹ s⁻¹; E_A,7 = (19 ± 8) kJ mol⁻¹; propionate: k₈(298 K) = (7.2 ± 0.4) × 10⁸ M⁻¹ s⁻¹; A₈ = (3.2 ± 0.2) × 10¹¹ M⁻¹ s⁻¹; E_A,8 = (15 ± 4) kJ mol⁻¹; glyoxylic acid: k₉(298 K) = (3.6 ± 0.2) × 10⁸ M⁻¹ s⁻¹, A₉ = (8.1 ± 0.4) × 10⁹ M⁻¹ s⁻¹, E_A,9 = (8 ± 3) kJ mol⁻¹, glyoxylate: k₁₀(298 K) = (2.6 ± 0.9) × 10⁹ M⁻¹ s⁻¹, A₁₀ = (6.0 ± 0.4) × 10¹⁵ M⁻¹ s⁻¹; E_A,10 = (36 ± 8) kJ mol⁻¹; pyruvic acid k₁₁(298 K) = (1.2 ± 0.4) × 10⁸ M⁻¹ s⁻¹; A₁₁ = (1.0 ± 0.1) × 10¹² M⁻¹ s⁻¹; E_A,11 = (23 ± 4) kJ mol⁻¹; pyruvate: k₁₂(298 K) = (7 ± 2) × 10⁸ M⁻¹ s⁻¹; A₁₂ = (1.3 ± 0.1) × 10¹² M⁻¹ s⁻¹; E_A,12 = (19 ± 4) kJ mol⁻¹; oxalate (monoanion): k₁₃(298 K) = (1.9 ± 0.6) × 10⁸ M⁻¹ s⁻¹; A₁₃ = (2.5 ± 0.1) × 10¹² M⁻¹ s⁻¹; E_A,13 = (23 ± 4) kJ mol⁻¹; oxalate (dianion): k₁₄(298 K) = (1.6 ± 0.6) × 10⁸ M⁻¹ s⁻¹, A₁₄ = (4.6 ± 0.5) × 10¹⁴ M⁻¹ s⁻¹; E_A,14 = (36 ± 10) kJ mol⁻¹; malonate (monoanion): k₁₅(298 K) = (6 ± 1) × 10⁷ M⁻¹ s⁻¹, A₁₅ = (3.2 ± 0.4) × 10⁹ M⁻¹ s⁻¹; E_A,15 = (11 ± 5) kJ mol⁻¹; succinic acid k₁₈(298 K) = (1.1 ± 0.1) × 10⁸ M⁻¹ s⁻¹, A₁₈ = (8 ± 1) × 10⁹ M⁻¹ s⁻¹; E_A,18 = (11 ± 6) kJ mol⁻¹ and succinate (dianion): k₁₉(298 K) = (5.0 ± 0.5) × 10⁸ M⁻¹ s⁻¹, A₁₉ = (5.0 ± 0.4) × 10¹⁰ M⁻¹ s⁻¹; E_A,19 = (11 ± 5) kJ mol⁻¹ were determined.