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Dynamics of collisions of OH radicals with organic liquid surfaces

OH自由基与有机液体表面碰撞的动力学

基本信息

批准号：
EP/G029601/1
负责人：
Kenneth McKendrick
金额：
$ 81.95万
依托单位：
Heriot-Watt University
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2009
资助国家：
英国
起止时间：
2009 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=EP%2FG029601%2F1
关键词：
Dynamics collisions OH radicals organic

项目摘要

This proposal concerns the chemical reactions that take place at the boundary between a gas and a liquid.A lot is already known about what happens when molecules react in gases. Because the molecules are spaced relatively far apart, when they do collide each pair interacts effectively in isolation. Reactions of gases at the surfaces of solids are more complex because of the much larger number of atoms involved. However, this is simplified by the solid's rigidity, which normally prevents the gases from penetrating below the outer layer of atoms. Solid structures also tend to be regular, making it easier to treat them theoretically.Contrast this with reactions at the boundary between a gas and a liquid. Much less is known about what happens there. At an atomic scale, the surface is much looser and softer, and the boundary is much less sharp. Molecules attacking from the gas may be able to penetrate to different depths, with varying densities of surrounding molecules. Because there are no regular repeating units, a large number of atoms need to be treated theoretically.We will study a particular class of gas-liquid reactions using a new experimental method that we have developed. We will create OH radicals, one of the key species in combustion and atmospheric chemistry, and collide them with a range of organic liquids. The liquids will contain different functional groups, from saturated (alkanes) and unsaturated (alkenes) hydrocarbons, to oxidised (aldehydes, ketones, carboxylic acids) molecules. It is known that the mechanisms of OH reactions with these types of molecules in the gas phase differ fundamentally. For alkanes, the OH pulls an H atom directly from a single C-H unit. In contrast, OH adds to C=C double bonds in alkenes, forming energized intermediates that require a collision with another molecule to be stabilised. The reactions with oxidised molecules are distinct again, because of the special 'hydrogen-bonding' forces between OH and oxidised sites. We aim to discover what consequences these distinct mechanisms have on the reactivity of OH at different liquid surfaces. We will do this by detecting the escaping OH using laser-spectroscopy. This reveals not only how much OH has reacted (by difference from the scattering from an inert liquid), but also what form of internal (rotational and any vibrational) energy the escaping OH carries away. The information content will be enhanced by the important technical development of creating a well-directed 'molecular beam' of OH, revealing how fast and in what direction the scattered molecules are moving. Overall, this will give a particularly complete signature of the OH that escapes. The experimental results, complemented by computational 'molecular dynamics' modelling of the structure of the liquid surfaces, will allow us to address a number of intriguing questions. How much of the OH makes a direct encounter, with one, or at most a few 'bounces' at the outer layers, coming off in a well-defined direction? In contrast, how much becomes temporarily trapped, leaving in a random direction having given up most of its energy? How does the balance between these outcomes, and between either and chemical reaction, depend on how fast the OH is moving initially? Crucially, how do they vary between different liquids with distinct reaction mechanisms?The answers to these questions are currently unknown. This makes them fundamentally interesting. They are also practically important. One relevant example is reactions at the surfaces of microscopic aerosol particles in the atmosphere. Even trace levels of organic molecules tend to accumulate on the outer surfaces of aqueous droplets. Their oxidation, by OH and other species, is an important step in the processing of organic pollutants. It also has climatic consequences, e.g. by affecting the ability of the droplets to take up further water and act as cloud-condensation nuclei .

这一建议涉及发生在气体和液体交界处的化学反应。当分子在气体中发生反应时会发生什么，我们已经知道了很多。由于分子的间距相对较远，当它们碰撞时，每对分子都能有效地相互作用。气体在固体表面的反应更为复杂，因为涉及的原子数量要多得多。然而，由于固体的刚性，通常可以防止气体穿透原子外层以下，这简化了这一过程。固体结构也往往是规则的，这使得理论上更容易处理它们。将此与气体和液体交界处的反应进行对比。人们对那里发生的事情知之甚少。在原子尺度上，表面要宽松和柔软得多，边界也不那么尖锐。来自气体的分子可以穿透到不同的深度，周围分子的密度也不同。因为没有规则的重复单位，大量的原子需要从理论上进行处理。我们将用我们开发的一种新的实验方法来研究一类特殊的气液反应。我们将制造氢氧根，氢氧根是燃烧和大气化学中的关键物质之一，并将其与一系列有机液体碰撞。这些液体将含有不同的官能团，从饱和（烷烃）和不饱和（烯烃）碳氢化合物到氧化（醛、酮、羧酸）分子。众所周知，氢氧根与这些类型的分子在气相中的反应机制有根本的不同。对于烷烃，氢氧根直接从单个碳氢单元中拉出一个氢原子。相反，氢氧根在烯烃中加入C=C双键，形成带电的中间体，需要与另一个分子碰撞才能稳定。氧化分子的反应又不同了，因为OH和氧化位点之间有特殊的“氢键”力。我们的目的是发现这些不同的机制对OH在不同液体表面的反应性有什么影响。我们将用激光光谱学来探测逃逸的氢氧根。这不仅揭示了氢氧根反应了多少（通过惰性液体散射的差异），还揭示了逃逸的氢氧根带走了什么形式的内部（旋转和任何振动）能量。信息的内容将通过一项重要的技术发展得到增强，这项技术可以创造出定向良好的氢氧根“分子束”，揭示出分散的分子运动的速度和方向。总的来说，这将给出逃逸的氢氧根的一个特别完整的签名。实验结果，加上液体表面结构的计算“分子动力学”模型，将使我们能够解决许多有趣的问题。有多少氢氧根直接与一个，或最多几个，在外层发生“反弹”，沿明确的方向脱落？相比之下，有多少被暂时困住，在放弃大部分能量的情况下，在一个随机的方向上离开？这些结果之间的平衡，以及任何一种化学反应之间的平衡，如何取决于氢氧根最初移动的速度？关键是，它们在不同反应机制的不同液体之间是如何变化的？这些问题的答案目前还不得而知。这让它们从根本上变得有趣。它们实际上也很重要。一个相关的例子是大气中微小气溶胶颗粒表面的反应。即使是微量的有机分子也倾向于积聚在液滴的外表面。它们被OH和其他物质氧化是处理有机污染物的一个重要步骤。它还会对气候产生影响，例如，影响水滴吸收更多水分的能力，并起到云凝结核的作用。