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 .

该提议涉及气体和液体之间边界处发生的化学反应。关于分子在气体中反应时会发生什么，人们已经了解很多。由于分子之间的距离相对较远，因此当它们发生碰撞时，每对分子都会有效地孤立地相互作用。由于涉及的原子数量更多，固体表面的气体反应更加复杂。然而，固体的刚性使这一点变得简单，固体的刚性通常会阻止气体渗透到原子外层以下。固体结构也往往是规则的，这使得从理论上处理它们变得更容易。将其与气体和液体之间边界的反应进行对比。人们对那里发生的事情知之甚少。在原子尺度上，表面更加疏松和柔软，边界也不那么尖锐。来自气体的攻击分子可能能够渗透到不同的深度，周围分子的密度也不同。由于不存在规则的重复单元，因此需要从理论上处理大量原子。我们将使用我们开发的新实验方法来研究特定类别的气液反应。我们将产生 OH 自由基（燃烧和大气化学中的关键物质之一），并将它们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）烃到氧化（醛、酮、羧酸）分子。众所周知，这些类型的分子在气相中发生 OH 反应的机制根本不同。对于烷烃，OH 直接从单个 C-H 单元中拉出一个 H 原子。相比之下，OH 会加到烯烃中的 C=C 双键上，形成带电中间体，需要与另一个分子碰撞才能稳定。由于 OH 和氧化位点之间存在特殊的“氢键”力，与氧化分子的反应再次不同。我们的目的是发现这些不同的机制对不同液体表面的 OH 反应性有何影响。我们将通过使用激光光谱检测逸出的 OH 来做到这一点。这不仅揭示了 OH 发生了多少反应（与惰性液体的散射不同），还揭示了逸出的 OH 带走了何种形式的内部（旋转和任何振动）能量。创造定向良好的 OH 分子束的重要技术发展将增强信息内容，揭示分散分子移动的速度和方向。总体而言，这将给出逃逸的 OH 的特别完整的签名。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 OH 会直接遭遇，在外层发生一次或最多几次“反弹”，并沿明确的方向脱落？相比之下，有多少东西暂时被困住，在放弃大部分能量后沿随机方向离开？这些结果之间以及化学反应之间的平衡如何取决于 OH 最初移动的速度？至关重要的是，它们在具有不同反应机制的不同液体之间有何变化？这些问题的答案目前尚不清楚。这使得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往往会积聚在水滴的外表面。它们被 OH 和其他物质氧化，是有机污染物处理的重要步骤。它还具有气候后果，例如通过影响水滴进一步吸收水分并充当云凝结核的能力。