Division of Geological and Planetary Sciences and ‡Division of Engineering and Applied Science, California Institute of Technology , Pasadena, California 91125, United States.
J Am Chem Soc. 2017 Apr 19;139(15):5367-5377. doi: 10.1021/jacs.6b12838. Epub 2017 Apr 11.
Approximately 500 Tg of 2-methyl-1,3-butadiene (isoprene) is emitted by deciduous trees each year. Isoprene oxidation in the atmosphere is initiated primarily by addition of hydroxyl radicals (OH) to C or C in a ratio 0.57 ± 0.03 (1σ) to produce two sets of distinct allylic radicals. Oxygen (O) adds to these allylic radicals either δ (Z or E depending on whether the allylic radical is cis or trans) or β to the OH group forming six distinct peroxy radical isomers. Due to the enhanced stability of the allylic radical, however, these peroxy radicals lose O in competition with bimolecular reactions. In addition, the Z-δ hydroxy peroxy radical isomers undergo unimolecular 1,6 H-shift isomerization. Here, we use isomer-resolved measurements of the reaction products of the peroxy radicals to diagnose this complex chemistry. We find that the ratio of δ to β hydroxy peroxy radicals depends on their bimolecular lifetime (τ). At τ ≈ 0.1 s, a transition occurs from a kinetically to a largely thermodynamically controlled distribution at 297 K. Thus, in nature, where τ > 10 s, the distribution of isoprene hydroxy peroxy radicals will be controlled primarily by the difference in the relative stability of the peroxy radical isomers. In this regime, β hydroxy peroxy radical isomers comprise ∼95% of the radical pool, a much higher fraction than in the nascent (kinetic) distribution. Intramolecular 1,6 H-shift isomerization of the Z-δ hydroxy peroxy radical isomers produced from OH addition to C is estimated to be ∼4 s at 297 K. While the Z-δ isomer is initially produced in low yield, it is continually reformed via decomposition of the β hydroxy peroxy radicals. As a result, unimolecular chemistry from this isomer contributes about half of the atmospheric fate of the entire pool of peroxy radicals formed via addition of OH at C for typical atmospheric conditions (τ = 100 s and T = 25 C). In contrast, unimolecular chemistry following OH addition at C is slower and less important.
每年落叶树会排放约 500 太克的 2-甲基-1,3-丁二烯（异戊二烯）。大气中异戊二烯的氧化主要是通过羟基自由基（OH）加成到 C 或 C 上引发的，加成比为 0.57±0.03（1σ），生成两组不同的烯丙基自由基。氧（O）要么加成到这些烯丙基自由基的 δ（Z 或 E，取决于烯丙基自由基是顺式还是反式），要么加成到 OH 基团的 β 位，形成六种不同的过氧自由基异构体。然而，由于烯丙基自由基的稳定性增强，这些过氧自由基会与双分子反应竞争失去 O。此外，Z-δ 羟基过氧自由基异构体经历单分子 1,6 H 迁移异构化。在这里，我们使用过氧自由基反应产物的异构体分辨测量来诊断这种复杂的化学。我们发现，δ 与 β 羟基过氧自由基的比例取决于它们的双分子寿命（τ）。在 τ≈0.1 s 时，在 297 K 下，从动力学控制到热力学控制的分布会发生转变。因此，在自然界中，τ>10 s 时，异戊二烯羟基过氧自由基的分布将主要由过氧自由基异构体的相对稳定性差异控制。在这个区域，β 羟基过氧自由基异构体占自由基池的约 95%，比初始（动力学）分布高得多。在 297 K 下，OH 加成到 C 生成的 Z-δ 羟基过氧自由基异构体的分子内 1,6 H 迁移异构化估计约为 4 s。虽然 Z-δ 异构体最初的生成产率较低，但它会通过 β 羟基过氧自由基的分解不断重新生成。因此，在典型的大气条件下（τ=100 s，T=25°C），通过 OH 加成到 C 形成的整个过氧自由基池中，该异构体的单分子化学贡献约为一半。相比之下，OH 加成到 C 后的单分子化学较慢且不太重要。
