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.甲氧基酚（Methoxyphenols）是经生物质燃烧排放的大气化学关键物种。本研究采用激光闪光光解（laser flash photolysis）结合密度泛函理论（density functional theory），系统探究了6种甲氧基酚与2种相关酚类的水相OH自由基反应的温度依赖性。实验测得的速率常数范围为(1.1–1.9) × 10^10 L·mol⁻¹·s⁻¹，速率常数大小排序为：k(3-MC) > k(Cre) ≈ k(Syr) ≈ k(MEP) > k(Res) > k(3-MP) > k(2-EP) ≈ k(2-MP)。研究团队基于获得的温度依赖速率常数推导了该类反应的动力学参数，得到平均阿伦尼乌斯活化能（Arrhenius activation energy）为16.9 kJ·mol⁻¹。针对每个反应体系计算了扩散速率常数，并与实验实测值进行对比。整体来看，所有反应的速率常数均接近完全扩散控制（所有反应的kdiff = (1.4–1.5) × 10^10 L·mol⁻¹·s⁻¹）。本研究通过实验结果建立了构效关系（structure–function relationship），可用于预测其他酚类化合物的未知速率常数。上述研究成果有望提升包括水相自由基化学机理在内的各类大气化学模型的预测能力。