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

• 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; Dynamics; collisions; OH; radicals; organic

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 .

该提案涉及在气体和液体之间边界上发生的化学反应。对于分子在气体中反应时会发生什么，批次已经知道。因为分子的间隔相对较远，所以当它们确实碰撞时，每对分离有效相互作用。由于所涉及的原子数量越大，气体在固体表面的反应更为复杂。但是，这是通过固体的刚性来简化的，固体的刚度通常可以防止气体在原子外层下方穿透。固体结构也往往是规律的，使理论上的治疗更容易。对抗气体和液体之间的反应。关于那里发生的事情，知之甚少。在原子尺度上，表面更宽松，更柔软，边界的尖锐要少得多。从气体攻击的分子可能能够渗透到不同的深度，而周围分子的密度变化。由于没有常规的重复单元，因此需要从理论上进行大量原子。我们将使用我们开发的新实验方法研究一类特定的气体液体反应。我们将创建OH自由基，这是燃烧和大气化学中的关键物种之一，并与一系列有机液体相撞。液体将包含不同的官能团，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，OH反应与气相中这些类型的分子的机制从根本上有所不同。对于烷烃，OH直接从单个C-H单元中拉出H原子。相比之下，OH在烯烃中增加了C = C双键，形成了需要与另一个分子碰撞才能稳定的中间体。由于OH和氧化位点之间具有特殊的“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机制对不同液体表面上OH的反应性产生了什么后果。我们将使用激光光谱法检测逃逸OH来做到这一点。这不仅揭示了OH的反应（与惰性液体的散射相差），还揭示了逃脱OH的内部内部（旋转和任何振动）能量的形式。通过创建指导良好的OH的“分子束”的重要技术发展，将增强信息含量，从而揭示了散射分子移动的速度和方向的速度和方向。总体而言，这将为OH逃脱的OH提供特别完整的签名。实验结果与液体表面结构的计算“分子动力学”建模相辅相成，将使我们能够解决许多有趣的问题。 OH有多少直接相遇，一个或最多是在外层的几个“弹跳”，朝着明确的方向出现？相比之下，有多少暂时被困，留下了一个随机方向，放弃了大部分能量？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，它们在不同液体之间如何具有不同的反应机制？这些问题的答案目前未知。这使他们从根本上有趣。它们实际上也很重要。一个相关的例子是大气中微观气溶胶颗粒表面的反应。甚至有机分子的痕量水平也倾向于在水滴的外表面上积聚。 OH和其他物种的氧化是加工有机污染物的重要一步。它还具有气候后果，例如通过影响液滴占据进一步水并充当云态核的能力。