Dispersion interactions in isolated molecules and molecular aggregates analyzed by IR/UV and Raman/UV double resonance spectroscopy

在这个项目中，我们分析了各种具有竞争结构安排的分子系统，其中伦敦色散力对能量偏好很重要。此外，只有当色散相互作用足够强时才稳定的二聚体被考虑在内，如取代的六苯基乙烷二聚体，例如氢原子之间的距离异常短。色散力的影响和数量将在光谱技术的帮助下在分子束实验中得到振动频率和电子能量，并与量子化学方法进行比较。要研究的一组系统是醚-醇簇，其中醇分子可以与醚氧或π云形成氢键，并且由于分散相互作用量的变化，不同的醇侧链能够控制对接偏好。另外，一个附着的芳香醇分子可以与不同的烷基侧链相互作用，这是由分散相互作用的强度驱动的。研究的另一类簇是不对称酮（包括受保护的氨基酸）与醇分子的聚集。在这里，醇分子的侧链相对于羰基氧原子的两个孤对可能有不同的取向。所有对醚-醇和酮-醇团簇的分析都描述了几乎等能结构之间的临界平衡，即提出了难以进行理论预测的情况，因为关于能量差的不确定性可以在零点能量差的区域内。我们将分子束研究重点放在中性和部分离子簇的电子基态上，但也通过应用各种组合质量和异构体选择性IR/UV技术以及受激拉曼/UV技术来研究中性物种的激发态。此外，将开发和应用一种新的组合红外/拉曼/紫外变体，这对异构体选择性测量很重要。我们的实验研究将与互补光谱方法、理论和合成领域的其他工作组密切合作。合成基团还为我们提供了特定的氘化或氟化化合物。提供与理论预测相比较的各种实验结果是一个普遍的目标。通过这种方法，可以改进和发展（新的）理论方法，以便更好地量化伦敦色散，特别是包括电子激发态的分析。

In this project we analyze a variety of molecular systems with competing structural arrangements in which London dispersion forces are important for the energetic preferences. Furthermore, dimers that are only stable if dispersion interactions are strong enough are considered, like substituted hexaphenyl ethane dimers with e.g. unusually short distances between H atoms. Both the influence and quantity of dispersion forces will be investigated with the help of spectroscopic techniques in molecular beam experiments yielding vibrational frequencies and electronic energies in comparison to quantum chemical methods. One set of systems to be investigated are ether–alcohol clusters in which the alcohol molecule can form a hydrogen bond to the ether oxygen or to a pi cloud and where a varying alcohol side-chain is able to control the docking preference due to changed amounts of dispersion interactions. Alternatively, an attached aromatic alcohol molecule can interact with different alkyl side chains, driven by the strength of dispersion interactions. A further class of investigated clusters are aggregates of asymmetric ketones (including protected amino acids) with alcohol molecules. Here, different orientations of the side-chains of the alcohol molecule with respect to the two lone pairs of the carbonyl oxygen atom are possible. All analyses, both for the ether–alcohol and ketone–alcohol clusters, describe a critical balance between nearly isoenergetic structures, i.e. difficult cases for theoretical predictions are presented, since the uncertainty with respect to energetic differences can be in the region of the difference of zero point energies. We focus our molecular beam investigations on the electronic ground state of neutral and partly ionic clusters, but also on the excited state of the neutral species by applying a variety of combined mass- and isomer-selective IR/UV techniques as well as stimulated Raman/UV techniques. Furthermore a new combined IR/Raman/UV variant which is important for isomer-selective measurements will be developed and applied. Our experimental investigations will be performed in close cooperation with other working groups in the field of complementary spectroscopic methods, theory, and synthesis. The synthetic groups also provide us with specifically deuterated or fluorinated compounds. It is a general aim to offer a variety of experimental results in comparison with theoretical predictions. By this an improvement and development of (new) theoretical methods can be achieved in order to get a better quantification of London dispersion, including especially the analysis of electronically excited states.