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Onset of charge separation in clusters: size-selective infrared spectroscopy of uncharged solute-solvent clusters

簇中电荷分离的开始：不带电溶质-溶剂簇的尺寸选择红外光谱

基本信息

批准号：
EP/E062970/1
负责人：
Andrew Michael Ellis
金额：
$ 54.82万
依托单位：
University of Leicester
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2007
资助国家：
英国
起止时间：
2007 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=EP%2FE062970%2F1
关键词：
Onset charge separation clusters size

项目摘要

The dissolution of substances in solvents is familiar to everyone. When a solid dissolves in a solvent it breaks down at the molecular scale into simple building blocks, such as small ions or molecules, which mix intimately with the solvent molecules. While much is known about this process, it is not easy to gain detailed knowledge of what is happening at the molecular scale. One way of tackling this is to use the 'cluster approach', whereby a small cluster of solute and solvent atoms/molecules in the gas phase is subjected to detailed study, e.g. by spectroscopy. If the cluster sizes are not too large, a supporting theoretical study is feasible and the combination of theory and spectroscopic data can then be interpreted to extract detailed information, such as the arrangement of solvent molecules around the solute. If the spectra from different sized clusters can be distinguished and assigned, then one potentially has the means to explore how the cluster properties change with the number of solvent molecules. This makes it possible to follow the evolution from a small solute-solvent cluster through to cluster sizes where the solute-solvent behaviour may begin to resemble the bulk solution.The cluster philosophy is well-established and in recent years it has begun to be applied to the detailed spectroscopic study of ion-solvent clusters. Charged clusters are especially amenable to this approach because different sized clusters are easily separated by mass spectrometry. By combining this mass selectivity with new tuneable infrared laser technology, important and exciting new information on ion-solvent clusters has been extracted by several research groups in the past few years using mass-selective infrared spectroscopy.The work proposed here takes a different approach. Our aim is to apply size-selective infrared spectroscopy to uncharged solute-solvent clusters. Two systems are targeted: (1) metal atom solutes combined with common solvent molecules such as water and ammonia; (2) alkali halide molecules (MX) in water and ammonia. In both cases there is a tendency for charge separation to occur in bulk solutions. For highly electropositive metals, such as the alkalis, the metal atom can release an electron into the solvent to form a solvated electron. The extent to which this occurs in clusters will depend on the identity of the metal, the identity of the solvent, and the number of solvent molecules. It is well-known that the alkali halides, MX, dissolve in bulk aqueous solutions to form the solvent-separated ions, M+ and X-, but in small clusters this ionic dissociation may be incomplete. We want to answer the following basic question: how many solvent molecules are required before the MX salt molecule fully dissociates into M+ and X- ions? Our aim is to employ infrared spectroscopy to extract this information.A challenge for this work is to achieve mass-selectivity in the spectroscopy of uncharged clusters. Our chosen approach is to record action spectra by observing the impact of the IR laser excitation on the mass-selected ion signals originating from UV laser photoionization. Three related action techniques will be available to tackle this problem, all requiring essentially the same equipment.The techniques we propose here are not limited to metal-solvent and alkali halide-solvent clusters. Other solute-solvent systems, such as base-solvent, acid-solvent and indeed other salt-solvent systems are potentially observable by this means. Consequently, this unique project will also serve as a stepping-stone for a longer term programme investigating solute-solvent interactions in neutral clusters and their link to solvation mechanisms.

物质在溶剂中的溶解是大家所熟悉的。当固体溶解在溶剂中时，它在分子尺度上分解成简单的结构单元，如小离子或分子，它们与溶剂分子密切混合。虽然对这个过程了解很多，但要获得分子尺度上发生的事情的详细知识并不容易。解决这一问题的一种方法是使用“簇方法”，即对气相中的溶质和溶剂原子/分子的小簇进行详细研究，例如通过光谱学。如果簇的大小不是太大，则支持理论研究是可行的，然后可以解释理论和光谱数据的组合以提取详细信息，例如溶质周围的溶剂分子的排列。如果不同大小的团簇的光谱可以区分和分配，那么人们就有可能探索团簇性质如何随溶剂分子数量而变化。这使得它可以遵循从一个小的溶质-溶剂集群的溶质-溶剂的行为可能开始类似于散装solution.The集群的大小的演变是行之有效的，近年来，它已开始被应用到详细的光谱研究的离子-溶剂集群。带电的团簇特别适合这种方法，因为不同大小的团簇很容易通过质谱法分离。通过将这种质量选择性与新的可调谐红外激光技术相结合，在过去的几年里，几个研究小组利用质量选择性红外光谱技术提取了关于离子溶剂团簇的重要和令人兴奋的新信息。我们的目标是将尺寸选择性红外光谱应用于不带电的溶质-溶剂团簇。有两个系统是针对性的：（1）与水和氨等常见溶剂分子结合的金属原子溶质;（2）水和氨中的碱金属卤化物分子（MX）。在这两种情况下，在本体溶液中都有发生电荷分离的趋势。对于高正电性的金属，如碱金属，金属原子可以释放一个电子到溶剂中形成溶剂化电子，这种情况在簇中发生的程度取决于金属的性质，溶剂的性质和溶剂分子的数量。众所周知，碱金属卤化物MX溶解在本体水溶液中以形成溶剂分离的离子M+和X-，但在小簇中，这种离子解离可能是不完全的。我们想回答以下基本问题：在MX盐分子完全解离成M+和X-离子之前，需要多少溶剂分子？我们的目标是利用红外光谱来提取这些信息，这项工作的一个挑战是实现不带电团簇光谱的质量选择性。我们选择的方法是通过观察红外激光激发对紫外激光光电离产生的质量选择离子信号的影响来记录作用光谱。三个相关的行动技术将可用于解决这个问题，所有需要基本上相同的设备。我们在这里提出的技术不限于金属溶剂和碱金属卤化物溶剂集群。其它溶质-溶剂体系，例如碱-溶剂、酸-溶剂和实际上其它盐-溶剂体系通过该方法是潜在可观察的。因此，这个独特的项目也将作为一个长期计划的垫脚石，调查中性集群中的溶质-溶剂相互作用及其与溶剂化机制的联系。