Kinetic and Theoretical Study of the Atmospheric Aqueous-Phase Reactions of OH Radicals with Methoxyphenolic Compounds

Methoxyphenols,
which are emitted through biomass burning, are
an important species in atmospheric chemistry. In the present study,
temperature-dependent aqueous-phase OH radical reactions of six methoxyphenols
and two related phenols have been investigated through laser flash
photolysis and the density functional theory. The rate constants obtained
were in a range of (1.1–1.9) × 1010 L mol–1 s–1 with k(3-MC)
> k(Cre) ≈ k(Syr) ≈ k(MEP) > k(Res) > k(3-MP)
> k(2-EP) ≈ k(2-MP). We
derived
the parameters of these reactions from the obtained T-dependent rate constants and found a mean Arrhenius activation energy
of 16.9 kJ mol–1. The diffusion rate constants were
calculated for each case and compared to the measured ones. Generally,
the rate constants are found to be close to fully diffusion-controlled
(kdiff = (1.4–1.5) × 1010 L mol–1 s–1 for all
reactions). A structure–function relationship was established
through the measurement result, which could be used for predicting
unknown rate constants of other phenolic compounds. All of these findings
are expected to enhance the predictive capabilities of models, such
as the chemical aqueous-phase radical mechanism.