以紫外線吸收光譜法測量甲基乙烯基酮氧化物(一種共振穩定的克里奇中間體)的反應動力學

Abstract

由烯類的臭氧化反應產生的克里奇中間體是大氣中很活潑的氧化劑，會和二氧化硫、二氧化氮、水蒸氣、有機酸、無機酸等氣體分子反應，或單分子分解產生氫氧自由基，進而影響大氣的氧化能力及酸雨、氣膠等之形成。 異戊二烯是大氣中含量最豐富的烯類，臭氧化異戊二烯將產生最簡單的克里奇中間體(CH2OO)、甲基乙烯基酮氧化物(CH3(C2H3)COO，簡稱MVKO)及甲基丙烯醛氧化物(H2C=C(CH3)CHOO)。MVKO及H2C=C(CH3)CHOO為共振穩定的克里奇中間體，目前直接觀測此種克里奇中間體的研究並不多，我們預期共振穩定結構將影響此種克里奇中間體的反應動力學行為。 我們實驗室使用Barber et al. (J. Am. Chem. Soc., 2018, 140, 10866)所發表的方法來產生MVKO (ICH2CHCICH3 + h→ CH3(C2H3)CI + I，CH3(C2H3)CI + O2 → CH3(C2H3)COO + I)，並根據Vansco et al. (J. Chem. Phys.,2018, 149, 244309) 及Caravan et al.(包括我們實驗室) (Proc. Natl. Acad. Sci. U.S.A., 2020, 117, 9733-9740) 報導的MVKO紫外光吸收光譜，藉著測量MVKO對340 nm紫外光吸收度隨時間的變化，來觀察其反應動力學。我們發現在高壓(>100 Torr)下，MVKO有個較慢的生成過程(約1-2 ms)，我們推測此過程可能是加成物的分解反應(CH3(C2H3)CIOO → CH3(C2H3)COO + I)，據此提出的動力學模型可描述MVKO濃度隨時間的變化。實驗測得加成物分解反應具有正溫度效應，活化能為(14.0±0.9) kcal mol1；此值與我們實驗室 (Lin et al., Phys. Chem. Chem. Phys., 2020, 22, 13603) 報導的理論計算的加成物分解所需能量14.0 kcal mol1相近。加成物較MVKO穩定，但推測由於加成物不像MVKO具有共振穩定結構，加成物與MVKO的能量差僅有14.0 kcal mol1，因此我們才能在微秒尺度觀測到加成物分解為MVKO的反應。 我們測量MVKO跟二氧化硫的反應，求得反應速率常數在298 K為(4.0±0.6) x 1011 cm3 s1，且在4-703 Torr之間沒有明顯的壓力效應，活化能為(3.1±0.8) kcal mol1，負的活化能代表此反應的過渡態能量低於反應物。我們也量測甲基乙烯基酮氧化物的單分子分解反應，求得反應速率常數在299 K為(71±18) s1，且在100-503 Torr沒有明顯的壓力效應，活化能為(8.3±2.5) kcal mol1。跟小型克里奇中間體比較，MVKO在299 K的單分子反應速率常數比最簡單的克里奇中間體(CH2OO, (0.19±0.07) s1 at 297±1 K)快，但比syn-甲基取代克里奇中間體(syn-CH3CHOO, 122 s1)跟雙甲基取代的克里奇中間體((CH3)2COO, 361 s1)慢(Chem. Soc. Rev., 2017, 46, 7483-7497.)。依據實驗結果，我們推測MVKO在大氣中的主要分解途徑為單分子分解反應。

Criegee intermediates, known as carbonyl oxides, are very reactive species generated from ozonolysis of alkenes. Criegee intermediates can react with atmospheric gases, such as SO2, NO2, H2O, organic acid, inorganic acid, volatile organic compounds (VOCs), etc. and the unimolecular decomposition of Criegee intermediates generate OH radicals. These processes play important roles in atmospheric chemistry. Isoprene is the most abundant alkene in the atmosphere. Ozonolysis of isoprene generates the simplest Criegee intermediate (CH2OO), methyl vinyl ketone oxide (CH3(C2H3)COO, MVKO), and methacrolein oxide (H2C=C(CH3)CHOO, MACRO). Our group used the method reported by Barber et al. (J. Am. Chem. Soc., 2018, 140, 10866) to generate MVKO (ICH2CHCICH3 + h→ CH3(C2H3)CI + I，CH3(C2H3)CI + O2 → CH3(C2H3)COO + I). Utilizing the UV absorption spectrum reported by Vansco et al. (J. Chem. Phys.,2018, 149, 244309) and Caravan et al.(including our group) (Proc. Natl. Acad. Sci. U.S.A. 2020, 117, 9733-9740), we studied the kinetics of MVKO with its UV absorption at 340 nm. We found that at high pressure (>100 Torr), there ia a slow process of MVKO formation (1-2 ms). We assume that this slow formation of MVKO is from the decomposition of the adduct (CH3(C2H3)CIOO → CH3(C2H3)COO + I). Based on this adduct mechanism, our kinetic model agrees very well with the experimental observations of MVKO kinetics. The activation energy of the decomposition of the adduct was measured to be (14.0±0.9) kcal mol1, which is similar to the calculated energy difference between CH3(C2H3)CIOO and (MVKO + I) reported by our group (Lin et al., Phys. Chem. Chem. Phys., 2020, 22, 13603). The adduct is more stable than MVKO. However, because the adduct has no resonance-stabilized structure like MVKO, the energy difference between the adduct and MVKO is only 14.0 kcal mol1, and the rate coefficient of the formation of MVKO from the adduct can reach ~103 s1 at room temperature. We studied the kinetics of the reaction of MVKO with SO2; the observed reaction rate coefficient is (4.0±0.6) x 1011 cm3 s1 at 298 K with an insignificant pressure effect in range 4-703 Torr. The activation energy was measured to be (3.1±0.8) kcal mol1. A negative activation energy means that the transition state energy is lower than that of the reactants. We also studied the unimolecular decomposition of MVKO, and the rate coefficient was determined to be (71±18) s1 at 299 K with an insignificant pressure effect for 100-503 Torr. The measured activation energy is (8.3±2.5) kcal mol1 at 302 Torr and 278-319 K. Comparing with small Criegee intermediates, the measured unimolecular decomposition rate coefficient of MVKO at 299 K is larger than that of CH2OO ((0.19±0.07) s1 at 297±1 K), and smaller than those of syn-CH3CHOO (122 s1) and (CH3)2COO (361 s1). According to the experimental results, we conclude that the dominant removal pathway for MVKO in the atmosphere is its unimolecular decomposition.