The ground-state rearrangement and decomposition of methyl vinyl ketone (MVK) and methacrolein (MACR) has been investigated using quantum chemical calculations and RRKM theory/master equation simulations. MVK and MACR absorb actinic radiation at around 380–280 nm, and we have identified a number of isomerization pathways with barriers that are accessible from the longer wavelength end of this range (visible/near-UV). Assuming that radiationless transitions dominate, master equation simulations of the reactions on the vibrationally excited ground-state potential-energy surface predict that isomerization to 2-hydroxybutadiene and 1-hydroxymethylallene from MVK, and isomerization to dimethylketene from MACR, are the major tropospheric reaction channels. Despite these processes having low quantum yields, they are prevalent because of the coincidence of high absorption cross sections with significant solar photon fluxes at around 320–330 nm, where photodissociation does not occur. This work suggests that photoisomerization may be an important process in the photolysis of these compounds in the troposphere, particularly for MVK, which, in comparison with MACR, has both a shorter lifetime with respect to photolysis and a longer lifetime with respect to reaction with the ^•OH radical.

中文翻译：

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对流层中甲基乙烯基酮和甲基丙烯醛的光异构化：基态反应途径的理论研究

使用量子化学计算和RRKM理论/主方程模拟研究了甲基乙烯基酮（MVK）和甲基丙烯醛（MACR）的基态重排和分解。MVK和MACR吸收约380-280 nm的光化辐射，并且我们已经确定了许多异构化途径，这些屏障具有可从该波长范围的较长波长端（可见/近紫外）进入的障碍。假设无辐射跃迁占主导地位，振动激发的基态势能表面反应的主方程模拟预测，MVK异构化为2-羟基丁二烯和1-羟甲基丙二烯，MACR异构化为二甲基烯酮是对流层反应的主要通道。尽管这些过程的量子产率很低，它们之所以盛行是因为高吸收截面与320-330 nm左右的明显太阳光子通量一致，而不会发生光解离。这项工作表明，光异构化可能是对流层中这些化合物光解的重要过程，特别是对于MVK，与MACR相比，MVK的光解寿命较短，而与MACR的反应寿命较长。^• OH自由基。