以紫外光吸收光譜法測量小型克里奇中間體的反應動力學

Abstract

臭氧化反應是消耗大氣中不飽和碳氫化合物的主要反應之一。克里奇中間體 (羰基氧化物) 會在臭氧化反應中生成，進一步可能產生氫氧自由基或與大氣中其它分子反應。我們團隊利用克里奇中間體在紫外光波段的強烈吸收，對最簡單的克里奇中間體CH2OO和二甲基取代克里奇中間體 (CH3)2COO做動力學探討；包含(CH3)2COO的單分子分解反應和兩者對水蒸氣、C2H4和(CH3)2C=C(CH3)2的雙分子反應。我們控制反應管中的溼度並發現CH2OO的衰減速率對[H2O]呈平方關係，指出與水二聚體之反應為主要發生的過程(k(H2O)2 = (7.4±0.6)x1012 cm3s1)，但(CH3)2COO與水蒸氣的反應慢到無法測量(kH2O < 1.5x1016 cm3s1)。另一方面，由於氫原子的穿隧效應，(CH3)2COO在一般的大氣條件下會進行分子內氫原子轉移而分解。我們測量並分析可能的副反應的反應物濃度效應並決定(CH3)2COO在298K的熱分解速率為kth,H(298K) = (361±49) s1。此穿隧效應也藉由對(CD3)2COO做相同的測量與分析而確認(kth,D(298K) < 100 s1)。我們也探討了克里奇中間體與烯類分子[C2H4、(CH3)2C=C(CH3)2]之間的反應行為。CH2OO和C2H4 的反應速率係數被測量為(6.8±0.7)x1016 cm3s1，且在50  760 Torr 之間沒有壓力效應。另外(CH3)2COO的衰減速率對C2H4的濃度呈現一奇特的現象，當C2H4的濃度大於1x1016 cm3時，(CH3)2COO的衰減速率會上升至一定值。我們目前還無法解釋這個現象。

Ozonolysis reaction is one of the main removal channels of unsaturated hydrocarbons in the atmosphere. Carbonyl oxide, also known as Criegee intermediate, is produced in ozonolysis reaction and thought to play a role in OH radical formation and react with atmospheric gases. Our group have probed CH2OO and (CH3)2COO by utilizing their strong UV absorption. The unimolecular decomposition of (CH3)2COO and their bimolecular reactions with water vapor, C2H4 and (CH3)2C=C(CH3)2 have been studied. We controlled the humidity in the reactor and found that the observed decay rate of CH2OO showed a quadratic dependence on [H2O], and thus assigned water dimer reaction to be the main pathway in the CH2OO decay, k(H2O)2 = (7.4±0.6)x1012 cm3s1, while (CH3)2COO reaction with water vapor was too slow to measure (kH2O < 1.5x1016 cm3s1). On the other hand, (CH3)2COO will isomerize and decompose under ambient conditions via fast tunneling of hydrogen atom. We measured and analyzed the concentration dependences for the reactants of possible side reactions to extract the thermal decomposition rate coefficients of (CH3)2COO to be (361±49) s1 at 298 K. The tunneling mechanism was confirmed by the small decomposition rate of (CD3)2COO, kth(298K)< 100 s1. For the reactions of Criegee intermediates with alkenes, the rate coefficient of CH2OO + C2H4 has been measured to be (6.8±0.7)x1016 cm3s1 with negligible pressure dependence (50  760 Torr). Furthermore, a strange kinetic behavior was observed in the decay rate of (CH3)2COO when adding C2H4; the (CH3)2COO decay rate increases to a saturation level for [C2H4] ≥ 1x1016 cm3. We have no good explanation for this observation yet.