Temperature Dependence of Aqueous-Phase Decomposition of α-Hydroxyalkyl-Hydroperoxides

Abstract

Ozonolysis of unsaturated organic species with water produces α-hydroxyalkylhydroperoxides (α-HHs), which are reactive intermediates that lead to the formation of H 2 O 2 and multifunctionalized species in atmospheric condensed phases. Here, we report temperaturedependent rate coefficients (k) for the aqueous-phase decomposition of α-terpineol α-HHs at 283−318 K and terpinen-4-ol α-HHs at 313−328 K. The temporal profiles of α-HH signals, detected as chloride adducts by negative-ion electrospray mass spectrometry, showed singleexponential decay, and the derived first-order k for α-HH decomposition increased as temperature increased, e.g., k(2882) × 10 −3 s −1 for α-terpineol α-HHs at pH 6.1. Arrhenius plot analysis yielded activation energies of 17.9 ± 0.7 (pH 6.1) and 17.1 ± 0.2 kcal mol −1 (pH 6.2) for the decomposition of α-terpineol and terpinen-4-ol α-HHs, respectively. Activation energies of 18.6 ± 0.2 and 19.2 ± 0.5 kcal mol −1 were also obtained for the decomposition of α-terpineol α-HHs in acidified water at pH 5.3 and 4.5, respectively. Theoretical kinetic and thermodynamic calculations confirmed that both water-catalyzed and proton-catalyzed mechanisms play important roles in the decomposition of these α-HHs. The relatively strong temperature dependence of k suggests that the lifetime of these α-HHs in aqueous phases (e.g., aqueous aerosols, fog, cloud droplets, wet films) is controlled not only by the water content and pH but also by the temperature of these media.

Fates of Organic Hydroperoxides in Atmospheric Condensed Phases

“…The formed aldehyde species are hydrated into geminal diols or isomerize into functionalized lactols in acidic aqueous media, consistent with experimental results: 56,58 Next, we measured the rate constant for the decomposition of α-HHs derived from ozonolysis of α-terpineol or terpinene-4-ol in water at different temperatures. 58 Arrhenius plot analysis yielded activation energies (E a ) of 17.9 ± 0.7 (pH 6.1, Figure 7) and 17.1 ± 0.2 kcal mol −1 (pH 6.2) for the decomposition of αterpineol and terpinen-4-ol α-HHs, respectively. 58 The uncertainty was obtained from linear-regression fitting.…”

“…We have also found that the decomposition of α-HHs occurs over the course of minutes to hours, with lifetimes (τ 1/e ) that are dramatically affected by the water content, 57,59 pH, 56 and temperature 58 of the reaction solution, as well as the structure of the α-HH. 57,59 For example, α-HHs derived from ozonolysis of α-pinene with water were found to decay with τ 1/e = 62 min in a 20:80 (vol:vol) water:acetonitrile solution, but with τ 1/e = 14 min in a 60:40 solution.…”

Decomposition mechanism of α-alkoxyalkyl-hydroperoxides in the liquid phase: temperature dependent kinetics and theoretical calculations

“…The present calculations also confirmed that H 2 O 2 could be an important byproduct of the decomposition of α-AHs in the aqueous phase, consistent with the previous theoretical result for α-Tp α-HHs. 15 A previous iodometry analysis of a mixture of 1 mM α-Tp + 0.2 mM NaCl + 0.06 mM O 3 + 0.1 mM HCl revealed that approximately 70% of O 3 was converted into H 2 O 2 and other peroxides in water. 38 Another experimental study on the aqueous-phase decomposition of α-acyloxyalkyl-hydroperoxides derived from α-pinene CIs with carboxylic acids found that H 2 O 2 is a key decomposition product.…”

“…The present calculations also confirmed that H 2 O 2 could be an important byproduct of the decomposition of -AHs in the aqueous phase, consistent with the previous theoretical result for -Tp -HHs. 15 A previous iodometry analysis of a mixture of 1 mM -Tp + 0.…”

“…16 a-AHs possess -OOH and -OR 0 groups attached to the same carbon-atom. Given that atmospheric mixing ratios of short-chain alcohols exceed a few parts per billion by volume (ppbv) at forest sites, 12,15,16 CIs could be partly converted by alcohols into a-AHs under specic conditions (e.g., at low relative humidity). In addition to the direct source, alcohols are formed during the oxidation of VOCs, such as incorporation of OH into C]C double bonds.…”