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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Enhanced Aqueous Photochemical Reaction Rates after Freezing
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Enhanced Aqueous Photochemical Reaction Rates after Freezing

机译：冻结后提高的水性光化学反应速率

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摘要

Sunlit snow/ice is known to play an important role in the processing of atmospheric species,including photochemical production of NO_x,HONO,molecular halogens,alkyl halides,and carbonyl compounds,among others.It has been shown that a liquid-like(quasi-liquid or disordered)layer exists on the surface of pure ice and that this quasi-liquid layer is also found on the surface of ambient snow crystals and ice at temperatures similar to polar conditions.However,it is unclear what role the liquid-like fractions present in and on frozen water play in potential photochemical reactions,particularly with regard to organic substrates.Here,we report a detailed study of enhanced rates of photochemical nucleophilic substitution of p-nitroanisole(PNA)with pyridine,a well-characterized and commonly used actinometer system.Reaction rates were enhanced by a factor of up to ~40 when frozen at temperatures between 236 and 272 K.Reaction rates were dependent on temperature and solute concentration,both variables that control the nature of the liquid-like fraction in frozen water.The results obtained indicate that a major portion of the organic solutes is excluded to the liquid-like layer,significantly impacting the rate of the photochemical nucleophilic substitution reaction studied here.Also,the direct comparison of liquid-phase kinetics to reactions occurring in frozen water systems is drawn into question,indicating that a simple extrapolation of liquid-phase mechanisms to snow/ice may not be valid for certain reactions.
机译：众所周知，阳光普照的雪/冰在大气物质的加工中起着重要作用，包括光化学生产NO_x，HONO，分子卤素，卤代烷和羰基化合物等。液态或无序的层存在于纯冰的表面上，并且在类似于极性条件的温度下，这种准液态层也存在于周围的雪晶和冰的表面上。存在于冷冻水中和在冷冻水中的部分在潜在的光化学反应中发挥作用，特别是在有机底物方面。在此，我们报告了对吡啶的对硝基苯甲醚（PNA）的光化学亲核取代速率提高的详细研究，该吡啶具有很好的表征和普遍用途。在236至272 K的温度下冷冻时，反应速率提高了约40倍。反应速率取决于温度和溶质浓度，两者都不同能够控制冷冻水中液体状馏分的性质。获得的结果表明，大部分有机溶质被排除在液体状层之外，这极大地影响了此处研究的光化学亲核取代反应的速率。有人将液相动力学与冷冻水系统中发生的反应的直接比较提出了疑问，这表明将液相机理简单推算到雪/冰中可能不适用于某些反应。

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