Project Details
Projekt
Dispersion interactions in isolated molecules and molecular aggregates analyzed by IR/UV and Raman/UV double resonance spectroscopy
Applicant
Privatdozent Dr. Christoph Riehn, since 5/2021
Subject Area
Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
Term
from 2015 to 2022
Project identifier
Deutsche Forschungsgemeinschaft (DFG) - Project number 271505332
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.
DFG Programme
Priority Programmes
Ehemaliger Antragsteller
Professor Dr. Markus Gerhards, until 4/2021 (†)
