Liu Tianlin, Lester Marsha I
Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, United States.
J Phys Chem A. 2023 Dec 28;127(51):10817-10827. doi: 10.1021/acs.jpca.3c05859. Epub 2023 Dec 18.
Alkene ozonolysis generates transient carbonyl oxide species, known as Criegee intermediates, which are a significant nonphotolytic source of OH radicals in the troposphere. This study demonstrates that unimolecular decay of -methyl-substituted Criegee intermediates proceeds via 1,4 H atom transfer to vinyl hydroperoxides, resulting in OH fission to O-O products or, alternatively, OH roaming to hydroxycarbonyl products. Newly generated Criegee intermediates are shown to yield hydroxycarbonyls with sufficient internal excitation to dissociate via C-C fission to acyl and hydroxymethyl (CHOH) radicals. The stabilized Criegee intermediates and unimolecular products are rapidly cooled in a pulsed supersonic expansion for photoionization detection with time-of-flight mass spectrometry. CHOH products are identified by 2 + 1 resonance-enhanced multiphoton ionization via the 3p Rydberg state upon unimolecular decay of CHCHOO, (CH)COO, (CH)(CHCH)COO, and (CH)(CH═CH)COO (methyl vinyl ketone oxide). The stabilized Criegee intermediates are separately detected using 10.5 eV photoionization. This study provides the first experimental evidence of roaming in the unimolecular decay of isoprene-derived methyl vinyl ketone oxide and extends earlier studies that reported stabilized hydroxycarbonyl products.
烯烃臭氧化反应会生成瞬态羰基氧化物物种，即所谓的克里吉中间体，它们是对流层中羟基自由基的一个重要非光解来源。本研究表明，甲基取代的克里吉中间体的单分子衰变通过1,4氢原子转移至乙烯基氢过氧化物进行，导致羟基分裂为氧 - 氧产物，或者，羟基漫游至羟基羰基产物。新生成的克里吉中间体被证明会产生具有足够内部激发能的羟基羰基化合物，从而通过碳 - 碳裂变分解为酰基和羟甲基（CHOH）自由基。稳定的克里吉中间体和单分子产物在脉冲超声速膨胀中迅速冷却，以便用飞行时间质谱进行光电离检测。通过CHCHOO、(CH)COO、(CH)(CHCH)COO和(CH)(CH═CH)COO（甲基乙烯基酮氧化物）单分子衰变时经由3p里德堡态的2 + 1共振增强多光子电离来鉴定CHOH产物。使用10.5 eV光电离分别检测稳定的克里吉中间体。本研究提供了异戊二烯衍生的甲基乙烯基酮氧化物单分子衰变中漫游的首个实验证据，并扩展了早期报道稳定羟基羰基产物的研究。
