Bond energies of weakly bound molecules

基本双分子反应在气相中的反应速度通常由试剂和产物之间的能垒高度决定。根据阿累尼乌斯速率定律，大多数反应随着温度的升高而以指数级的速度进行。这是因为有足够能量越过势垒的分子的比例由玻尔兹曼分布函数决定。然而，一些反应表现出负的温度依赖性，其中反应速度随着温度的降低而增加。当反应无障碍时，这种所谓的非阿累尼乌斯行为就会发生。一类有趣的反应显示了Arrhenius在高温下的行为，但在低温下也会加快。这种行为归因于由于试剂之间的范德华作用力而在入射通道中存在长程弱结合络合物，而经常表现这种行为的一类重要反应是自由基与分子之间的反应。这些反应在地球大气层对流层顶的寒冷环境、气态行星的大气层和它们的一些卫星，以及星际空间中年轻恒星周围的气体云中特别重要。最近，在实验室中捕获这些弱结合物种并研究它们的性质已经成为可能。玛莎·莱斯特和她的同事在宾夕法尼亚大学进行了一项实验，通过用红外辐射脉冲激励这些络合物，并在络合物被激光诱导荧光加热时观察其中一个产生的碎片，来测量这些络合物的红外吸收光谱。通过计算解离光子的能量与在光碎片中观察到的最大内能之间的能量差，他们能够推导出该络合物的键离解能的上限。在氢三氧自由基HO-OO的情况下，他们这样获得的值足够大，以至于在地球上的大气中，大约四分之一的OH自由基预计会与分子氧络合，如果是真的，将引发我们对对流层顶化学的重大反思。另一方面，理论预测HO-OO的键离解能大约是实验所建议的值的一半。这种差异的原因很可能是因为假设解离是无障碍的，所需要的是对络合物的键能的直接测量。我们建议使用一种称为速度图成像的技术来实现这一点。尽管我们相当有信心地了解了在氢三氧基的情况下，理论和实验之间出现明显差异的原因，但我们的实验将是第一次直接测量离解能，并将提供评估该物种真正的大气重要性所需的基本数据。更广泛地说，我们建议研究一些其他类似的络合物，并通过测量光产物中的内部能量分布以及它们的反冲速度矢量与旋转角动量之间的关联等性质来获得关于它们的解离动力学的准确数据。我们研究的主要成果将是提供关于这些络合物的键能的数据，这些数据可以与量子力学电子结构计算进行比较，并输入到自由基和分子反应的化学动力学模型中。

The rate of reaction of elementary bimolecular reactions in the gas phase is usually determined by the height of the energy barrier between reagents and products. Most reactions proceed exponentially faster as the temperature is raised according to the Arrhenius rate law. This is because the fraction of molecules with sufficient energy to surmount the barrier is determined by the Boltzmann distribution function. Some reactions however exhibit a negative temperature dependence in which the rate of reaction increases as the temperature decreases. This so-called non Arrhenius behaviour occurs when the reaction is barrierless. An interesting class of reactions show Arrhenius behaviour at high temperatures but also speed up at low temperatures. This behaviour is attributed to the existence of long range weakly bound complexes in the entrance channel due to van der Waals forces between the reagents, and an important class of reactions which often exhibit this behaviour are those between radicals and molecules. These reactions are of particular importance in the cold environments of the tropopause of the earth's atmosphere, the atmospheres of the gaseous planets and some of their moons, and in the gas clouds surrounding young stars in interstellar space. Recently it has become possible to trap these weakly bound species in the laboratory and to study their properties. An experiment carried out at the University of Pennsylvania by Marsha Lester and her co-workers measures the infrared absorption spectra of these complexes by exciting them with a pulse of infrared radiation and observing one of the resulting fragments as the complex is heated using laser induced fluorescence. By calculating the energy difference between the energy of the dissociating photon and the maximum internal energy observed in the photofragment they are able to deduce an upper bound for the bond dissociation energy of the complex. In the case of the hydrotrioxy radical, HO-OO, the value they so obtain is large enough that in the upper earth atmosphere about a quarter of the OH radicals would be expected to be complexed with molecular oxygen and if true would provoke a significant rethink of our understanding of the chemistry of the tropopause. Theory on the other hand predicts a bond dissociation energy for HO-OO that is about half the value suggested by the experiment. The reason for the discrepancy is most probably due to the assumption that the dissociation is barrierless and what is required is a direct measurement of the bond energy of the complex. We propose to do this using a technique called velocity map imaging. Even though we are fairly confident that we understand the reason for the apparent discrepency between theory and experiment in the case of the hydrotrioxy radical, our experiment would be the first direct measurement of the dissociation energy and would provide the essential data needed to assess the true atmospheric importance of the species. More generally we propose to study a number of other similar complexes and obtain accurate data on their dissociation dynamics by measuring properties such as the internal energy distribution in the photoproducts and correlations between their recoil velocity vectors and rotational angular momenta. The principal output of our research will be to provide data on the bond energies of these complexes which may be compared to quantum mechanical electronic structure calculations and fed into chemical kinetic models of the reactions of radicals and molecules.