Combustion Research Facility, Sandia National Laboratories, Livermore, California 94551, United States.
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States.
J Phys Chem A. 2023 Apr 13;127(14):3221-3230. doi: 10.1021/acs.jpca.3c00356. Epub 2023 Apr 4.
The concentration of formic acid in Earth's troposphere is underestimated by detailed chemical models compared to field observations. Phototautomerization of acetaldehyde to its less stable tautomer vinyl alcohol, followed by the OH-initiated oxidation of vinyl alcohol, has been proposed as a missing source of formic acid that improves the agreement between models and field measurements. Theoretical investigations of the OH + vinyl alcohol reaction in excess O conclude that OH addition to the α carbon of vinyl alcohol produces formaldehyde + formic acid + OH, whereas OH addition to the β site leads to glycoaldehyde + HO. Furthermore, these studies predict that the conformeric structure of vinyl alcohol controls the reaction pathway, with the -conformer of vinyl alcohol promoting α OH addition, whereas the -conformer promotes β addition. However, the two theoretical studies reach different conclusions regarding which set of products dominate. We studied this reaction using time-resolved multiplexed photoionization mass spectrometry to quantify the product branching fractions. Our results, supported by a detailed kinetic model, conclude that the glycoaldehyde product channel (arising mostly from -vinyl alcohol) dominates over formic acid production with a 3.6:1.0 branching ratio. This result supports the conclusion of Lei et al. that conformer-dependent hydrogen bonding at the transition state for OH-addition controls the reaction outcome. As a result, tropospheric oxidation of vinyl alcohol creates less formic acid than recently thought, increasing again the discrepancy between models and field observations of Earth's formic acid budget.
与野外观测相比，详细的化学模型低估了地球对流层中甲酸的浓度。乙醛通过光互变异构作用转化为其不太稳定的互变异构体乙烯醇，随后由 OH 引发的乙烯醇氧化，被提议为一种缺失的甲酸来源，这可以改善模型与野外测量之间的一致性。在过量 O 中进行的 OH + 乙烯醇反应的理论研究得出结论，OH 加成到乙烯醇的α 碳上生成甲醛+甲酸+OH，而 OH 加成到β 位则生成甘油醛+HO。此外，这些研究预测乙烯醇的构象结构控制着反应途径，β-乙烯醇促进α OH 加成，而α-乙烯醇则促进β 加成。然而，这两项理论研究对于哪种产物占主导地位得出了不同的结论。我们使用时间分辨多路复用光电子电离质谱法研究了这个反应，以定量产物分支分数。我们的结果，得到详细的动力学模型的支持，得出结论，糖醛产物通道（主要来自β-乙烯醇）占主导地位，而甲酸生成的分支比为 3.6:1.0。这一结果支持 Lei 等人的结论，即过渡态中构象依赖性氢键控制了 OH 加成的反应结果。因此，乙烯醇在对流层中的氧化产生的甲酸比最近认为的要少，这再次增加了模型与地球甲酸预算野外观测之间的差异。
